Quote:
Originally Posted by gregbradley
Richard's paper is quite clear on that point. Its only F-ratio that determines the airy diameter. Its near the beginning of his paper about page 3.
Greg.
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And that is true, but it doesn't tell the whole story. To prove that is the case, just imagine that both of the diffraction patterns in Richard's paper were sampled by 20 micron pixels. The resulting star shapes for both f8 and f2.8 would be the same - the energy for would both be contained in a single pixel. ie, the star shape in an image is
not determined by the FNo. in isolation.
When you allow the pixel size and focal length to vary depending on sampling requirements etc. and then you incorporate seeing (which is measured in angular terms), the final determinant of tightness of star skirts is the aperture - properly sampled big scopes have inherently tighter star skirts than properly sampled smaller ones, as illustrated by the angular star profiles in the figure attached to post #9. Of course we already know this from viewing images taken by 24 inch f8 scopes - they look way tighter than those from FSQ106s, even though the 106 is a faster scope. This is not to say that a fast scope is no advantage - after you have settled the sampling by choosing pixel size and focal length, a faster scope will give you a bigger aperture, resulting in tighter star skirts and more signal.