Sneak peak at the new spectrograph

My original choice for the spectrograph’s geometry turned out to be much worse than I had even dared expect. The difference between the angle of incidence and the angle of the diffracted light was far too great.

I (obviously) didn’t know at the time, that there’s something called the anamorphic factor.¹ The definition is as follows:

r = cos θi / cos θm

If the anamorphic factor is significantly different from 1, the image of the slit on the CCD will be deformed. I don’t fully understand the implications, but I could see that a “line” supposed to take up maybe 100µm was spread out over almost 1 mm. Ok, I might be exaggerating slightly, but it was very obvious that it was never going to work.

I spent some time not thinking about it(!?), and eventually came across T.J. Nelson’s page about a High resolution compact spectrograph, and decided that this was juuust exactly what I wanted to do.

His spectrograph is of a design proposed and theoretically treated by Gil and Simon² and first realised by Schieffer³. The following diagram is from Schieffer’s publication:

Image from Schieffer S.L. et al, Appl. Opt. 46, 3095-3101. P1 and P2 are the off-axis parabolic mirrors. M1-3 are regular plane mirrors.

My new spectrograph is still very much in the works, but so far the 90° off-axis parabolic mirrors have arrived:

and you can sort of get the idea of the layout:

Obviously some optics are missing at this point, and the grating has a plastic-box-proxy. The problems to solve are alignment of OAPs, folding mirrors, grating and CCD, and of course some practical considerations of how to fit everything inside the Modushop GX388 box.

[2] Gil M.A., Simon J.M., Appl. Opt. 22 (1983), p 152-158, New plane grating monochromator with off-axis parabolical mirrors.
[3] Schieffer S.L. et al, Appl. Opt. 46 (2007), p 3095-3101, High-resolution, flat-field, plane-grating, f/10 spectrograph with off-axis parabolic mirrors.


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