Hale-Bopp Imaging with the New Narrowband Comet Filters

R.L. Millis, D.G. Schleicher, T.L. Farnham (Lowell Obs.)

The new narrowband comet filter set (designated the ``Hale-Bopp set'') consists of 11 filters: five that isolate gas emission bands (OH, NH, CN, C tex2html_wrap_inline11 , and C tex2html_wrap_inline13 ), two for ions (CO tex2html_wrap_inline15 and H tex2html_wrap_inline13 O tex2html_wrap_inline15 ) and four that isolate associated continuum bands. Based on the knowledge gained from the IHW filter set, many improvements were made in the new filters, including a reduction of the continuum filter contamination from gas emissions. Also, advancements in the manufacturing process produced higher transmissions, squarer profiles and more robust, longer-lived UV filters. Forty-seven full or partial filter sets were distributed early in 1997. The status of both the standard star system and the calibration process will be presented.

Our own filters were extensively used for both photometry and imaging of Comet Hale-Bopp from January to May. Near-nucleus images were obtained in all wavelengths with the Lowell Hall 42'' telescope and a 2048 tex2html_wrap_inline21 CCD. The continuum images show arc structures that change from night to night and can be measured to obtain information about the nucleus rotation and dust velocities. The CO tex2html_wrap_inline15 images show detailed structures that comprise the extensive plasma tail, but the images taken through the H tex2html_wrap_inline13 O tex2html_wrap_inline15 filter exhibit little structure apart from what is seen in the dust images. The neutral gas images are dominated by a large contribution from the background continuum; however, even a preliminary (uncalibrated) continuum subtraction reveals many details hidden by the underlying dust.

Wide-field images of Hale-Bopp were also obtained through the narrowband filters, using the 2048 tex2html_wrap_inline21 CCD mounted on an 8-inch f/4 Takahashi telescope. These images have a 3 tex2html_wrap_inline31 field of view, which captures 10-15 million kilometers of the tail. In these images, the dust tail is well defined with a high signal-to-noise ratio but exhibits very little structure. The CO tex2html_wrap_inline15 images, on the other hand, show complex structure, which was observed to change on time scales as short as 30 minutes.