|
In the classic "Tools of the Astronomer", Harvard books...
p21 ""As sources of light, astronomical objects are of two kinds - point sources and extended sources. A star does not show any appreciable disk of its own but gives a more or less pointlike image in a photographic plate....Objects of the second type, such as nebulae and planets extend over appreciable areas. The telescopes which are required to image most efficiently these two types of objects are not the same.""
p109 "" The limiting magnitude depends on and increases with both the aperture and the f ratio. An f5 camera will photograph stars about three magnitudes fainter than an f1 camera.""
p110"" Thus either stars or nebulae may be selectively emphasized. One chooses a relatively long focal length and a large aperture to photograph the faintest stars, or a small f ratio to minimise the presence of stars in a photograph of nebulae"
With today's CCD/Cmos this still stands true.
|