Observations of Comet Hale-Bopp C/1995 O1 were taken with the optical polarimeter (Deshpande et al. 1985; Joshi et al. 1987) mounted at the Cassegrain focus of the 1.2 m telescope at Mt. Abu Infrared Observatory of Physical Research Laboratory, India.
The optical polarimeter is equipped with the IAU/IHW filter set. These filters were acquired about a decade ago and have been kept with care in a dry atmosphere to preserve them. However, to be sure of their transmission characteristics, we determined the transmission curves of these filters before the start of present observations. We found that (with the exception of the CO+ (4260 Å) filter curve) they compare very well with the transmission characteristics supplied by the manufacturers without any significant deterioration. The polarization in the three continuum bands (3650/80, 4845/65, 6840/90, all figures in Å) is reported here. It is now known that these continuum filters suffer from the contamination from a previously underestimated cometary molecular emission (Schleicher 1996). However, the degree of polarization and position angle are not expected to be affected significantly by small contamination. Also, the observations with these filters facilitate comparison with comets observed earlier with same set of filters, hence their continued use is justified.
The observations were conducted during the period from October 1996
to May, 1997. The apertures employed for these observations
were of size 52.4 and 26.5 arcsec.
The observations were made alternately
on the photocenter of the comet and the region of sky nearly
45 arcmin away, in the anti-tailward direction.
This procedure was used to take care of the sky polarization. Polarization
standard stars, HD 14433, 9 Gem, o Sco (Serkowski 1973), were
used to calibrate the observed position angle. A zero polarization star,
HD 202575, was used to determine the instrumental polarization which was
found to be negligible (
0.03%) compared to the errors
in the observation. The instrument's performance
was tested for 100% linear polarization using a Glan prism and it was found
to be 98.7% efficient. The details of the data reduction procedures are given
by Joshi et al. (1987).