Quote:
Originally Posted by Emuhead
The part thats gets me is, how this calculation can be true for different apertures.. i would totally get this if we were talking the same apertures (100mm only), but we have 2 different apertures 100mm & 120mm. I just cant get my head around how that one formula can be good for any 2 different apertures. Say a 50mm and a 1000mm, both were f/2. This calculation breaks here.. because 2^2/2^2 = 1. They both cant catch light at the same speed.. the 1000mm would be so much faster..
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It’s not necessarily about the amount of light that gets into the system. What it is really about is how that light is concentrated and the focal ratio determines how concentrated the light is.
As aperture increases the amount of flux per arcsec of sky increases BUT at the same time the amount of sky that each pixel covers decreases proportionally. Let’s take your Fuji as an example.
A 100mm (4”) F/2 will have an image scale of 4”/pixel so it’s very low resolution. The 1000mm (40”) F/2 will have an image scale of 0.4”/pixel which is quite high resolution. If I do 4^2/0.4^2 (image scale) it shows that each pixel in the 1000mm is covering 100x less area of sky than the 100mm. But the 1000mm is also capturing 100x more photons so it balances itself out.
If a 100mm telescope captures 100e- then my 50mm F/1.8 (35mm aperture) only captures 12.76e-/arcsec/s which is considerably less. The caveat is that each pixel covers 16 arcsec as opposed to the 1.463”/pixel of the 100mm Esprit which is 121x more area! It’s only capturing 7.8x less light though which is why it’s 7.8/121=15.4x faster.