Astron. Astrophys. 336, 769-775 (1998)

2. Observations and data reduction

The observations were performed with the 1.23 m telescope on Calar Alto Observatory, Spain between the 3rd and 14th of April 1997, comprising 6 days with acceptable observational conditions. We recorded time series with a duration of at least one hour and a temporal resolution of 20 and 30 seconds using a Gunn g and a Strömgren v filter. A total field of view of was observed with a spatial resolution of 2 arcsec. Each night the filter and/or pointing position was changed tracking either coma, dust tail or ion tail. Fig. 1 displays the observed positions, Table 1 summarizes the relevant information.

Fig. 1. Schmidt plate images () of comet Hale-Bopp with fields of view (indicated by squares) as observed in different nights. Coma (I) and ion tail (II) were observed through either filter in subsequent nights. Transition region (III) and dust tail (IV) were only observed in the `broad band' filter. Plates were taken by K. Birkle on Calar Alto.

Table 1. Overview on the obtained data sets

Two different sources contribute to the observed intensity signal: reflected sunlight and stimulated emission by resonant fluorescence. Thus the signal might be influenced by an oscillating solar wind (changes of density and of the magnetic field) but also by genuine activity of the comet (like changes in evaporation, and by effects of rotation). In addition, the signal might be affected by changes in the earth's atmosphere, by straylight, and by instrumental effects.

For the distinction of different processes, we alternated between two different filters in subsequent nights. Both filters transmit photons from stimulated emission and reflected sunlight, but with different contributions. We used a broad filter (Gunn g) to detect reflected sunlight and a narrow filter (Strömgren v) centered on a molecular spectral line (namely C₃ at 4100 Å). Fig. 2 shows the corresponding transmission curves. Unfortunately, all hitherto published spectra of Hale-Bopp taken in the appropriate frequency band were observed at rather large heliocentric distance (e.g. Fig. 2 in Fitzsimmons & Cartwright 1996 at 6.82 AU and Fig. 1 in Cartwright 1997 at 4.2 AU). Instead we use here a spectrum of comet Hyakutake (ESO Press Photo 22/96) taken at a heliocentric distance of about 1 AU. This spectrum was used to prepare the observations of Hale-Bopp, assuming a general similarity of the spectra of both comets.

Fig. 2. Transmission curves of both filters used in these observations plotted over the spectrum of comet Hyakutake (ESO Press Photo 22/96). Two different contributions were observed: stimulated emission in the C3 line dominating the Strömgren v filter as well as a mixture of reflected sunlight and emission from C2 with the Gunn g filter.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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