CCD V Photometry of Comet Hale-Bopp (1995 O1)
Prepared by Herman Mikuz and Bojan Dintinjana.
Image Processing and Data Reduction
Results of CCD V photometry of comet 1995 O1 (Hale-Bopp)
1. Observations
1.1 Instrumentation
Observations were performed at the Crni Vrh Observatory, Slovenia. The 1996 observations were collected with the 20-cm, f/2 Baker-Schmidt camera and ST-6 CCD, covering the field of 72 x 54 arc min. As it passed the conjunction in late December 1996, Hale-Bopp became visible again in early January morning sky. By the mid of January, it brightened to 3rd magnitude and its coma diameter growth to nearly one degree. At that time, we switched our observations to a new combination of short focus lenses and CCDs.

Table 1: Our combinations of lenses + CCDs

Figure 1: The combination of 90mm lens, V filter and ST-6 CCD attached to the 20-cm Baker-Schmidt camera

In order to measure comparison stars and the comet in the standard Johnson V band, we used V photometric filter as proposed by Bessel (1990, 1995).
1.2 Observational method
Our observational methods were similar to the technique we used in faint comets photometry (Mikuz & Dintinjana 1994). Because of the low resolution of our imaging systems (typically ~30-60 arc sec/pixel), the comparison star images were udersampled, since they covered only a few pixels. This often result in some inconsistency in instrumental magnitudes which we overcome by taking at least 10 consecutive images of comparison stars.
At bright comets, the images became saturated yet on very short exposures. The exposure times depends on the sensitivity of CCD and were in the range of 1-20 secons (ST-6) and <5s with Wright detector. The integration times for the comparison stars were considerably longer. Obviously it is necessary to be careful and avoid the saturation of stellar as well as comet image. Another way to avoid the saturation is by slightly defocusing the lens. However, the lens should remain defocused to the same amount until the whole photometric sequence is completed.
Because of a large FOV, at least two comparison stars were easily found within the frame. In case they were more than 2o away from comet, the extinction corrections was applied (Green 1992). We used comparison stars from the "Bright Star Catalogue" (Hoffleit, 1982), as it contains precise V-band photometry for stars down to 7th magnitude. During the final reduction, the comparison-star instrumental magnitude is corrected to the same integration time as the comet and the magnitudes are then compared.
2. Image processing and data reduction
The image processing and data reduction procedures were basically the same as described in our paper about faint comets photometry (Mikuz & Dintinjana 1994). The instrumental magnitudes of C/1995 O1 were obtained with a newly developed FitsPro computer software written for MS Windows. This software includes a user-friendly routine for CCD aperture photometry and was adapted to meet some specific requirements encountered during the CCD photometry of comets.
Before the FitsPro is ran, we need to carefully determine the radius of star/comet aperture in pixels as well as inner/outer radius of the surrounding sky background. The program counts the units inside the specified aperture, subtracts the sky counts - leaving only the star/comet counts - and converts them into instrumental magnitudes.
We found that our V magnitudes are best fitted to ICQ data if we include the comet coma to full extent. In order to meet this requirement, we carefully determined the aperure radii by using histogramic analysis. Also the aperture radii was adopted according to daily changes in coma size. During the comet brightest phase, the choosen apertures were in the range of 30-40 arc min.
3. Results of CCD V photometry of comet 1995 O1 (Hale-Bopp)
Altogether 61 CCD V measurements were obtained, covering the period March 1996- April 1997. Of total 61 observations, 35 were obtained during the January-April when the comet was at its brightest. In order to check the quality of our results, we plotted them against the ICQ m1 estimates, extracted from their web page. Additionally, the estimates by D. W. E. Green are also displayed separately since we found that they best fit to ICQ data. We consider his estimates are very reliable as they are all extinction corrected and made in strict accordance with ICQ standards. Clearly, the CCD V results are in very good agreement with ICQ data and Green's estimates in particular throughout the analyzed observing period.

Figure 2: Total CCD V magnitudes plotted against the ICQ total m1 estimates

4. References
1. Bessell, M. S. (1990). P.A.S.P. 102, 1181.
2. Bessell, M. S. (1995). CCD Astronomy 2, No. 4, p. 20.
3. Green, D. W. E. (1992). ICQ 14, No. 3, p. 55.
4. Hoffleit, D. (1982). The Bright Star Catalogue (Yale University Observatory).
5. Mikuz, H., Dintinjana, B. (1994). ICQ 16, No. 4, p. 131.
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