Quote:
Originally Posted by Startrek
Mark
Here’s a few notes ( not in order ) from Craig Starks document
F ratio determines photon density which is explained as follows -
Aperture = total photons
Focal length = spread of photons
F Ratio = density of photons
Photon density per pixel well rather than total photons collected is more important
There is a balance between Aperture and focal length. As we go up in focal length for same Aperture past the point of over sampling, you will lose SNR without gaining any spatial information.
Scopes that have higher focal ratios of say f8 to f10 spread photons thinly which reduce the photon count per pixel and reduces SNR. Running a scope at a lower F ratio will make each pixel cover more Sky which provides better images.
Finally
Quote - Photon count for an extended object is driven by F ratio. Image scale is driven by focal length. Want more resolution at the same pixel wise SNR ?, boost the Aperture but keep the same F ratio. Want more SNR in your images and your willing to trade of some spatial information ( area ) or your already asking for more spatial information ( area ) than your conditions will allow ? Then drop your F ratio !!
Cheers
Martin
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Forgot to mention
Camera type , pixel size, well depth , Gain, sensor temperature, QE , colour or mono etc etc... is a separate issue not discussed but is directly associated with the optical physics above