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and as always aperture is king
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For astrophotography, f/ratio is Emperor. For instance, a massive 16" RC at f/8 will yield only 1/3 the number of photons per unit area of CCD sensor compared a small 3" apo at f/4.8. Or, put another way, a 12" newt at f/4 will give exactly the same number of photons per unit area of sensor as a 6" newt at f/4.
So why bother to go to larger apertures for AP? Two words: image scale. In the above example, the RC has a focal length almost 9 times longer than the little apo, and the 10" newt has one 1.66 times longer than its 6" counterpart. That means that the pixel resolution increases, and if your CCD pixel size matches the focal length, you get more detailed 'zoomed in' images of galaxies etc.