Explanatory Supplement to the IRAS Additional Observations of 9P/Tempel Delivered to University of Maryland February 27, 2001 Russ Walker Vanguard Research, Inc. 5321 Scotts Valley Drive, Suite 204 Scotts Valley, CA 95066 Revised March 30, 2005 by PDS-SBN, University of Maryland to reflect modifications requested during the PDS Peer Review Background for Radiance and Noise Maps ====================================== The bulk of the IRAS observation time was spent in the survey mode that systematically mapped the sky with a series of overlapping and confirming scans. However, IRAS devoted almost 40% of its observing time to Additional Observations (AO), that is, pointed observations of selected fields of interest. Circumstances of the Tempel 1 AOs are given in Table 1. The observation labeled SOP 287, OBS 13 was made pre- perihelion, the rest were post-perihelion. Perihelion occurred on 1983-07-09. Table 1. Circumstances of the Tempel 1 Additional Observations While construction of comet images from survey scans requires compensation for comet and spacecraft motion during the large time interval (about 6000 sec.) between scans, the duration of an AO is usually less than 800 seconds. Motion compensation may not be necessary if the comet's apparent motion is sufficiently small. Table 2. Details of the AO MACROS used for Tempel 1 Observations. Table 2 notes: 1. #S is the number of scan legs in the MACRO. 2. Off is the cross-scan offset between scan legs in arcsec. 3. TAO is the total duration of the AO in seconds. 4. Tobs is the actual time observing. The rest of the time was used to make fine attitude updates on reference stars at the beginning and at the end of the AO. 5. delRA and delDEC are the apparent motion of the comet during Tobs in arcsec. Table 3. Details of the AO MACROS used for Tempel 1 Observations, Continued Table 3 Notes: 1. #S is the number of scan legs in the MACRO. 2. Date is the UTCS start time converted to UTC date. 3. Flux Grid is a unique number used by in IPAC's IRAS AO archive to identify a flux map. 4. Intens Grid is a unique number used by in IPAC's IRAS AO archive to identify an intensity map. Tables 2 and 3 gives some details of the AO MACROS used for the Tempel 1 observations and the apparent motion of Tempel 1 during each AO. In all cases, the apparent motion of the comet is smaller than the 15- arcsec default pixel size for HIRES processing (see description below). This pixel size is several times smaller than the best achievable HIRES resolution. The AO is completed in 0.14 of an IRAS orbit. This can produce a maximum of 1.1 arcsec parallax for a comet 1 AU away, small compared to the pixel size and best resolution. Thus, I decided that we could process the Temple 1 AOs using the HIRES program at IPAC, which assumes that the comet is fixed in inertial space during the observation. HIRES employs the Maximum Correlation Method (MCM) (Aumann, Fowler and Melnyk, 1990) to construct resolution-enhanced coadded images of the observed fields. Details of the HIRES processing can be obtained from the IPAC/IRAS website, http://irsa.ipac.caltech.edu/IRASdocs/hires_proc.html from which are excerpted the following description: HIRES is a three-step process: 1. A program called AOCRDD is used to retrieve the raw AO data. The IRAS raw scan data are known as CRDD (Calibrated, Reconstructed Detector Data) and the full set of CRDD is stored in a FITS table format merged with the pointing information. The collection of the raw data in FITS table format is known as the Level 1 Archive. 2. A program called LAUNDR is used to clean up the CRDD. 3. A program called YORIC applies the MCM algorithm to the LAUNDR'd data. For HIRES, YORIC is iterated, progressively reconstructing a better model of the sky. In default processing YORIC is iterated 20 times. The HIRES processing defaults were chosen to provide the best performance (highest resolution and freedom from artifacts) for the majority of targets. These were adopted for the Tempel 1 images. They are: 1. The image size is 1 degree by 1 degree. 2. The pixel size is 15 arcseconds. This is not the effectiv resolution of the image. 3. All four bands (12, 25, 60 and 100 microns) are processed. 4. The data are de-striped with detector baseline removal and flux bias is applied. 5. The beam sample maps use a regular grid of 10-Jansky sources on a flat background. 6. All types of ancillary maps are produced. 7. The algorithm is iterated 20 times, with maps produced at the 1st, 5th, 10th and 20th iterations. A better model of the sky is reconstructed for each iteration. HIRES outputs several diagnostic files in addition to the surface brightness map. These ancillary maps are important to the interpretation of the HIRES results. The maps produced are: 1. surface brightness map 2. coverage map 3. photometric noise map 4. correction factor variance map 5. HIRES'd beam sample maps 6. simulator input 'spike' plus background model for beam sample maps 7. detector track maps 8. beam width report for the last iteration 9. processing log file The complete set for one AO consists of 76 images and 12 ASCII files. However, only the radiance maps, noise maps, and effective resolution (beam width) tables are included in this archive. HIRES accepts electronic requests for automated image reprocessing. Visit http://irsa.ipac.caltech.edu/IRASdocs/iras_data_requests.html for an electronic request form. IPAC documentation states that the last version of HIRES dates to June 1997. Radiance Map, Noise Maps, and Effective Resolution Tables - File Designations and Content =========================================================== The radiance and noise maps based on Additional Observations were identified by the satellite operations plan (SOP) and observation (OBS) number. The IRAS observations were made in four spectral bands centered nominally at 12, 25, 60, and 100 microns. Images are supplied for the first (01) and 20th iteration (20) and effective resolution (beam size) tables are supplied for the 20th iteration. Walker retained the images for the 5th and 10th iterations because the images were inferior to the 20th. The following file names were adopted: Radiance and Noise Maps in /data/ao/images/: Radiance Maps (40) = sSOP_oOBS_Band_Iteration#_radiance.fit Noise Maps (40) = sSOP_oOBS_Band_Iteration#_noise.fit Effective Resolution (Beam Size) Tables in /data/ao/effres/: Eff. Res. Tables (20) = sSOP_oOBS_Band_Iteration#_effres.tab The numbers in parenthesis above are the number of files supplied by Walker and included in this archive. The Radiance and Noise Maps are in FITS image format. They have extensive headers that provide all the data that you need to use the images. The following is an example from a 20th iteration of a 100-micron image. SIMPLE = T / Written by IDL: 14-Feb-2001 12:59:15.00 BITPIX = -32 / IEEE single precision floating point NAXIS = 2 / NAXIS1 = 241 / Cross-scan grid dimension NAXIS2 = 241 / In-scan grid dimension BLOCKED = T / TAPE MAY BE BLOCKED IN MULTIPLES OF 2880 EXTEND = T / TAPE MAY HAVE STANDARD FITS EXTENSIONS OBJECT = '9P/TEMPEL 1' / Object imaged by YORIC CTYPE1 = 'RA---CAR' / Decreases in value as sample index increases CUNIT1 = 'deg' / RA unit CRVAL1 = 194.0374908 / RA at origin (deg) CRPIX1 = 121. / X-axis origin (cell) CDELT1 = -0.004166666884 / Pixel X-width (deg) CROTA1 = 0. / Twist(celest clk) angle undefined for X-axis CTYPE2 = 'DEC--CAR' / Increases in value as line index increases CUNIT2 = 'deg' / DEC unit CRVAL2 = -0.6277800202 / Dec at origin (deg) CRPIX2 = 121. / Y-axis origin (cell) CDELT2 = 0.004166666884 / Pixel Y-width (deg) CROTA2 = 0. / Rotates +NAXIS2 into Lat axis (angle COMMENT measured positive CCW from COMMENT +NAXIS2 to +Lat) (degrees) BUNIT = 'W/cm2/sr' / Watts/cm2/steradian BLANK = -2000000000 / Value for empty cell BSCALE = 1.00000 / TRUE=TAPE*BSCALE+BZERO BZERO = 0.000000 / DATAMAX = 3.0684367E-10 / Watts/cm2/sr (in-band radiance) max valid phys DATAMIN = -6.7676536E-13 / Watts/cm2/sr (in-band radiance) min valid phy SIGMA = 6.800000072E-13 / Median detector noise (Watts/cm2/sr) SIGORIG = 0.6800000072 / Median detector noise (MJy/ster) BIAS = 0. / Median pixel flux subtracted EQUINOX = 1950. / EME50 RADECSYS= 'FK4 ' / TELESCOP= 'IRAS ' / IRAS Satellite INSTRUME= 'YORIC ' / HiRes imaging program developed COMMENT for IRAS data at IPAC, Pasadena, CA DATE = '02/15/2001' / Date of final processing (m/d/y) ITERNO = 20 / Number of iterations done VERSION = '2.11 960805' / Version of YORIC COMMENT This is a "HiRes" image generated by the Maximum Correlation Method COMMENT (see Aumann, Fowler, and Melnyk, Astron. J. Vol. 99, No. 5, COMMENT p. 1674, 1990) COMMENT YORIC: Offset compensation was used for destriping COMMENT The destriping method was the same in all input observations. FBIAS = 1.06500002E-14 / Flux bias used (Watts/m2) FBIASL = 1.06500002E-14 / Flux bias from Laundr (Watts/m2) COMMENT Laundr flux bias is the maximum in all input observations FMIN = -1.746099975E-14 / Min normalized flux (Watts/m2) in Laundr FBPCT = 0.009999999776 / FBIAS percentile (dec. fraction) in Laundr DETMIN = 1 / Detector on which FMIN was found in Laundr FILENAME= 's287_o13_100um_20_radiance.fit' / OUTPUT FILE NAME INFIL01 = 'data/tmpl_s287o13s' / INPUT FILE NAME COMMENT SOP = 287 OBS = 13 COMMENT Input Destripe: Baseline removal COMMENT Laundr version: 1.39 951004 COMMENT Raw data were Laundred on 11/22/ 0 COMMENT Pass 3 CRDD were used; Level 1 Archive usage: NONE COMMENT Data are on the AC scale COMMENT AOCRDD VERSION 4.2 940603; date of run: 11/22/00 COMMENT Jan 2003, PDS Small Bodies Node modified the header: COMMENT 1) Removed the unused NAXIS3 = 1 keyword. COMMENT 2) Corrected several keywords and values. COMMENT 3) Added keyword comments for clarification. COMMENT 4) Added observation-specific keywords at end of header. COMMENT 5) Image size is 1 degree by 1 degree. COMMENT 6) Pixel size is 15 arcsec by 15 arcsec. COMETMAX= 1.69721E-11 / Watts/cm2/sr (in-band radiance) PIXMAXX = 120 / x pixel number of max comet radiance PIXMAXY = 94 / y pixel number of max comet radiance WAVE = 100 / Wavelength (microns) MEAN_JD = 2445503.780 / Mean Julian date of combined obs MEANDT = '1983-06-18' / Mean UTC date of combined obs MEANTM = '06:43:12.000' / Mean UTC time of combined obs SOP = 287 / Satellite operations plan number OBSNUM = 13 / Observation number within SOP RHO = 1.507 / Sun-Comet distance (AU) DELTA = 0.853 / Earth-Comet distance (AU) PHASE = 40.1 / Phase angle (S-T-O) (deg) MACRO = 'DPS52B' / AO MACRO used NLEGS = 6 / Number of scan legs by MACRO XSCANOFF= 18 / Cross-scan offset between scan legs(arcsec) TOBS = 428 / Actual time observing (sec) TAO = 764 / Total duration of AO observation (sec) DELRA = 6.1 / App. motion of target during Tobs(arcsec) DELDEC = 7.1 / App. motion of target during Tobs(arcsec) UTCSTART= 2445503.7754 / UTC at start of obs (JD) UTCSTOP = 2445503.7843 / UTC at end of obs (JD) STARTDT = '1983-06-18' / UTC date at start of obs STARTTM = '06:36:37.000' / UTC time at start of obs STOPDT = '1983-06-18' / UTC date at end of obs STOPTM = '06:49:21.000' / UTC time at end of obs SCSTART = 77610997 / IRAS native time at start of obs SCSTOP = 77611761 / IRAS native time at end of obs NO_SCAN = -9999.00 / Array elements outside scan set to -9999 END The image is 241 by 241 pixels (NAXIS1 by NAXIS2). The coordinate system is equatorial (RA, DEC (B1950)). The comet is not centered in the image. The pixel size is 15 by 15 arcsec. However, this is not the effective resolution of the image. The effective resolution is provided by the effective resolution table associated with the image. These tables are discussed at the end of this section. The maximum radiance of the comet (COMETMAX) occurs at PIXMAXX and PIXMAXY. MEAN JD is the Julian date at the mid-time of the AO. You will also encounter the time variable UTCS. This is IRAS time. It may be converted to Julian date by JD = 2445335.5d0+(UTCS - 63072000.0d0) / 86400.0d0 The UTCS start and stop times for each image were converted to UTC Julian dates and Gregorian dates and included in the FITS headers. The maps are 1 degree by 1 degree, but the IRAS scan is only about 30 arcminutes wide and therefore does not fill the image area. The array elements that lie outside of the scanned area a have the FITS keyword NO_SCAN set to -9999.00. Note that the original value for SIGMA was in units of mega Jansky per steradian. This value was converted to the in-band radiance by multiplying the original SIGMA value in the FITS header by (13.48, 5.16, 2.58, and 1.00) x 10-12 for the 12, 25, 60, and 100 micron bands respectively. The original SIGMA value is retained in the SIGORIG keyword. Although the images are binned into 15 arcsecond pixels for HIRES processing, the actual resolution within the image is dependent on (among other things) the number of iterations. HIGHRES creates a grid of 10 Jansky point sources spaced throughout the image field and then processes these sources as if they were scanned by the AO. HIRES produces beam width reports give the full width at half maximum of the best fitting gaussian to the resulting profiles. These can be used as an estimate of the effective resolution as a function of position within the field. The Effective Resolution tables are fixed-width, ASCII tables. The following example is a table for the last iteration (20th) for a 100-micron image. A table header is included here to document the contents; it is not included in the archival table. The pixel coordinates are with respect to IRAF. The Maj and Min columns contain the full-width half-max axes of a Guassian fit. The PosAng column gives the position angle, east of north. The values in the Spike and Background columns are model inputs. The number of pixels into which spike is smeared is on the order of Spike/Peak. Pixel RA Dec Maj Min PosAng RA/DecErr Peak Spike Backgrnd X Y " " deg " " Jy/pix Jy Jy/pix 25 25 12:57:45.0 -01:01:40 17 17 180 0 0 -0.004 10.00 0.000 73 25 12:56:57.0 -01:01:40 17 17 180 0 0 -0.004 10.00 0.000 121 25 12:56:09.0 -01:01:40 128 82 118 -2 1 0.205 10.00 0.001 169 25 12:55:21.0 -01:01:40 129 84 115 1 3 0.199 10.00 0.002 217 25 12:54:33.0 -01:01:40 162 79 120 4 -3 0.125 10.00 0.000 25 73 12:57:45.0 00:49:40 17 17 180 0 0 -0.004 10.00 0.000 73 73 12:56:57.0 00:49:40 82 71 11 -13 7 0.022 10.00 0.000 121 73 12:56:09.0 00:49:40 149 97 120 -3 4 0.144 10.00 0.000 169 73 12:55:21.0 00:49:40 130 86 119 -2 1 0.177 10.00 0.001 217 73 12:54:33.0 00:49:40 17 17 180 0 0 -0.004 10.00 0.000 25 121 12:57:45.0 00:37:40 17 17 180 0 0 -0.004 10.00 0.000 73 121 12:56:57.0 00:37:40 162 81 118 -3 0 0.115 10.00 0.000 121 121 12:56:09.0 00:37:40 132 86 118 0 -1 0.167 10.00 0.000 169 121 12:55:21.0 00:37:40 168 87 118 -4 2 0.116 10.00 0.000 217 121 12:54:33.0 00:37:40 17 17 180 0 0 -0.004 10.00 0.000 25 169 12:57:45.0 00:25:40 17 17 180 0 0 -0.004 10.00 0.000 73 169 12:56:57.0 00:25:40 129 86 119 2 0 0.177 10.00 0.002 121 169 12:56:09.0 00:25:40 133 86 119 -4 2 0.196 10.00 0.000 169 169 12:55:21.0 00:25:40 89 71 167 -7 10 0.051 10.00 0.000 217 169 12:54:33.0 00:25:40 17 17 180 0 0 -0.004 10.00 0.000 25 217 12:57:45.0 00:13:40 158 82 118 15 -10 0.119 10.00 0.000 73 217 12:56:57.0 00:13:40 153 86 124 -17 6 0.153 10.00 0.001 121 217 12:56:09.0 00:13:40 145 89 118 2 -2 0.162 10.00 0.001 169 217 12:55:21.0 00:13:40 17 17 180 0 0 -0.004 10.00 0.000 217 217 12:54:33.0 00:13:40 17 17 180 0 0 -0.004 10.00 0.000 Aperture Photometry =================== The data set that Walker delivered to PDS included aperture photometry files. However, the original photometry files were be replaced by photometry data derived and published by the science team for the Deep Impact Mission. The revised photometry data are archived as a separate data set within the PDS as DATA_SET_ID DI/IRAS-C-FPA-5-9P-PHOT-V1.0. However, Walkerıs discussion of his aperture photometry analysis is included in this report for future reference. Aperture Photometry - File Designations and Content =================================================== The IRAS AOs are identified by their satellite operations plan (SOP) and observation (OBS) number. The IRAS observations were made in four spectral bands centered nominally at 12, 25, 60, and 100 mircons. Images are supplied for the first (01) and 20th iteration (20). R. Walker retained the images for the 5th and 10th iterations because the images are inferior to the 20th. The following file names were adopted for the AO photometry tables: Radiance Phot Tables (20) = sSOP_oOBS_Band_Iteration#_radiance.tab Noise Phot Tables (20) = sSOP_oOBS_Band_Iteration#_noise.tab The numbers in parenthesis above are the number of files that you should have received. Bands b1, b2, b3, and b4 are the 12, 25, 60, and 100 micron bands respectively. Aperture Photometry - Methodology ================================= The radiance (and noise) photometry files are the result of summing the radiance (and variance) from all the pixels that are within a selected distance from the pixel of maximum comet radiance. There are three columns in the files. The first is the distance (radius) that includes the pixels summed. Zero is the central pixel, etc. The second column is the summed radiance (or noise) in units of Watts cm-2. The third column is the number of pixels actually included in the sum. Most of the coma flux should be contained within a few pixels radius. There is 0.846125 x 10-8 steradian per 15" square pixel. The photometry files delivered to PDS-SBN in December 2000 for the Survey Scan Images used the image median to estimate the background and subtract that from the image prior to photometry. This is usually a reasonable thing to do for stars, etc. However, for the comet the dust tail may be a significant source of background (or foreground) for estimates of the coma flux (see Figure 1a). In the case of a point source in a uniform background one expects the aperture photometry curve to rise to an asymptotic value if the background has been properly subtracted (Figure 1b). R. Walker modified his photometry program to iterate the background radiance until the profile tends toward asymptotic at radii greater than 30 pixels from the maximum comet radiance. He chose to call this "coma aperture photometry" and the value of the asymptote an estimate of the total coma flux. This is given in Table 4 for the AO data and Table 5 for the survey data. Table 4. Results of "Coma" Aperture Photometry on the AO Observations Figure 1a. Aperture photometry of the 12 um survey radiance image #339 with median background subtraction as delivered on December 29, 2000. Figure 1b. Aperture photometry of the 12 um survey radiance image #339 with iterative background subtraction (coma photometry). Table 5. Results of "Coma" Aperture Photometry on the Survey Observations The survey photometry has been redone using the new software and radiance survey "coma" photometry files have been included in the present delivery. Comments on Colors and Flux Measures ==================================== One can convert the in-band radiance (w cm-2 sr-1) to flux density units (Jy sr-1) by dividing the in-band radiance by (13.48, 5.16, 2.58, and 1.00) x 10-18 for the 12, 25, 60, and 10 micron bands respectively. This must then be corrected for the spectrum of the source, for example: for a blackbody at 250K the flux densities must be divided by 0.87, 1.11, 1.19, and 1.07 for the 12, 25, 60, and 100 micron bands respectively. It is recommended that model spectral radiances be integrated over the IRAS spectral response and compared to the observed in-band radiances rather than converting the observations to flux densities. Colors (flux ratios) are a more complicated problem. Even after running the MCM high resolution process the spectral bands have different resolutions. Ratios made from the 12 and 25 micron bands will have meaning since these two bands have virtually the same resolution at all iteration steps. However, if one wants to compare radiance ratios, say, R(12 micron)/R(60 micron), R(25 micron)/R(60 micron), etc. one will not obtain the desired result due to the different beam sizes involved. There are two sophisticated ways to approach this problem: 1) Cross-band simulation wherein a new set of images is made by scanning the high-resolution images with a simulation of the IRAS focal plane, running MCM on these new images to produce images that have the same resolution and can then be ratioed. 2) Prior-knowledge technique wherein say the 60-micron high-resolution image is used as the starting point for constructing a 100-micron image and vice versa. This technique results in an image of higher resolution than the cross-band technique. Both of these methods are described by Fowler and Aumann (1994). A simple approach would use the beam-size information in the above table to estimate the two-dimensional gaussian beam profile necessary to convolve with the higher resolution image to produce an image at the resolution of the lower resolution image. These can then be ratioed for comparison to calculate spectral ratios. For example: one wishes to ratio a 20th iteration 12-micron image with a 10th iteration 60-micron image; the FWHM of the gaussian kernel would be 1) cross-scan: sqrt(86^2 - 37^2) = 77.6 and 2) in-scan: sqrt(50^2 - 23^2) = 44.4. The 12-micron map is then convolved with the 77.6 x 44.4 kernel and the two maps ratioed. Values for other numbers of iterations and band combinations can be interpolated in the table. The assumption that the output PSF of MCM is gaussian is not really a bad one. In addition, the noise images are good measures of the relative noise from one part of an image to another. They are poor estimates of the absolute noise level. The photometric noise map indicates the internal photometric error of the detector samples resulting from the averaging of overlapping samples. It does not include absolute errors such as calibration errors. This applies to both the survey and AO data. Beichman, et al (1988) estimates that the absolute errors in the IRAS absolute calibration are 2%, 5%, 5%, and 10% at 12, 25, 60, and 100 microns respectively due to uncertainty in the models used. These numbers should be RSSd with 3% uncertainty in the standard stars used and 4% uncertainty for spectral bandpass errors. I have removed some systematic errors by re-calibrating IRAS in accordance with Cohen, et al (1992). Thus an estimate of the uncertainties of the radiance data can be had by combining the above absolute errors with the noise map values in the root sum squared sense. Some Interesting Plots ====================== Figure 2 is a plot of the "coma" fluxes in Tables 3 and 4 divided by the pre-perihelion "coma" flux (s287 o13). Curvature in these plots is an indication of spectral variations at differing heliocentric distances. The heliocentric distances are labeled on the plot. One expects the spectrum to change due to variations in grain temperature, size distribution, and possibly composition. Figure 2. Tempel 1 spectrum relative to its pre-perihelion spectrum at 1.507 AU. Figure 3 is a plot of the relative "activity" of Tempel 1 as a function of heliocentric distance. I have defined the "activity" as del^2 x Flux (del), where del is the geocentric distance of the comet. This quantity is a measure of the dust production rate, or perhaps more properly, the resident time of dust within the beam. I have normalized these data to the "activity" of the pre-perihelion observation. The heliocentric distances are labeled. Figure 3. Activity of Tempel 1. References ========== Aumann, H.H., Fowler, J.W., Melnyk, M., 1990, "A Maximum Correlation Method for Image Construction of IRAS Data", Astron. J. 99, 1674-1681 Beichman, et al., 1988, "IRAS Catalogs, Vol 1. Explanatory Supplement" Cohen, M., Walker, R.G., Barlow, M.J., Deacon, J.R., 1992, "Spectral Irradiance Calibration in the Infrared. I. Ground-based and IRAS Broadband Calibrations", Astron. J. vol. 104, no. 4, 1650-1657 Fowler, J.W. and Aumann, H.H., 1994, "Science with High Resolution Far-Infrared Data, Proceedings of a Workshop Help 14-16 June 1993", JPL Publication 94-5, (S. Terebey adn J. Mazzarella, Eds) Melnyk, M., 1990, "Technical Reference to the Full Resolution Survey Coadder (FRESCO)", IPAC User's Guide, Ed. 4, Part VR, California Institude of Technology