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The "brightness" of light reaching the CCD chip is the same at any given F ratio. Your camera lens set to F4 and my 10 inch F4 have the same brightness on the CCD. For both systems, the same exposure at the same ISO will give the same brightness. The difference is focal length and consequently field of view. The camera lens will take in a large part of the sky, the telescope field of view will be much narrower. When using long focal length (more magnification) it takes a big mirror or lens to match the brightness of a wide angle lens. It's pretty much the same rules ( focal ratio) that exist for daylight photography - if you were to take a single shot.
Focal length, aperture and focal ratio. It's a bit like ohms law. You can calculate the third by knowing any of the other two. If something is F4 then the focal length must be 4 times the aperture.
So how do we get hours and hours without overexposing? We take short sub frames and stack them to simulate a long exposure, otherwise we would overexpose. You can always wind the gain or ISO down also to prevent overexposing.
Stacking lots of short sub frames works on the principle of signal to noise ratio. Cameras are noisy. Fortunately a lot of it is random so with multiple sub frames the random noise gets cancelled out, but the signal which is constant remains.
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