2 Observations

Observations of comet C/1995 O1 (Hale-Bopp) were carried out with the 0.50 m Swedish Vacuum Solar Telescope (SVST) on La Palma. Images were taken during the period April 21-25, 1997, three weeks past perihelion. At the site, the value of the cometary phase angle made optical observations possible during a period of at most one hour in a gradually darkening twilight sky, while the altitude of the comet decreased from $20\hbox{$^\circ$}$ to $7\hbox{$^\circ$}$. Ephemeris data (Yeomans 1997) is given in Table 1.

  

Table 1: Ephemeris data for Hale-Bopp during the observing run. Also given is the equivalent distances of the telescope diffraction limit D at $\lambda~550\ \rm nm$ ($0\hbox{$.\!\!^{\prime\prime}$}28$) and CCD pixel angular size S ($0\hbox{$.\!\!^{\prime\prime}$}24$) at the geocentric distance of the comet. The last two columns give a UT and the position angle of the radius vector (direction to Sun) at that time

In its nominal configuration, the SVST operates at focal ratio f/45 and focal length F 22.3 m, measured at $\lambda~550\ \rm nm$ (Scharmer 1985). For increased field coverage, auxiliary compressing optics were installed for an F 7.7 m, f/15, configuration. A Kodak Megaplus 1.6 CCD 10-bit camera with a KAF-1600 chip having $1\,536\times1\,024$ imaging pixels of $9\times9$ micrometer size was used. The camera operated at 0-12 dB gain with no cooling at a stable ambient temperature of $20\hbox{$^\circ$}\rm C$.

Dust continuum imaging was performed with Ealing 550nm and 830 nm band-pass filters, having FWHM of 43 and 38 nm, and transmission maxima of 80 and 83$\%$, respectively. The steep chromaticity curve of the telescope's achromatic objective prevents useful imaging with filters of wider band-pass than this, while narrow-band filters would imply unrealistically long exposure times for the present target, considering the noise characteristics of the uncooled chip. Exposures were made of a $4\hbox{$.\mkern-4mu^\prime$}4\times6\hbox{$.\mkern-4mu^\prime$}6$ field centered on the psuedo nucleus, with an image scale of $0\hbox{$.\!\!^{\prime\prime}$}24$ per pixel equalling the diffraction limit of the instrument at $\lambda~520\ \rm nm$. Exposure times were in the range 1-15 s with photo-centre saturation occurring at the longer exposures. A total of 320 images were obtained, which were subsequently averaged into 37 data images for analysis.

A short description of the optical setup is in order. The converging beam of the system exited horisontally along the surface of a levelled optical table, where auxiliary optics were mounted. Focussing was achieved by moving the table along the direction of the optical axis. The beam was diaphragmed at the first focus to minimize effects of scattered light. A single lens of F 200 mm imaged the first focal plane, and a 70 mm camera objective lens mounted on the CCD head completed the compressive optics setup. The 25 mm diameter imaging filters were positioned in between the lenses.

Contribution of $\rm C_2$ emission to the total flux from the comet within the $\lambda~550\ \rm nm$ filter passband is estimated to be negligible at a $0.5\%$ level, based on a low-dispersion spectrum by Avila et al. (1997) obtained on April 24.