Observers: Francois Colas, Jean Lecacheux
Location: Pic du Midi Observatory, France
Date: May 2, 1997 20:40 UT
May 2nd : one month after perihelion, Hale-Bopp keeps great interest.
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We continue our systematic survey of both ion and atomic sodium tails in spite of low elevation and increasing twilight. On May 2nd around 20 h 40 UT we have made a 26 minutes exposure through our 2.8 nm wide interference filter centered on the 579.3 nm sodium D line and our 50 mm photographic lens. Contrary to our previous assumption, we find evidence of image pollution by faint H2O+ emissions in our filter bandpass. This effect explains changing features in the sodium tail from night to night, especially its odd binary appearance of May 1st evening. To correct the sodium image, we now subtract the ion contribution using a blue wide-band exposure made some minutes before. So the faint CO+/H2O+ streamers are completely removed, and even the previous fan shape of the sodium tail disappears. The sodium feature remains alone, about 9 degrees long (90 000 000 km), and incredibly narrow.
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Even if the comet brightness is fading, the mysterious shells around the nucleus are still active. More, the simple concentric system is now becoming more complex : on this image taken on May 2nd evening, at least three different shell systems are visible. The bad news is that the shell contrast has faded during the last week, requiring now stronger image processing . This image was obtained with the 105 centimeter telescope with a Gunn 6 filter.
Contact: Francois Colas (francois.colas@bdl.fr)
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