Pic du Midi Observatory Image of Comet Hale-Bopp
24K
Observers: Francois Colas, Jean Lecacheux
Location: Pic du Midi Observatory, France
Date: April 15, 1997
Hidden comet in the comet ! April 15th
As the sunlighted cometary dust observed through different broadband
filters in the visible spectrum exhibits the same spatial repartition
of luminosity, it is possible to cancel the dust tail accurately by
subtracting two CCD images taken in different colours, providing that
we have chosen the good cooefficient of proportionality. Here we have
subtracted a R image to a B image, the result of the process is an
enhancement of the ion tail, whose brightness is manly due to the
blue-violet CO+ emissions. Although modern photographic emulsions give
excellent detection of the external ion tail, its near-nucleus part is
hopelessly drowned in a ocean of light due the dust luminosity. Only
computer processing of CCD images can restore it.
This very simple method is able to reveal the hidden comet in the
comet. It can be applied by amateur astronomers and is sufficient to
make a rough hour by hour survey of the ion tail extensions and
morphology. But it could be erroneous to derive quantitative flux
measurements, as the red emissions of the H2O+ ion are subtracted in
the resulting images, and consequently appear in negative ! A correct
scientific work need observations with costly narrow-band CO+ , H2O+
and continuum interference filters.
Contact: Francois Colas (francois.colas@bdl.fr)
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