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It can actually be quite complicated but here is a quick example. You have a 10" F/5 and a 10" F/10. On both telescopes you use a KAF8300 which has 5.4 micron pixels.
On the 10" F/5 you are imaging at 0.877"/pixel while on the 10" F/10 you are at 0.44"/pixel.
This means that every pixel is capturing four times the area (four times as many photons) on the F/5 than the F/10. Now the confusing part is as follows. Let's say the 10" F/4 used the ASI1600 with 3.8 micron pixels and you had one of the older KAF 1024x1024 15 micron sensors (Amdor still work with them I believe).
In this case the 10" F/4 is at 0.617"/pixel while the 10" F/10 is at 1.218"/pixel. In this instance the slower telescope is in essence near four times faster as each pixel captures near four times the surface area.
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