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If you're not prepared to wait / fork out the cash for a colour scientific CMOS sensor, it might be worth 
giving a modern DSLR a go - their low light performance is getting seriously impressive. I've been able to 
get good images of 100nm green fluorescent beads (Invitrogen) using my EOS500D on the side port of our 
fluorescence microscope. I think I needed an exposure time of ~1-2s for really good images, but could still 
discern the beads both when using the cameras 'live view' (video rate preview with mirror locked up) mode 
and in full images taken at lower integration times (I think I went down to ~20 ms, but would have to check 
back in the files to be certain). It's got a 1.8" CMOS sensor which means the chip area was somewhat 
larger than the FOV when used without any intermediate optics.

cheers,
David