How much detail can telescopes actually provide? For example, could the numbers / letters on a postage stamp in a randomly specified location be clearly visible from space.
This is to settle a discussion with a friend that piqued my curiosity.  
 A: Two major issues here. Well, maybe three...


*

*Optical limits of the instrument. Think Rayleigh Criterion, but beware of the existence of interferometric methods (hard to do in the optical for now, but...). It's going to take a big lens to image the a postage stamp even from low Earth orbit, and you might expect to find a spy satellite a bit higher up than that.

*Stuff in the way. The "twinkle" you see in stars is related to atmospheric interference. There are methods to compensate. Trying to look through clouds in the optical is a lost cause.

*Is the platform stable? If your optics are not well isolated from any vibration of your platform the camera will jump around.

A: Under excellent conditions, a telescope on Earth can see details with an angular size as small as 1 arcsec.  This tells us that the greatest distance at which you could see a detail as small as a postage stamp (0.02 m or 2 cm) is about 4,126 m or around 4 km.  So I infer that a telescope that is 4 km above Earth's surface could resolve the postage stamp (in excellent viewing conditions.)  Now, I am guessing that the number on a stamp might be as big as 1/5 the width of the stamp, so the telescope would need to be 5 times closer, about 0.8 km.  I used the small-angle formula to determine this.  Reference:  Universe (2010) Freedman, R.
