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DIY spectrometer test bed


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#1 Astrobodger



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Posted 02 April 2013 - 09:47 PM

Here’s my  DIY spectrometer test bed.  It is built from very basic cheap materials such as laminate flooring, hose clips etc. Scraphead Challenge style astro-bodge with just basic hand tools. The design is very close to the classic spectrometer principle of using separate collimation and imaging lenses. I got the lenses from eBay or car boot sales. Proper spectroscopes can cost thousands. I tried to build a Littrow spectroscope before, but that is less forgiving.


The reason I call this a test bed is because I specifically wanted something very generic that I could use to evaluate different gratings, angles, distances and lenses.


The platform consists of a hinge made from laminate flooring. The lenses are held in place using heavy duty clips and aligned using a laser colimator. I take no responsibility for lenses or CCD cameras falling off if you try this!


The camera can be swung through a wide range of angles relative to the colimator assembly (that is attached to the telescope focuser). The transmission grating sits on a small table that can be orientated an an angle independent of the camera. This allows me to replace the 300 l/mm transmission grating with my 1200 l/mm reflection grating (still working on a holder for this).



The assembled unit is shown below. Note that the colimation lens is actually mounted the wrong way around. This only works with non-zoom (prime) lenses. My colimation lens is a 128mm F2.8 Cosina (cheap & cheerful). Luckily it has a 42mm thread that fits the t-thread of the 5mm extension.



The side shot below should help explain how the thing works



This shows the camera swung at a large angle



The slit assembly uses a t-thread adaptor from Astroboot with an illuminated Surplus Shed adjustable slit inside.



My first attempt used a broken craft knife blade. Crude but useful to prove the concept.



Not shown, is the length of foam/rubber sheet that I wrap around the spectrometer body to shield the optics from stray light and dew!


Using the spectro test bed


You need guiding when using a slit spectrometer. You have to keep the star image on a slit thats probably about 30 microns wide. This is quite a challenge on my well worn EQ5.


  1. Check that the camera lens assembly is focused at infinity (e.g on a star)
  2. Ensure your view finder and guidescope are accurately aligned with the telescope.
  3. Mount the spectrometer with the slit fully open and with the camera straight.
  4. Locate your target and focus the telescope (on the zero order image).
  5. Illuminate the slit and focus the image of the slit. Refocus the telescope if necessary.
  6. Calibrate your guiding (e.g. using PHD)
  7. Slew on 1x speed so that have the target is obsocured by the static half of the slit.
  8. Reduce the slit width to a narrow width that is stable to small guiding errors. If you set it too narrow you will keep losing the target.
  9. Allow the guiding to 'recover' each time you adjust anything.
  10. Swing the camera to the required angle too see the spectra spread on the screen.
  11. Allow the guiding to 'recover' each time you adjust anything.
  12. Focus the colimating lens to produce the thinest spectra (at your region of interest as some chromatic aberation wil be visible)
  13. Allow the guiding to 'recover' each time you adjust anything.
  14. Cover the optics with the foam sheet and secure with velcro or gaffa tape to sheild light and moisture.



Areas for improvement

The ease of access is also the weakness. Some kind of enclosure around the grating and lenses will prevent dew.


When using a transmission grating, I need to swing the camera. With a reflection grating, I could just rotate the grating from zero order to the desired angle. Less impact on guiding.


Some kind of flip mirror to check on the target is still on the slit would help.  I have some ideas that involve a Pritstick ;)


Thanks for looking,


  • Neil likes this

Celestron 8" F5 Newt on EQ5

Spectrographs 300 l/mm & 1200 l/mm, SA100

Mammut L429, Canon 1000d, Canon 450d



#2 Neil



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Posted 03 April 2013 - 01:25 PM

Very interesting project John,


Thanks for sharing the details of it with us. I can't help thinking that a reflection grating rather than a transmission grating would simplify the project? Are they more expensive or just difficult to get hold of?


Then you could just change the angle of the grating and keep both of the lenses in a fixed position. Or have I misunderstood?


The one thing that I still don't understand is the spectral order? Everyone talks about the '0' order but what actually is it?


Best regards




#3 gasman


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Posted 03 April 2013 - 08:33 PM

Excellent `Bodge` that John :)  I`m sure I`ve seen that design somewhere before. I`d like to have a go at making one of those but just toying with treating myself to the new Alpy spectroscope and how to get it past SWMBO.

Neil - zero order is simply the actual star that is used as point `0` on the graph. Its not really needed if you know some wavelengths of the peaks and troughs on your graph. The wavelength scale starts at zero and goes up in value from left to right assuming the `zero order star` is on the left





#4 Astrobodger



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Posted 03 April 2013 - 10:25 PM

Thanks for the comments chaps.


Neil - Alsolutely,  a refelection grating allows the colimation and image lenses to be fixed (See area for improvement point 2). That's why I am already working on a special holder for my 1200 l/mm reflection grating. Transmission gratings are cheaper and easier to mount.


Steve - I previously posted an effort (using a £3 educational grating that used a similar principle to swing a DSLR) a while ago on Ken's site. Results were not good. That was before I discovered the sa100.




Celestron 8" F5 Newt on EQ5

Spectrographs 300 l/mm & 1200 l/mm, SA100

Mammut L429, Canon 1000d, Canon 450d



#5 Starlock


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Posted 04 April 2013 - 05:08 PM

Good stuff John, a nicely illustrated write up, I think I've learnt a thing or two:)

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