Data Set Overview ------------------ This data set contains images of comet Borrelly, obtained by Tony Farnham at the McDonald Observatory on five different observing runs during the comet's 2001 apparition: 21-23 Sep 2001 12 Nov 2001 04-07 Dec 2001 07-08 Feb 2002 17-18 May 2002 The images from Sept 21 and 22 bracket the Deep Space 1 spacecraft encounter. The December data were obtained at the 2.1-m Otto Struve telescope and the data from the other four runs were obtained at the 2.7-m Harlan J. Smith telescope. In both cases, the Imaging Grism Instrument (IGI), a 5:1 focal reducer and a TeK 1024x1024 CCD were used. On the 2.7-m telescope, this configuration produces a 7 arcmin vignetted field of view with 0.57 arcsec pixels. On the 2.1-m the field of view is 6 arcmin, with 0.48 arcsec pixels. For most of the observing runs, both a Harris V and a Mould R filter were used for imaging. On 7 Dec 2001 and 17 May 2002, only R images were obtained. An extensive discussion of the data can be found in Farnham, T.L. & A.L. Cochran, 'A McDonald Observatory Study of Comet 19P/Borrelly: Placing the Deep Space 1 Observations into a Broader Context' Icarus, 160, 398-418, 2002. Processing ---------------- The data were processed in a standard manner. The bias was removed in a two-step process: In the first step, the overscan region was used to remove the bulk of the bias level, and in the second step, a master bias frame (created by averaging many bias frames) was used to remove the residual for each individual pixel. Flat fielding was done using twilight sky flats that were medianed together to remove any stars. Note that the data from May 2002 (UT020517 and UT020518 files) was obtained during twilight, and much of the structure (dust specks on the filter, etc) in the images did not completely flat field out. Calibration -------------- On nights that were photometric, Landolt standard stars were observed at several different airmasses to provide a measurement of the atmospheric extinction and a means of obtaining an absolute flux calibration. (Landolt, A., UBVRI Photometric Standard Stars in the Magnitude Range 11.5-16.0 Around the Celestial Equator. Astron. J. 104, 340-371, 1992) The standard star observations were used to compute the extinction coefficient (in mag/airmass) and to determine the instrumental zero point offset (in mag) for each filter. These were then applied to the images, using the following relation, to convert to observed fluxes: F = F_0 * P/t * 10^(-0.4*(25.0 - X*E - DM)) where F is the calibrated flux for each pixel (erg/sec/cm^2/Angstrom), F_0 is the flux for a 0 mag star (erg/sec/cm^2/Angstrom), P/t is the count level per second for each pixel, X is the airmass, E is the extinction coefficient (mag/airmass) and DM is the instrumental zero point offset (mag). The values used for the calibration are given below. Flux of zero magnitude star (erg/sec/cm^2/Angstrom) Filter F_0 ------ --------- V 3.526e-9 R 1.702e-9 Extinction coefficients and zero point offsets Date E_V DM_V E_R DM_R ----------- ----- ----- ----- ----- Sep 23 2001 0.161 1.189 0.117 1.549 Nov 12 2001 0.128 1.138 0.106 1.141 Dec 4 2001 0.291 1.310 0.211 1.751 Dec 5 2001 0.178 1.131 0.142 1.562 Feb 7 2001 0.115 1.334 0.073 1.347 Feb 8 2001 0.138 1.282 0.101 1.245 Data --------- The data consist of images from the different observing runs. The images are contained in four directories. The first (basic) contains the bias subtracted, flat fielded images of comet Borrelly from all of the observing runs. The second (standards) contains the images of the standard star fields that are used to compute the extinction coefficients and instrumental zero point offsets. The third (calibrated) contains the images from photometric nights calibrated to observed flux values. The fourth (processing) contains the bias frames and the flat field images that were used to process the data. Review ====== This data was peer reviewed on May 29, 2003.