Is there a known optical design for a beam compressor? With my little knowledge of optics I have come across some 'known' designs such as the Double Gauss for example, is there a 'beam compressor'.
My requirements are to reduce an incoming parallel wavefront (source at infinity) to either an outgoing parallel beam (ie afocal), by a factor of around 100 within 10cm or coming to focus at an f-number around 4 or 5.
I have access to Zemax but am currently just searching in the dark worried I might be re-inventing the wheel...
 A: A beam expander takes a parralel beam of light and outputs another parallel beam. A factor of 100 is a little optomistic - even assuming you had perfect optics you would inevitably need an exit lens (or mirror)  100x larger in diameter than the incoming beam. 
Unless the incoming beam is very very small - and then you hit a whole other bunch of optics problems - the output element is going to be large and expensive.
There are 'standard designs' but these are not going to scale to 100x.
Possible an astronomical Cassegrain telescope is the easiest practical system?
A: You can use two parabolic mirrors (either one concave + one convex or two concave) in an afocal Gregorian-telescope arrangement but with multiple reflections. Every second reflection would give you an extra magnification. 
For example, if you take two parabolic mirrors with 100mm and 20mm focal length (5:1) then you will get after the second reflection 5x, fourth reflection 5x5x=25x, and sixth reflection 5x5x5x=125x magnification. 
No aberrations if you use parabolic mirrors but at every reflection you will loose some power. With a 95% reflection mirror (at some wavelength), after the sixth reflection you will end up with a total loss of 11.5%.
Alignment is not easy either. 
