Astron. Astrophys. 341, 912-917 (1999)

2. The observations and the data reduction

The CCD imaging of the two target comets was performed with the 3.5m telescope of the Max-Planck-Institut für Astronomie at the Calar Alto Observatory in Spain. Comet 26P/GS was observed in mid September 1993 about 1 year after its perihelion passage, Comet 73P/SW3 in late December 1994 about 3/4 year before perihelion. Table 1 lists the observing geometry and gives information on the prevailing atmospheric conditions during the observations. Both comets were observed through Johnson V and R filters. The detector, a 1024 1024 pixel Tektronic CCD (maximum quantum efficiency = 90 percent around 650 nm, gain = 3.9 electrons/ADU, read-out noise = 2 ADU) of 24 µm pixel size, provided a 6.93 6.93 arcmin field of view (pixel resolution = 0.406 arcsec) at the f/3.5 prime focus of the telescope. During comet imaging the telescope followed the differential motion of the target. The standard star fields were taken from the list of Christian et al. (1985) and Odewahn et al. (1992).

Table 1. Observing geometry of the comets and atmospheric conditions during the observations

For Comet 26P/GS, altogether, we could obtain 43 V and R exposures of (in nearly all cases) 1200 s duration each. For the final data evaluation, however, we only used the 23 R and 3 V exposures of the 2 photometric nights of Sept. 14/15 and 16/17. The V images for the measurement of the V-R colour index were always taken immediately after an R exposure. In the case of 73P/SW3, a total of 10 R and 4 V exposures of either 1200 s or 900 s could be taken during the first 2 of the 4 nights indicated in Table 1.

The first steps of the data reductions were bias level subtraction, flatfield division as well as cosmics and bad columns removal. For the absolute photometry the counts of the objects and of the standard stars were measured by aperture integration and sky subtraction in the images (avoiding overexposed stars). The photometric reduction was executed using a computer code which follows the principles described by Sterken & Manfroid (1992). The basic routine was provided by J. Manfroid (University Liege, Belgium), but was considerably modified and supplemented for the application described (Rainer, 1997).

From the 23 selected R images of 26P/GS, 17 frames could be used for the lightcurve analysis, the rest was discarded because of high statistical errors due to star blends of the comet image or high sky background. For 73P/SW3 all 10 R filter exposures were taken for the lightcurve inspection although their intrinsic quality is not as good as the data for 26P/GS are. Additionally, relative photometry between the comets and reference objects was exercised using stars and galaxies which were visible in all frames of a single observing night.

For the deep coma search, all available R exposures of the respective comet (without star blends) were aligned to have the peak maximum of the object at the same pixel and were thereafter coadded. For the improvement of the signal-to-noise ratio (S/N) in the low level magnitude range, pixel smoothing (3 3 pixels) and linear rebinning by a factor of 0.625 was applied to the coadded images. Finally, we obtained coadded images of 9.5 h total integration time for Comet 26P/GS and of 3.5 h total integration time for Comet 73P/SW3 (Figs. 1 and 2). The detection limit of a potential coma and tail signal could be improved to 27 mag/arcsec² for 26P/GS and to 25.5 mag/arcsec² for 73P/SW3. The image processing was executed using the MIDAS (Munich Image and Data Analysis Software) package of the European Southern Observatory ESO.

Fig. 1. Coadded image of Comet 26P/Grigg-Skjellerup. In total 9.5 h of broadband R filter exposures from the period 14-17/9/1993 are coadded. North is up, East to the left, the field of view is 51 51 arcsec.

Fig. 2. Coadded image of Comet 73P/Schwassmann-Wachmann 3. In total 3.5 h of broadband R filter exposures from the period 27-30/12/1994 are coadded. North is up, East to the left, the field of view is 70 70 arcsec.

Fig. 3. Comparison of 73P/SW3 and star image profile. The R filter images of P/SW3 were aligned once centered on the comet and once on the stars and were in each case co-added. From the resulting two sum frames subimages of the comet (comet image) and of a star similar in brightness (star image) were extracted. The comet and the star image were scaled to the same maximum (pixel value = 1) and minimum brightness (average pixel value = 0 on the neighbouring sky background) and profile cuts were taken (for the comet perpendicular to the position angle of the tail direction, for the star perpendicular to the trail direction). The plot shows the comet (continuous line) and the star profile (dashed line) close to the sky background. Obviously, a faint and very condensed coma was surrounding the nucleus of the comet.

© European Southern Observatory (ESO) 1999

Online publication: December 16, 1998
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