>
>I may have to cool the detector and I am 'familiar' with helium cryogenic
>pumps as featured on shall we say .. satellites used for watching other
>peoples activity :)

I always wanted to paint a test target on my roof.
Squares, crosses at various sizes.  Maybe a cute message.


In your position, I would be very tempted to use a prism, combined with the
PMTs.
The physical size isn't small though, unless the wavelengths are pretty
much spread out.  You can put a slotted mask over the PMT, so that it only
looks at the right wavelength.

My father-in-law had a spectrometer that worked that way, though he had
more than a few photons lying around.  Process control in a steel mill.
"Yeah, toss in 10' of half-inch copper, and a couple of car batteries".

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