Hunting and Imaging Comets by Martin Mobberley

Author:Martin Mobberley
Language: eng
Format: epub, mobi
Publisher: Springer New York, New York, NY

Going Faint

The larger the aperture you are using the deeper you will go, but size is not the only consideration. Sharp focusing is essential to get the lowest stellar magnitude possible and incoming comets are often very stellar in appearance. An image that is even fractionally out of focus will result in that extra light from the comet being smeared out into the sky background. Remember, when imaging to the limit of your equipment the faintest objects detectable may be less than 1% above the background sky brightness. They are literally sitting on top of a wall of sky glow. At the darkest sites in the world the background night sky will have a glow of between magnitude 21 and 22 per square arc-second. Regardless of the size of your telescope you are faced with this almost impenetrable barrier. The situation is far worse from a typical amateur observatory though. A really good amateur rural site might enjoy magnitude 20 per square arc second skies, but even in modest towns this can drop to 18, 17 or even 16 depending on how much light pollution there is (see Fig. 7.5). For comet recovery the amateur really needs to be in a semi-rural location with skies darker than magnitude 19 per square arc-second. At such a site the dark adapted eye will still see the sky as grey, rather than black, but the river of the Milky Way will still be easily visible to the naked eye, along with, say, objects like the Andromeda Galaxy, M31. Having a background sky brightness of magnitude 19 per square arc second does not mean that you are limited to imaging stellar objects of magnitude 19. It is the ability of your system to extract signal from noise that is crucial. Even below magnitude 20 a steady flow of photons will be hitting your CCD chip, but will almost be drowned in the tsunami of orange photons from the sky. The key is to focus the stars precisely, track them accurately and then use long exposures so that the small signal you want gradually emerges from the much bigger random noise signal of the sky. A magnitude 20 comet, even if vaguely stellar in appearance, will be spread into a blob many arc-seconds across and so will actually have a sky brightness of the order of magnitude 23 per square arc-second; that’s four magnitudes (or 40 times) fainter than a magnitude 19 per square arc-second sky. But the image should not be thought of as being totally buried under that sky background. It is slightly more reassuring to think of it as sitting atop the wall of light pollution.

Fig. 7.5.Light pollution can severely hamper your ability to reach faint magnitudes: Image by Gary Poyner.

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