Skip Navigation: Avoid going through Home page links and jump straight to content

Pic du Midi Observatory Image of Comet Hale-Bopp


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 (

comethome.gif Comet Hale-Bopp Home Page