Optics nerds: What's the easiest, lowest cost way to build something that focuses a lot of light from a fairly wide (say 90 degree, give or take a bit) FOV into a spectrometer with a SMA 905 fiber input?
Goal is to collect UV-VIS-NIR spectra of the night sky (particularly interested in both light pollution and auroras) over as much of the 200-1200nm range as I can get with low-cost optics (i.e. I don't want to spend extra to get a bit further outside visible, but will take what I can get easily).
Since the device will be operated outside at night, it can be open frame (no need for any exterior light-shield tube, only mechanical support components).
My initial thought is some kind of 80/20 based frame holding a cheap Fresnel lens at one end, with the spectrometer mounted at the focal point (no fiber, directly bolted to a bracket at the focal point) with a cosine corrector on the input to increase the size of the entrance pupil and provide a bit of tolerance for misalignments.
@azonenberg The one annoying thing of optics is that angles and sharpness and whatnot come at a tradeoff. As the light of the night sky will come from different directions in your FOV, that angle difference between different sources will persist throughout the optics pathway - even with perfect optics.
A good spectrometer uses angular separation of light to identify wavelengths, you actually want a singular focused source to work with.
@azonenberg you could focus the entire night sky onto a focal plane, yes, but then your fiber input would act like an aperture to select specific cutouts of it - while it sounds like you want to average out the spectrum of the night sky?
It's a tricky thing, and I don't know what would help it. I feel like FOV of the spectrometer and spectrum clarity are inherent tradeoffs of the optical path, as a large FOV means low source coherence and thusly lower coherent spectrum.
@xaseiresh I want the spectrum of a large area. The cosine corrector has a PTFE diffuser in it, so my thought was that if I were to focus the image of the entire night sky down onto this ~4mm disk, I'd have fairly uniform representation of the whole sky at the other end of the fiber with all spatial information discarded.
@xaseiresh That's why the plan was to put a cosine corrector on the fiber input and focus the light from the sky down onto the surface of the corrector, giving me a nice parallel input to the spectrometer.
PMMA looks like a pretty good choice for large, cheap, relatively broadband optics.
It's got a fairly flat ~90% transmission for a few-mm sheet (good enough, I can make up for the loss by increasing objective diameter) from around 370-1100nm then falls off sharply into the UV and has a small dip in the IR before rising again.
So it'll cover the vast majority of my spectrometer's capacity from mid UV-A into NIR, just losing the UV-B/C on the far short end of my wavelength range (which the atmosphere doesn't transmit all that well anyway).
@RichiH This system isn't something you'd want to use for alerting because the large objective diameter would collect way too much light, possibly to the point of damaging the spectrometer, during daylight.
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