Public Lab Research note

Proposal: SLR spectrometer attachment

by warren | July 20, 2013 17:10 20 Jul 17:10 | #8912 | #8912

Lots of folks have talked about doing a higher-end spectrometer attachment based on an SLR lens mount. This is a good idea -- we can model it and share the model on Shapeways so people can 3D print it.

To start out with, I did a quick survey of open source Thingiverse models of camera lens mounts and lens caps:

One thing I'm not sure about -- will take some testing -- is whether we want to make an attachment for the front of a lens -- lens-cap style -- or to attach directly to the body of the camera, which would have no lens. I'm not sure the latter would work with our current spectrometer design... the geometry may be pretty different. I also don't have an SLR :-P


I think this would be extremely helpful as a more easily calibrated and thus more accurate spectrometer (compared with the current "piece of CD-ROM in a box with a camera" type DIY setup. With a rigid and fixed system like this, you could take a calibration curve spectrum once (RGB lasers?) and use that to quantify the subsequent spectra. Check out Samir Kharusi who has done this very well: He used it to crease spectra of stars and planets: A setup with results like that would be fantastic. Hopefully a cheap diffraction grating film would work as well as the blazed grating he used and a rigid plastic tube can be used/attached instead of a cardboard box. Not sure how you'd mass-produce a thin slit but the 2-razor blades approach works and isn't difficult. I had a quick go at this last week with a 2 razor blade slit at the end of a cardboard tube and using a CD-ROM with reflective coating removed as a transmissive DIY diffraction grating. Seems to work. Benjamin

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Pentax K-mount please? The base connector could be the simple K-mount-to-Pentax-thread rings that are available and cheap, for example.

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@BKF11 - calibration works super well with our current setup; it's sub-nanometer. The big problem with an SLR (besides expense) is that they don't have a USB interface, so it ends up being cumbersome to record and analyze data.

@hank - do you have any 3d models or even just really good measurements for those? The screw threading in particular...

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Pentax No models or measurements; this may help, if the description of the thread type is what you need:

T-Rings for Questar Telescopes Only - Astronomics‎ The 42mm x 1mm pitch male threads of the camera coupling set allow the coupling to be connected directly to camera bodies using a Pentax screw thread lens ...

I could buy and send you one of these adapters (they're available much cheaper than this Amazon listing, it's just handy):

Those take the old Pentax screw mount lenses and allow them to mount on newer bayonet-type cameras.

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Not sure what is mounted to what in the 3D image above, but the two camera mounting methods are different: 1) "in convergent beam" - grating parallel to image chip w/o lens and 2) "objective grating" - grating parallel to front of lens. #1 avoids the lens (spectrum limits) but causes blurring as the focus changes across the spectrum (i.e. across the imager chip). #2 has a smaller aperture plus glass in the path but the spectrum is in focus.

However, in either case you have to include the slit in the design. These SLR methods are most often used for astronomy where true collimated light from a star, off-axis to the grating+lens, is the source. With light-bulbs and such, a precision slit at the end of a dark tube (zero reflections) with the lens focused on the slit, would be required. Depending on the lens, this suggests a more elaborate design to house slit, grating and lens adaptor. Finally, note that the grating must be adjustable to keep its orientation aligned with the imager chip. Could be two parts (lens+grating) and (tube+slit) but they need to be light tight and aligned (slit with grating and grating with chip).

Hope this is helpful. Cheers, Dave

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