If you want to make a real RGB projector, which I assume you do, you will need two special mirrors that are called dichroic glass. These are a bit like electronic low pass filters (sort-of). I have three in my projector, but that's because I'm weird and I wanted violet in there as are a fouth color (pretty much a useless novalty). Anyway... One of them passes light above red like a piece of clear glass but it reflects red like a mirror. The other one is also a high pass and it passes below blue (red + green) but it reflects like a mirror at blue. These things are typically round and about 25mm in diameter. I think I got mine for about $30 each. But you need mounts for them too, I paid $150 for both the mounts and the dichros. I would have to say I got a pretty good deal cause I got mine from a guy who already had them in the USA. The ones I've seen that are in the USA are more expensive and the cheaper deals on the same ones seem to cost a lot to ship from out of the country. You don't really need a conventional first surface mirror for the green but your projector frame might well be outfitted and set up for one as this makes it much easier to align. The idea is that the beams are combined at 90 degree angles. The green goes straight through the first mirror and the red bounces off of it from 90 degrees to one side, the green and the red come together into one beam. This bean goes straight through the next dichro and the blue bounces off of one side at 90 degreens from the straight beam, this is the combination of all three and that's what hits your galvo mirrors. There are other possible combinations of these things, but what I have is very common. It works and it's what I have.
