LARGE-SCALE PHENOMENA NETWORK                                                                            
                                                                                                                                  
                                                                                                                                  
I. INTRODUCTION                                                                                                                   
                                                                                                                                  
        In order both to conduct a second trial run and to support the 1985                                                       
September 11 encounter of the International Cometary Explorer (ICE) with Peri-                                                    
odic Comet Giacobini-Zinner (G-Z), the International Halley Watch was broadened                                                   
in scope to include observations of G-Z.  For most of the IHW networks, the                                                       
observation interval was limited essentially to 1985 July-September.  Supporting                                                  
the ICE mission was an especially important activity for the Large-Scale Phe-                                                     
nomena Network (LSPN) because the spacecraft was being targeted to pass through                                                   
the comet's plasma tail; observations of the length, width, and variability of                                                    
the tail via wide-field photography were seen to be important to a full inter-                                                    
pretation of the in situ data.                                                                                                    
                                                                                                                                  
        The ICE mission was successful in nearly every sense, as the plasmas and                                                  
magnetic fields were measured directly for the first time in any comet (Bame et                                                   
al. 1986; Smith et al. 1986; also see Brandt et al. 1985, von Rosenvinge et al.                                                   
1986).  Remote data of several types taken near and at the time of the                                                            
encounter not only helped planners make late changes to the mission profile                                                       
during the final approach to encounter on 1985 September 11, but also assisted                                                    
scientists in the post-encounter analysis of the spacecraft observations.                                                         
                                                                                                                                  
        An example of the latter is that the width of the so-called plasma                                                        
sheet measured by ICE was found to correspond approximately to the tail                                                           
width observed in ground-based images and spectrograms (Slavin et al. 1986;                                                       
Meyer-Vernet et al. 1986), thereby confirming the idea of cometary plasma tails                                                   
as visible manifestations of the plasma sheet, and not the much broader                                                           
ensemble of magnetic lobes (with embedded plasma sheet).  An example of                                                           
pre-encounter support was provided by IUE: observations of the comet's gas                                                        
production rate in the weeks before encounter by M.F. A'Hearn and colleagues                                                      
allowed calculations of the expected size of the bowshock.  These values were                                                     
then used by mission controllers to ensure that the Deep Space Network (DSN)                                                      
was operating at the theoretical shock-crossing times. These are but two                                                          
examples which illustrate the synergism which often operates between in situ                                                      
and remote data.                                                                                                                  
                                                                                                                                  
        The purpose of this overview is to describe the content of the Large-                                                     
Scale Phenomena (L-SP) Giacobini-Zinner archives--printed and digital CD-ROM                                                      
(compact disk, read-only memory)--assembled from observations made by the LSPN                                                    
Observers.  Sections I-IV were written by M.B. Niedner, and Section V by A.                                                       
Warnock and Niedner.                                                                                                              
                                                                                                                                  
                                                                                                                                  
II. OBSERVATIONS OF COMET GIACOBINI-ZINNER BY THE LARGE-SCALE PHENOMENA NETWORK                                                   
                                                                                                                                  
        Table I lists the L-SP Discipline Specialist personnel who made essenti-                                                  
ally full-time contributions to the G-Z archive for a period of a year or more;                                                   
others are thanked in the acknowledgement section at the end of this overview.                                                    
The team received 118 wide-field images of G-Z from 9 different observing sites.                                                  
The images are of all types--glass plates, film copies, prints, and slides--and                                                   
were taken on 36 nights between 1985 July 8 and September 26.  Information about                                                  
the observers, instruments (including their aperture, type, f ratio and scale),                                                   
and observatories (including their system codes; cf. Sec. III below) is listed                                                    
in Table II.  The numbers of images per UT day are given in Table III.                                                            
                                                                                                                                  
                                                                                                                                  
Table I.  Discipline Specialist Team of the Large-Scale Phenomena Network                                                         
________________________________________________________________________________                                                  
                                                                                                                                  
Team Member              Affiliation                    Responsibility                                                            
________________________________________________________________________________                                                  
                                                                                                                                  
Malcolm B. Niedner, Jr.  Laboratory for Astronomy       Discipline Specialist                                                     
                           and Solar Physics (LASP)                                                                               
                         NASA-Goddard Space Flight                                                                                
                           Center (GSFC)                                                                                          
                         Greenbelt, MD 20771                                                                                      
                         U.S.A.                                                                                                   
                                                                                                                                  
John C. Brandt           Laboratory for Atmospheric     Discipline Specialist                                                     
                           and Space Physics                                                                                      
                         University of Colorado                                                                                   
                         Boulder, CO 80309                                                                                        
                         U.S.A.                                                                                                   
                                                                                                                                  
Juergen Rahe             Dr. Remeis Sternwarte          Discipline Specialist                                                     
                         Universitat Nurnberg-Erlangen                                                                            
                         D-8600 Bamberg                                                                                           
                         Federal Republic of Germany                                                                              
                                                                                                                                  
Daniel A.                LASP                           Senior Team Member                                                        
  Klinglesmith III       NASA-GSFC                                                                                                
                                                                                                                                  
Archibald Warnock III    ST Systems Corporation         Senior Software                                                           
                         Lanham, MD 20784                 Specialist                                                              
                         U.S.A.                                                                                                   
                                                                                                                                  
Barbara B. Pfarr         ST Systems Corporation         Archive Manager                                                           
                                                                                                                                  
Nancy E. Podger          ST Systems Corporation         Software Assistant                                                        
                                                                                                                                  
Joan E. Isensee          ST Systems Corporation         Data/Software Assistant                                                   
                                                                                                                                  
Steve B. Howell          ST Systems Corporation         Software Assistant                                                        
                                                                                                                                  
Edwin J. Grayzeck, Jr.   Interferometrics, Inc.         CD-ROM Specialist                                                         
                         Vienna, VA 22180                                                                                         
                         U.S.A.                                                                                                   
________________________________________________________________________________                                                  
                                                                                                                                  
                                                                                                                                  
Table II.  Observer List by Observatory, Telescope Parameters, and System Codes                                                   
________________________________________________________________________________                                                  
                                                                                                                                  
Observatory           Telescope          Longitude   Latitude    Observer(s)                                                      
                                        [deg/min/s] [deg/min/s]                                                                   
________________________________________________________________________________                                                  
                                                                                                                                  
Hoher List Obs.       30 cm Astrograph   006/50/58   +50/09/47   Geffert, M.                                                      
Hoher List            f/5                                                                                                         
Fed. Rep. of Germany  137 arcsec/mm                                                                                               
SYSTEM = 30170300                                                                                                                 
                                                                                                                                  
National Astr. Obs.   70 cm Schmidt      024/43/30   +41/43/00   Ivanova, V.,                                                     
Rojen                 f/2.8                                       Shkodrov, V.                                                    
Bulgaria              120 arcsec/mm                                                                                               
SYSTEM = 35000301                                                                                                                 
                                                                                                                                  
Chorzow Obs.          20 cm Astrograph   018/59/30   +50/17/33   Wlodarczyk, I.                                                   
Chorzow               f/5                                                                                                         
Poland                206 arcsec/mm                                                                                               
SYSTEM = 35530100                                                                                                                 
                                                                                                                                  
Joint Obs. for        36 cm Schmidt      252/48/41   +33/59/06   Arnold, G.                                                       
  Comet. Res.         f/2                                         Bair, L.                                                        
Socorro, NM           298 arcsec/mm                               Brandt, J.                                                      
U.S.A.                                                            Marr, E.                                                        
SYSTEM = 37020100                                                 Moore, E.                                                       
                                                                  Niedner, M.                                                     
                                                                                                                                  
E. E. Barnard Obs.    30 cm Schmidt      254/37/30   +39/52/30   Emerson, G.                                                      
Golden, CO            f/1.8                                                                                                       
U.S.A.                380 arcsec/mm                                                                                               
SYSTEM = 35001604                                                                                                                 
                                                                                                                                  
A. J. Dyer Obs.       60 cm Schmidt      273/11/41   +36/03/09   Heiser, A.                                                       
Nashville, TN         f/3.5                                                                                                       
U.S.A.                99 arcsec/mm                                                                                                
SYSTEM = 35001601                                                                                                                 
                                                                                                                                  
Goddard Research      20 cm Cel. Schm.   283/10/23   +39/01/16   Niedner, M.,                                                     
  Laboratory          f/1.5                                       Schmidt, R.                                                     
Greenbelt, MD         680 arcsec/mm                                                                                               
U.S.A.                                                                                                                            
SYSTEM = 35001610                                                                                                                 
                                                                                                                                  
Mt. Cuba Astr. Obs.   61 cm Baker Cam.   284/21/59   +39/47/04   Buckley III, J.                                                  
Greenville, DE        f/4                                         Buckley Jr, J.                                                  
U.S.A.                85 arcsec/mm                                Sharp, W.                                                       
SYSTEM = 37880100                                                 Westergard, B.                                                  
                                                                                                                                  
Calar Station         80 cm Schmidt      357/27/50   +37/13/45   Kohoutek, L.                                                     
National Astr. Obs.   f/3                                                                                                         
Calar Alto Mt.        86 arcsec/mm                                                                                                
Spain                                                                                                                             
SYSTEM = 34930200                                                                                                                 
________________________________________________________________________________                                                  
                                                                                                                                  
                                                                                                                                  
Table III.  Distribution of Images by Day                                                                                         
________________________________________________________________________________                                                  
                                                                                                                                  
Date    Number of    Date    Number of    Date    Number of    Date    Number of                                                  
1985 UT  Images      1985 UT  Images      1985 UT  Images      1985 UT  Images                                                    
________________________________________________________________________________                                                  
                                                                                                                                  
Jul.  8     1        Aug. 17     3        Sept. 8     2        Sept.17     4                                                      
     12     2             21     1              9     5             18     4                                                      
     13     6             22     3             10     1             19     1                                                      
     17     2             23     3             11     5             20     2                                                      
     19     1             24     2             12     4             21     1                                                      
     20     2             27     1             13     7             22     1                                                      
     25     5        Sept. 5     7             14     3             24     3                                                      
Aug.  8     1              6     8             15     2             25     1                                                      
     13     1              7     9             16    13             26     1                                                      
________________________________________________________________________________                                                  
                                                                                                                                  
                                                                                                                                  
        Considering that the L-SP Network is comprised of over 100 observato-                                                     
ries, the image yield was not very high for G-Z, but this was perhaps not un-                                                     
expected.  In a comparative sense, Giacobini-Zinner was not a bright or large-                                                    
scale phenomena object, but on many dates it did possess a detectable plasma                                                      
tail.  Although week-to-week differences were seen in the length and brightness                                                   
of the plasma tail--differences which seemed to follow the overall trend of the                                                   
gas production rate--it is also true that G-Z was not observed (and was not                                                       
expected) to display any of the rapid plasma-tail changes, such as disconnection                                                  
events, folding tail rays, etc., which characterize the brighter comets and                                                       
which make dense coverage absolutely essential for studying them.  Taken in this                                                  
context, we feel that the L-SP Network's response to the G-Z opportunity was                                                      
excellent, and that the G-Z archive contains a useful body of data for scien-                                                     
tific purposes.                                                                                                                   
                                                                                                                                  
                                                                                                                                  
III. ORGANIZATION AND CONTENT OF THE PRINTED ARCHIVE                                                                              
                                                                                                                                  
        Contained in the printed version of the G-Z archive is a one line table-                                                  
format summary for each of the 118 images received by the L-SP Discipline Spe-                                                    
cialist Center and an annotated hardcopy image for 23 of the observations.  For                                                   
each image the parameters listed are shown in Table IV.                                                                           
                                                                                                                                  
                                                                                                                                  
Table IV.  Entries in the Printed Archive                                                                                         
________________________________________________________________________________                                                  
                                                                                                                                  
Heading      Description                                                                                                          
________________________________________________________________________________                                                  
                                                                                                                                  
Date(UT)     Date (day & fraction of day) of middle of observation                                                                
LSPN#        Large-Scale Phenomena Network number--a unique number for each                                                       
               observation. (The lead digit 3 indicates that the Discipline is                                                    
               Large-Scale Phenomena)                                                                                             
PlateID      ID number provided by the submitting observatory on the original                                                     
               plate jacket or reverse side of print                                                                              
Ap           Telescope aperture size [m]                                                                                          
Scale        Scale of the original plate [arcsec/mm].  For copy prints submitted                                                  
               by observers, the scale is that of the original plate, not of the                                                  
               print                                                                                                              
Instrument   Type of instrument used                                                                                              
FOV          Field of view of the original plate [deg].  For copy prints submit-                                                  
               ted by observers, the listed FOV is that of the original plate,                                                    
               not of the print                                                                                                   
ExpM         Exposure time [min]                                                                                                  
Emul         Type of emulsion used for original image                                                                             
Filter       Type of filter used                                                                                                  
Hyp          Plate hypersensitization (Y or N)                                                                                    
Cal          Calibration data accompanying the image (Y or N)                                                                     
St           Status of image.  A (for Archive) indicates that the image was di-                                                   
               gitized and is included in the CD-ROM video archive.  DS (for                                                      
               Discipline Specialist) indicates that the image was not scanned,                                                   
               but that the DS either has the original plate, a film copy, or a                                                   
               photographic print                                                                                                 
System       Observatory System Code (see Table II and general appendix listing                                                   
               system codes for all networks)                                                                                     
Observer(s)  Name(s) of the observer(s)                                                                                           
________________________________________________________________________________                                                  
                                                                                                                                  
                                                                                                                                  
        For the most part, the hardcopy images in the printed archive are of                                                      
original plates and films which were scanned and deposited on the CD-ROM. There                                                   
are exceptions, however: several of the prints received showed more of the                                                        
comet than some of the scanned plates, and were included in hardcopy form                                                         
to illustrate the state of comet Giacobini-Zinner during important times.                                                         
Moreover, mostly for practical reasons, several of the scanned images do not                                                      
have hardcopy representations in the printed archive.  It is hoped that this                                                      
slight deviation from one-to-one between the scanned images and the hardcopy                                                      
photographs in the printed archive will not cause confusion.  A quick check of                                                    
each hardcopy LSPN# in the table will show whether the original image was                                                         
scanned (St = A) or not (St = DS).                                                                                                
                                                                                                                                  
        The hardcopy images were generated from film copies made of the original                                                  
glass plates or prints; none are hardcopy of actual digital images. The                                                           
distance scale below each image equals the angular length on the sky of the                                                       
quoted linear distance along the prolonged radius vector (PRV).  In other                                                         
words, the scale provides approximately correct distances along the plasma                                                        
tail, and not in the sky plane (at the distance of the comet).                                                                    
                                                                                                                                  
                                                                                                                                  
IV. ORGANIZATION AND CONTENT OF THE DIGITAL (CD-ROM) ARCHIVE                                                                      
                                                                                                                                  
A. Scanning Criteria at the L-SP Discipline Specialist Center                                                                     
                                                                                                                                  
        Although the total number of G-Z images was not large, careful                                                            
consideration was still given to which plates and original film copies were to                                                    
be digitized, because scanning time for G-Z was in a very real sense                                                              
competition for the much larger set of Halley imagery.  The selection                                                             
criteria were:  plate quality, date of observation, and UT day fraction.  The                                                     
first of these is obvious, the rationale for the last two being the desire to                                                     
have reasonably good day-to-day coverage, and no more than two images on any                                                      
given day, separated by at least 12 hours (since the comet was slowly-varying).                                                   
The selection of images to be scanned was made by M.B. Niedner.                                                                   
                                                                                                                                  
       Application of the scanning criteria to the entire set of glass plates                                                     
and first generation film copies resulted in 24 digitized images of comet                                                         
Giacobini-Zinner spanning 1985 July 20-September 26. The remaining 94 images                                                      
were either unscanned (failing one or more of the 3 criteria) or unscannable                                                      
(e.g., photographic prints). As a result of both the criteria tests and the                                                       
temporal unevenness of network submissions, coverage in the scanned images is                                                     
not as dense around the time of encounter as had been hoped.  For details on                                                      
the actual microdensitometer procedures used on the G-Z images, consult Sec. V.                                                   
                                                                                                                                  
        In the event that users of the CD-ROM archive have a need for additional                                                  
digitized plates, it is worth noting that, unless it has been otherwise agreed                                                    
to, the original glass plates will have been returned by the DS to the                                                            
observers who obtained them.  Given that this is the most probable situation,                                                     
it would be most efficient to first contact the observer.  If the plate is                                                        
residing at the DS Center indefinitely, the DS would in most instances be able                                                    
to digitize the image and supply it to the user on magnetic tape.                                                                 
                                                                                                                                  
B. Scanned L-SP Images of Giacobini-Zinner on CD-ROM                                                                              
                                                                                                                                  
        The scanned L-SP images of comet Giacobini-Zinner were deposited on                                                       
CD-ROM as three separate files: a PDS (= Planetary Data System) label, a FITS                                                     
(Flexible Image Transport System) header, and the actual binary image data.  The                                                  
PDS label is a relatively short (10-15 lines) "header" which allows users with                                                    
PDS software (e.g., IMDISP; ref: IMDISP User's Manual, M. Martin et al., JPL)                                                     
to access the image data with a personal computer. The FITS header, a sample of                                                   
which is given in Table V, contains all the "data about the data", and allows                                                     
the user to access the image with large astronomical data packages such as AIPS                                                   
and IRAF. It should be noted that the image data files have been compressed                                                       
using a previous pixel algorithm.  An algorithm for decompressing the images is                                                   
included on the CD-ROM in pseudo-code, and a hardcopy listing is published in                                                     
the printed archive.                                                                                                              
                                                                                                                                  
                                                                                                                                  
Table V.  Sample FITS header for Large-Scale Phenomena                                                                            
________________________________________________________________________________                                                  
                                                                                                                                  
Keyword    Content                Explanation                                                                                     
________________________________________________________________________________                                                  
                                                                                                                                  
SIMPLE  =                    T  / Basic FITS format                                                                               
BITPIX  =                   16  / 2-byte two's compl integers                                                                     
NAXIS   =                    2  / Number of axes, 2 = single image, 3 = multiple                                                  
NAXIS1  =                 1024  / Pixels per row                                                                                  
NAXIS2  =                 1024  / Lines per image                                                                                 
OBJECT  = 'P/GIACOBINI-ZINNER'  / Name of object (on some files: 'P/G-Z   ')                                                      
FILE-NUM=               300305  / Number of file sent to JPL, dnnnnn                                                              
DATE-OBS= '21/09/85          '  / Date of mid-exposure, dd/mm/yy                                                                  
TIME-OBS=              0.05199  / Time of observation, day fraction                                                               
DATE-REL= '04/06/87          '  / Data release date, dd/mm/yy                                                                     
DISCIPLN= 'LARGE SCALE PHEN  '  / IHW Network                                                                                     
LONG-OBS= ' 24/43/30         '  / Station longitude ddd/mm/ss (positive to east)                                                  
LAT--OBS= '+41/43/00         '  / STATION latitude sdd/mm/ss (s = + or -)                                                         
SYSTEM  = '35000301          '  / Observing system code                                                                           
OBSERVER= 'IVANOVA,V/SHKODROV,V             '  / Name(s) of observer(s)                                                           
SUBMITTR= 'NIEDNER,M         '  / LSPN submitter                                                                                  
SPEC-EVT=                    F  / Special events flag                                                                             
DAT-FORM= 'STANDARD          '  / Standard FITS data                                                                              
DAT-TYPE= 'IMAGE             '  / Type of FITS data                                                                               
LSPNSEQ =               194331  / Sequence # of plate received at L-SP Center                                                     
COMMENT                                                                                                                           
COMMENT                         / Submitting station                                                                              
OBSVTORY= 'NATIONAL AST. OBS.'  / Name of observatory                                                                             
LOCATION= 'ROJEN, BULGARIA   '  / Station location--address                                                                       
COMMENT                                                                                                                           
COMMENT                         / Instrument specifications                                                                       
TELESCOP= 'SCHMIDT           '  / Type of telescope                                                                               
APERTURE=                0.700  / Telescope aperture [m]                                                                          
FRATIO  =                2.800  / Effective telescope f ratio                                                                     
FOV1    =                5.000  / R.A. field of view [deg]                                                                        
FOV2    =                5.000  / Dec field of view [deg]                                                                         
COMMENT                                                                                                                           
COMMENT                         / Plate specifications                                                                            
PLTSZE1 =                0.160  / X plate size [m]                                                                                
PLTSZE2 =                0.160  / Y plate size [m]                                                                                
PLTSCALE=              121.112  / Plate scale [arcsec/mm]                                                                         
CALAVL  =                    T  / Calibration available                                                                           
HYPSEN  =                    F  / Hypersensitization                                                                              
EMULSION= 'IIIA-J            '  / Emulsion type                                                                                   
FILTER  = 'M                 '  / Type of filter used                                                                             
PLTYPE  =                    F  / Plate type, T = original, F = copy                                                              
COMMENT                                                                                                                           
COMMENT                         / Observation specifications                                                                      
EXPOSURE=                1800.  / Exposure time [s]                                                                               
HISTORY   OBSLOG  = '2850    '  / Original observatory plate                                                                      
HISTORY   CMTS-LOG= 'COMET IS FAINT; TAIL IS BROAD AND DIFFUSE, PROBABLY DUST. '                                                  
COMMENT                                                                                                                           
COMMENT                         / Tape specifications                                                                             
BSCALE  =                  1.0  / Scale factor, TRUE = TAPE * BSCALE + BZERO                                                      
BZERO   =                  0.0  / Offset for true pixel value                                                                     
BUNIT   = 'DENSITY           '  / Units of pixel values                                                                           
BLANK   =               -32767  / Value for undefined pixel                                                                       
COMMENT                                                                                                                           
COMMENT                         / Digitization specifications                                                                     
HISTORY   SCNAPR  =   20        / PDS scan aperture [micron]                                                                      
HISTORY   SCNSTP  =   20        / PDS scan step [micron]                                                                          
HISTORY   SKYDEN  =  412        / Sky background PDS density                                                                      
HISTORY   SKYUNF  = 'FAIR    '  / Sky background: POOR/FAIR/GOOD/EXCL                                                             
COMMENT                                                                                                                           
COMMENT                         / Astrometry information                                                                          
EQUINOX =               1950.0  / Equinox of coordinate system                                                                    
CTYPE1  = 'RA---ARC          '  / X axis units                                                                                    
CTYPE2  = 'DEC--ARC          '  / Y axis units                                                                                    
RA--CPME=         0.961125E+02  / R.A. plate center mid-exposure                                                                  
DEC-CPME=         0.102762E+02  / Dec plate center mid-exposure                                                                   
CRPIX1  =         0.600200E+03  / Reference point for AXIS1, samples                                                              
CRPIX2  =         0.263250E+03  / Reference point for AXIS2, lines                                                                
CDELT1  =        -0.672847E-03  / R.A. per pixel [deg]                                                                            
CDELT2  =        -0.672847E-03  / Dec per pixel [deg]                                                                             
CROTA1  =         0.317443E+00  / Rotation angle in R.A. [deg]                                                                    
CROTA2  =         0.317443E+00  / Rotation angle in Dec [deg]                                                                     
SENSE   =                    T  / Sense of rotation, T=counterclws., F=clockwise                                                  
CRVAL1  =         0.961125E+02  / R.A. at reference point [deg]                                                                   
CRVAL2  =         0.102762E+02  / Dec at reference point [deg]                                                                    
RA--HEAD=         0.961554E+02  / R.A. of comet head at mid-exposure [deg]                                                        
DEC-HEAD=         0.101321E+02  / Dec of comet head at mid-exposure [deg]                                                         
COMMENT                                                                                                                           
COMMENT                         / Processing and analysis information                                                             
DATE-REC= '04/06/87          '  / Date plate was received at GSFC, dd/mm/yy                                                       
HISTORY   DATE-PDS= '29/03/88'  / Date of plate digitization, dd/mm/yy                                                            
HISTORY   DATE-PRC= '04/04/88'  / Date processed/calibrated, dd/mm/yy                                                             
QUALITY = '2222              '  / Quality of plate,guiding,trails,seeing,transp.                                                  
HISTORY  CONVERTED TO FITS BY PROGRAM IHWP2F WED APR 06,1988                                                                      
HISTORY  PDS OPERATOR'S COMMENT FOLLOWS:                                                                                          
HISTORY 880329 ID194310 S200 A20 L1024 P1024                                                                                      
END                                                                                                                               
________________________________________________________________________________                                                  
                                                                                                                                  
                                                                                                                                  
        The meaning of the FITS keywords is fairly straightforward in the exam-                                                   
ple header given (see text descriptions to the right of the / in each line), and                                                  
most will not be belabored here.  Many of the keywords near the top of the                                                        
header--BITPIX, NAXIS, NAXIS1, NAXIS2, etc.--are standard, mandatory keywords                                                     
which have been described by Wells et al. (1981).  The reader is referred to                                                      
that paper for further details.                                                                                                   
                                                                                                                                  
        DATE-REL was originally intended to give the earliest possible release                                                    
date for imagery contributed by network members, but is now irrelevant since the                                                  
archive is complete and is being distributed.  The SYSTEM code denotes the ob-                                                    
servatory and the instrument used to obtain the image; refer to Table II and to                                                   
the general appendix listing for this information for all disciplines.  SPEC-EVT                                                  
has been set = F for all the G-Z images because nothing truly outstanding was                                                     
apparent in any of them. [In Halley, disconnection events, bursts of ray                                                          
generation, and major distortions of the plasma tail merit the designation                                                        
SPEC-EVT = T, but none of these were seen in G-Z].  PLTYPE designates whether                                                     
the image was an original or a copy made by the contributing observer.  In some                                                   
cases, when the observer sent the DS a high quality film copy only, it was                                                        
possible to scan that copy. The history keywords OBSLOG, CMTS-OBS, and LSPN-OBS                                                   
give the observatory plate ID, observer comments, and Discipline Specialist                                                       
assessment of the image, respectively.                                                                                            
                                                                                                                                  
         Most of the keywords in the astrometry section have names and meanings                                                   
identical to keywords defined by Wells et al. (1981), but in accordance with                                                      
their strong suggestion, the uses and conventions of CROTA1, CROTA2, and SENSE                                                    
will be defined precisely here.  All three keywords refer to the rotation of                                                      
the X-Y PDS (microdensitometer) coordinate frame relative to the axes of the                                                      
celestial coordinate system.  To reasonable accuracy, the +Y scanning axis is                                                     
in the direction of North at the plate center, for the epoch of 1950.0.  The +X                                                   
scanning axis is in the approximate direction of due East (again, at the plate                                                    
center for 1950.0). As a result of the inevitable human error in positioning                                                      
the plate on the PDS platen, however, there will be a non-zero rotation angle                                                     
offset (very small) between the two coordinate systems.  When the North                                                           
direction lies in the +X/+Y quadrant, i.e., is rotated in a counterclockwise                                                      
direction relative to the +Y axis, we say that the rotation angles CROTA1 and                                                     
CROTA2 are both positive (and of the same value).  A positive angle for such a                                                    
counterclockwise rotation is what is meant by SENSE = T (check the text to the                                                    
right of the SENSE keyword).                                                                                                      
                                                                                                                                  
        It will be noted from the values of RA-CPME, DEC-CPME, CRVAL1, and                                                        
CRVAL2 in the sample header, that we define the reference point on the plate to                                                   
be the tangent point or plate center.  The coordinates for the approximate                                                        
comet head center are given by RA-HEAD and DEC-HEAD except when the scanned                                                       
plate is an offset plate and the head is not present on the plate; in this case                                                   
these keywords are simply dropped from the header.  Note that the head center                                                     
coordinates are for reference purposes only and are not to be used for accurate                                                   
positions of the nucleus; well-saturated, wide-field plates are certainly not                                                     
suitable for that purpose! The QUALITY keyword gives a very subjective                                                            
assessment of the condition of each scanned image in terms of five categories                                                     
(plate, guiding, trails, seeing, and transparency); the scale is 1-5 (excellent                                                   
to poor).                                                                                                                         
                                                                                                                                  
C. Dataless Headers (Unscanned Images)                                                                                            
                                                                                                                                  
        Ninety-four of the images received by the L-SP Center were not scanned,                                                   
but FITS headers have been generated for them as well, and they reside on the                                                     
CD-ROM.  These headers contain a subset of the keywords listed above, mostly                                                      
those which describe platelog parameters (date, emulsion, filter, exposure                                                        
time, etc.).                                                                                                                      
                                                                                                                                  
                                                                                                                                  
V. PROCEDURES FOR DIGITIZING PLATES                                                                                               
                                                                                                                                  
A. Plate Materials                                                                                                                
                                                                                                                                  
        Most of the submissions to the Large-Scale Phenomena Discipline were in                                                   
the form of photographic plates or films.  Original glass plates or film were                                                     
most desirable for digitizing, but first generation film copies were also                                                         
acceptable. Original plates were digitized whenever possible according to the                                                     
scanning criteria outlined earlier, in Section IV.A.                                                                              
                                                                                                                                  
B. Hardware                                                                                                                       
                                                                                                                                  
        All digital images produced by the Large-Scale Phenomena DS Team were                                                     
generated on one of the two Perkin-Elmer PDS 1010A Microdensitometers (PDSs) in                                                   
the Laboratory for Astronomy and Solar Physics at GSFC.                                                                           
                                                                                                                                  
        The PDSs have been modified to enhance performance by replacing the                                                       
original logarithmic amplifier circuit with a faster one (Anderson et al. 1983),                                                  
and replacing the original analog-to-digital converter with one which provides                                                    
a sample-and-hold circuit in support of the faster log amplifier.                                                                 
                                                                                                                                  
        Each PDS is controlled by a DEC PDP-11/23 minicomputer running version                                                    
5.1 of the RT-11 operating system. The PDS is interfaced to the computer through                                                  
a bus converter board that allows the Unibus interface of the PDS to operate on                                                   
the Q-bus of the PDP-11/23.  The control programs were written in a combination                                                   
of Fortran IV and PDP-11 assembly language.  All scanning was performed with                                                      
the microdensitometers in density mode.                                                                                           
                                                                                                                                  
        The optical system of the microdensitometer passes a beam of light                                                        
through a pre-slit aperture which defines the size of the scanning spot.  The                                                     
beam is focused on the plate, then passes through a post-slit and a Fresnel lens                                                  
to illuminate a photomultiplier tube.  The output voltage from the photomulti-                                                    
plier tube is sent to a logarithmic amplifier, yielding an approximate photo-                                                     
graphic density.  The output voltage from  the log amplifier is converted into                                                    
PDS density units by a a 10-bit analog-to-digital converter.                                                                      
                                                                                                                                  
        The plate is scanned by moving the PDS platen, under computer control,                                                    
in a raster pattern.  As the plate is passed through the light beam, the                                                          
photomultiplier voltage is read at each pixel in real time.                                                                       
                                                                                                                                  
        The digital images were written in a PDS line-by-line format to magnetic                                                  
tape at 1600 bpi on Kennedy 9000-series dual density (1600/800 bpi) tape drives.                                                  
The PDS format tapes were converted to FITS format on the IBM 3081 mainframe at                                                   
Goddard, then checked for readability and accuracy on a VAX 11/750 computer in                                                    
the LASP before submission to the Lead Center.                                                                                    
                                                                                                                                  
C.  Preliminary Procedures                                                                                                        
                                                                                                                                  
        Before scanning a plate, the microdensitometer was used as a measuring                                                    
engine to produce an astrometric solution for the entire plate.  The astrometry                                                   
program assumes a Schmidt plate geometry, using the transformation equations                                                      
given by Dixon (1962).  The assumption of Schmidt plate geometry yields                                                           
acceptable results even for plates from other types of telescopes.                                                                
                                                                                                                                  
        Reference stars were selected from the AGK3 catalog for plates north of                                                   
-5 deg declination.  The Perth-70 catalog was used for southern declinations.                                                     
Although the SAO catalog is generally unacceptable for astrometric work, it was                                                   
occasionally used when none of the other catalogs yielded an adequate number of                                                   
reference stars to get a satisfactory plate solution.  The software corrects for                                                  
proper motions from the catalog date to the plate epoch, and precession may be                                                    
performed to any desired date.  All positions are given in 1950.0 coordinates.                                                    
                                                                                                                                  
        Mean astrometric error depends strongly on the individual plate scales                                                    
and somewhat on the number of acceptable reference stars available, but is                                                        
typically less than 10 arcsec (worst case) and frequently is on the order of                                                      
2 to 5 arcsec.                                                                                                                    
                                                                                                                                  
        The plate solution was used to measure the rotation angle of the                                                          
North-South meridian passing through the plate center, relative to the  Y-axis                                                    
of the microdensitometer.  The coordinates of the comet's head center were also                                                   
calculated, although this manual measurement provides only a crude location for                                                   
the nucleus embedded in a highly-saturated coma.                                                                                  
                                                                                                                                  
D.  Scanning                                                                                                                      
                                                                                                                                  
        After completion of the astrometry, the region of interest on the plate                                                   
was scanned.  The aperture size was selected to yield roughly the same angular                                                    
resolution per pixel, regardless of the original plate scale.  Typically, an                                                      
aperture of 40 or 50 microns was used on plates with plate scales around 100                                                      
arcsec/mm, and an aperture of 10 or 20 microns was used for plates with scales                                                    
greater than about 200 arcsec/mm.  Spatial resolution of the photographic                                                         
emulsions limits the smallest usable apearture to 5 or 10 microns in any event.                                                   
Generally, the step size was equal to the aperture size, so the pixels were                                                       
contiguous across the scan line with no oversampling.                                                                             
                                                                                                                                  
        The photomultiplier tube high voltage was set to yield 0 density on                                                       
clear glass, when clear glass was available.  Otherwise, the density was set to                                                   
around 0.1D on the background fog, primarily to avoid negative density values                                                     
in the resulting digital image.                                                                                                   
                                                                                                                                  
        The 10-bit analog-to-digital converter yields integers in the range 0 to                                                  
1023.  These may be converted to an approximate photographic density by dividing                                                  
by 200.                                                                                                                           
                                                                                                                                  
                                                                                                                                  
ACKNOWLEDGEMENTS                                                                                                                  
                                                                                                                                  
        Without the diligent efforts of LSPN observers to obtain and submit                                                       
observations of comet Giacobini-Zinner, these printed and CD-ROM archives for                                                     
Large-Scale Phenomena would not have been possible, at least not in their                                                         
existing substantial form.  It is a pleasure for the Discipline Specialists to                                                    
thank each and every contributing L-SP Network Observer both for submitted                                                        
observations of Giacobini-Zinner, and of Halley.  We are very hard at work on                                                     
the latter archives at the moment, and think that when it is finished it will                                                     
speak extremely highly of the cooperation of astronomers worldwide.                                                               
                                                                                                                                  
        Contributors to the L-SP Discipline effort who are not listed in Table                                                    
I, but who strongly deserve recognition as well as our thanks are: L.L. Taylor,                                                   
M.R. Greason, K. Bhagat, J. Gagliardi (all of ST Systems Corporation), J.                                                         
Sinclair (Royal Greenwich Observatory, Hailsham, East Sussex, U.K.), W. Liller                                                    
(Instituto Isaac Newton, Vina del Mar, Chile), and P.D. Usher (Pennsylvania                                                       
State University, University Park, PA 16802, U.S.A.).                                                                             
                                                                                                                                  
        J.C. Brandt and M.B. Niedner would like to express their thanks to E.P.                                                   
Moore of the Joint Observatory for Cometary Research (JOCR), Socorro, NM 87801,                                                   
U.S.A., for his hospitality during a run at JOCR in September 1985 observing G-Z                                                  
days after the historic ICE encounter and during the G-Z/Halley conjunction.                                                      
                                                                                                                                  
                                                                                                                                  
REFERENCES                                                                                                                        
                                                                                                                                  
Anderson, C., Slovak, M.H., and Michalski, D.E. (1983).  In Astronomical Micro-                                                   
        densitometry Conference, NASA CP-2317, edited by D.A. Klinglesmith (U.S.                                                  
        GPO, Washington, DC), p. 163.                                                                                             
Bame, S.J., Anderson, R.C., Asbridge, J.R., Baker, D.N., Feldman, W.C.,                                                           
        Fuselier, S.A., Gosling, J.T., McComas, D.J., Thomsen, M.F., Young, D.                                                    
        T., and Zwickl, R.D. (1986).  Science 232, 356.                                                                           
Brandt, J.C., Niedner, M.B., Jr., and Farquhar, R.W. (1985).  Advances in Space                                                   
        Research 5, No. 12, p. 3.                                                                                                 
Dixon, M.E. (1962).  Mon. Not. R. Astron. Soc. South Africa 21, 180.                                                              
Meyer-Vernet, N., Couturier, P., Hoang, S., Perche, C., and Steinberg, J.L.                                                       
        (1986).  Geophys. Res. Lett. 13, 279.                                                                                     
Slavin, J.A., Goldberg, B.A., Smith, E.J., McComas, D.J., Bame, S.J., Strauss,                                                    
        M.A., and Spinrad, H. (1986).  Geophys. Res. Lett. 13, 1085.                                                              
Smith, E.J., Tsurutani, B.T., Slavin, J.A., Jones, D.E., Siscoe, G.L., and                                                        
        Mendis, D.A. (1986).  Science 232, 382.                                                                                   
von Rosenvinge, T.T., Brandt, J.C., and Farquhar, R.W. (1986).  Science 232,                                                      
        353.                                                                                                                      
Wells, D.C., Greisen, E.W., and Harten, R.H. (1981).  Astron. Astrophys. Suppl.                                                   
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