Reading through John James’ Spectrograph Design Fundamentals it became increasingly clear that while a Czerny-Turner spectrograph has excellent properties with regards to resolution and elimination of some of the many abberations, it also presents a daunting number of degrees of freedom in its design.
I normally don’t think the phrase “too much of a good thing” applies to freedom, but there’s of course always an exception to the rule.
To put it briefly; I’ve decided upon a much simpler design. I don’t even know if it has a name, but I see that some astronomers employ a similar construction.
The schematic for my spectrograph is shown here (not as pretty as the diagrams on the astrosurf page, but you get the idea):
The ligth from the slit is collimated by an achromatic doublet (Thorlabs AC254-040-A) one focal length from the slit. Everything is mounted a 2″ long lens tube, greatly reducing the degrees of freedom on this side of the grating.
The focusing lens is an m-rokkor 90mm f/4 that I had lying around. It’s basically a Leica lens in disguise. It doesn’t image the spectrum of interest (500-4000 cm⁻¹) on quite the entire CCD – that would require a slightly longer focal length.
The grating sits in a rotating mount. I don’t plan to do any rotating though, except for calibration. The design of the rotating mount is rudely stolen from here.
And here are the pictures:
Some modifications are pending; I would still like to anodize the aluminium base black, but I’ve not come across a way to do it (that I like). The grating adjustment mechanism might also be changed to something simpler ..think a screw in a piece of plastic..
And of course I’m waiting for the circuit board for the TCD1304 to arrive, so until then you’ll have to use your imagination for that particular component.