Freedom Freedom Freedom, Oy!

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):

drawing11The 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:


DSC01963Some 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.


2 thoughts on “Freedom Freedom Freedom, Oy!

  1. Fernando C. Alvira says:

    Hi, I am follow your blog and I found this spectrometer. Have you put it to function? There is some things that I do not understand on it. Whay do you use a holografic grating? They have less efficience than the blesed. And another thing is, if you put a fosucisng lens in front of the CCD, how do you spect illuminate all the pixels??


  2. The design on this page proved to have very poor resolution, so you shouldn’t copy it exactly. The design is not flawed as such, but it would be better to use a camera lens with a greater focal length for capturing such a small portion of the spectrum (540-676nm) on the CCD.

    With the focal length here (90mm) the difference between the angle of incidence and the angle of the diffracted light was too great, causing the anamorphic factor to be too large resulting in widening of the image of the slit on the CCD.

    With regards to your other questions:
    1. Yes I used a holographic grating, even though the efficiency is lower than for a blazed grating. The choice between holographic and blazed grating is not only about effiency. It’s a topic on it’s own. In brief I can say that most raman spectrometers I’ve seen use holographic gratings, and I see no point in trying to outsmart the professionals.
    2. You /will/ need some kind of image-forming optics between the grating and the CCD, otherwise you won’t be able to focus the diffracted light on the CCD and you will end up with light of many different wavelengths hitting the same pixels. To minimize the loss of diffracted light to the lens, the aperture of the lens should be greater or equal to the diameter of the beam of the diffracted light. See here:
    Most spectrometers are constructed with mirrors and not lenses, but you must still have a focusing mirror with a diameter great enough to capture all the diffracted light.


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