Does anybody know of a good reference for determining the required diameter of an earth observing telescope's primary mirror? I am trying to find determine a rough estimate for a design I am working on. So far I have found the equation Sin[itex]\theta[/itex] = 1.220 [itex]\lambda[/itex]/D for angular resolution and [itex]\Delta[/itex]l = 1.220 d[itex]\lambda[/itex]/D for spatial resolution, where D is the Diameter, d is the distance, lambda is the wavelength. I've never really dealt with optics like this (minus like a class in high school long ago) but I do know my results are ridiculous at a 12 cm primary for a 500Km orbit My confusion comes from what the angle I use is. The satellite would be orbiting between 200 and 500 Km and be taking 25 x 25 km images in full colour. If anyone can help me with this it would be greatly appreciated.