By Greg Walton
The larger the telescope the brighter the image you see at the eyepiece and the faster we can take a photo. A telescope 200mm in diameter collects 4 times the light of a 100mm telescope. As you can see four 100mmm circles can fit in to a 200mm circle. See diagram below.
The inverse square law says when you double the diameter; the area will increase by 4 times. This work for us and we can take a shot in ¼ of the time. But if we double the magnification, we will double the F ratio and we will be back were we started, because we will be looking at an area of sky that is only ¼ of the size, which would contain ¼ of the light. See diagram below.
So it's a balancing act between diameter and F ratio which determines the brightness of the image.