Thread: F-ratio myth
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Old 23-02-2018, 05:50 PM
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Peter Ward
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Quote:
Originally Posted by markas View Post
I'm buying into this one quite late, but here goes anyway;

Firstly, the "f-ratio myth":
The amount of light entering the telescope is a function of aperture. Hence the number of photons arriving at the sensor plane is basically a function of aperture.

BUT, the concentration of photons hitting a pixel is also a function of focal length.

If you take Steve Moore's own equations for Signal and S/N you can arrive at a relatively simple relationship that pixel S/N ~ (pixel size/f-ratio)*sqrt(sub time) - all other things such as quantum efficiency, optical efficiency etc being equal.


Similarly, the Signal per pixel ~(px/f-ratio)^2 .

These are very useful rules of thumb in estimating exposure times and signal intensity for your system based on images obtained by other systems of known (px/f-ratio).

Secondly, photons and waves.

Very hard to comprehend, but the quantum mechanical reality is that if you interrogate a wave/particle with wave instruments, you'll get wave answers. If you interrogate the same wave/particle with particle instruments, you'll get particle answers.

An wave/particle in an optical train is well charasterised by wave behaviour, but the sensor asks particle questions and the particle explanation works.

Confused? We're not Robinson Crusoe - quantum mechanics is plain hard to understand. As the great quantum theorist Richard Feynman said "if you think you understand quantum theory, you don't"

With apologies!
Mark
All good. I've found many of the responses useful. Thanks to one and all for keeping the thread relevant and informative.

Now onto the subject of: life the universe and everything....
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