Quote:
Originally Posted by rmuhlack
This is a very pertinent discussion for me as well, as I have been thinking through these same sorts of ideas.
Question is what do you regard as a "standard" exposure length? If FL determines limiting magnitude, the isn't this where f ratio, QE and image scale now start to come into the equation, if the intention is to compare different setups?
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It doesn't really matter what you define as a "standard"
FL does not determine limiting magnitude.
If you use the example of a scope being used visually then our eyes can only integrate for a short time. This will be constant enough to compare the limiting magnitude for different scopes. The only way to increase the star limiting magnitude visually is to increase the diameter of the scope. Increasing the focal length has no action apart from slightly increasing the size of the airy disc as the optics become less perfect.
f ratio is mostly only important for extended objects not stars.
QE is obviously important to capturing the light more efficiently.
There is a practical limit to how deep a mag can be achieved. It has to do with the number of photons arriving from an object. If 1 photon arrives/m2/min for a very dim object then it is going to take an eternity with any size scope to measure this and get above the background noise. This may seem extreme but for some of the X-ray and gamma ray detections from the respective satelite observatories, only a very few photons are needed to constitute a +ve measurement. The nutrino detectors measured 5 nutrinos for the 1987e SN
http://www.sciencedirect.com/science...70269388916516
Optical scopes have more background noise making it more difficult.