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3D Printed Spectrograph

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#1 Tucker512

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Posted 19 September 2015 - 03:27 PM

So, in an effort to highlight the dangers of having Zemax, a 3D printer, and too much free time, I made a 3D-printed guided spectrograph. It actually works, and pretty well, so I thought I'd share some details. Pictures below if you want to skip to the "it works!" part.

 

The design is a classical spectrograph, using a 600 lines/mm reflection diffraction grating from Edmund Optics. The spectrograph features a "slit" consisting of a 35-micron laser-drilled pinhole from Lenox Laser. The pinhole is drilled into a very thin sheet of stainless steel, the surface of which acts as a mirror to send light to a guide camera.

 

Light comes into the spectrograph and is focused onto the pinhole plate. Most of the target starlight goes through the pinhole and into the main spectrograph optics. These consist of a 20mm diameter x 80mm focal length achromat from Anchor Optics. The achromat collimates the diverging beam which then hits the 1/2" diffraction grating. The nominal design has the grating tilted 9.5 degrees to the incoming beam. This diffracts light at 5700A at 30.5 degrees, meaning the outgoing beam is tilted a total of 40 degrees to the incoming beam. This angle and wavelength were chosen so that the H-alpha line would fit on the CCD, but more on that later. The diffracted beam then goes through another 20mm x 80mm achromat and is focused onto the CCD, in this case an Atik 490EX monochrome camera.

 

Meanwhile, some of the starlight that doesn't make it through the pinhole is reflected by the pinhole substrate, which is tilted 15 degrees, off a 1/2" flat mirror, and through two 12.5mm x 45mm achromats, also from Anchor. These relay the image of the pinhole and field of view of the telescope to the guide camera, a Starlight Xpress Lodestar.

 

Initial setup involves illuminating the pinhole plate (by shining a flashlight into the scope) and focusing the guide camera on the pinhole.  Focusing is done simply by sliding the cameras in and out of their 1.25" openings, but works surprisingly well. The XY pixel position of the pinhole is noted. Next a star is focused on the pinhole plate as seen by the guider. The guider is calibrated in the usual manner in MaxIm DL. Then when guiding is started, the XY guidestar coordinates are manually entered to match the coordinates of the pinhole. MaxIm then drives the guidestar right onto the pinhole! It guides surprisingly well! The first light test was done with a C11 at f/10. Despite the questionable surface quality of the pinhole substrate, the images look fine and a 9.1-mag companion to one of the stars was visible in a 0.1-second guide exposure! The stainless steel surface works quite well!

 

Focusing of the spectrograph camera is most easily done by pointing the telescope at a compact fluorescent lightbulb or other line source and focusing on the narrow lines. The source need not be focused in the telescope itself.

 

For wavelength calibration, I simply held a neon bulb in front of the telescope (and stuck the dust cover over it to block stray light) and took a long exposure. I wrote software that lets me identify the known lines and will produce a wavelength solution, as well as a program to extract the spectra and fit the wavelength solution. I use iSpec, a free Linux program that is available in a self-contained virtual system that can run under Windows. It can normalize the contiuum, find elements from lines, find the radial velocity of a star, etc.

 

I designed the 3D printed housing in two halves, top and bottom. This makes it feasible to print and easy to assemble by simply dropping the optics into their slots and then putting the top on! The two halves bolt together with screws and hex nuts. I couldn't print an accurate enough 1.25" nosepiece that wouldn't wiggle in the scope, so I revised the design to hold a metal T-thread-to-1.25" nosepiece. That helped considerably. I would change how I mounted the pinhole as I didn't have a precise way to center it. This changed the available wavelength range and eliminated H-alpha. It ended up spanning from 4065-6305A. Print time for each half was 6.5 hours at low resolution on a Makerbot Replicator 5th generation printer.

 

For first light I pointed it at a few bright stars of various spectral types, and a high-pressure sodium light. All look about as you would expect. Hydrogen Balmer lines in the B and A type stars, lots of metal lines in the K and M stars. The sodium doublet is resolved in the stars that show it. I calculate the resolution to be about R = 1000, or about 5A. That is pretty much what the theoretical calculations give, so I guess it works! Exposures are only 1-5 seconds, depending on the star. I plan to try some fainter stars next time out.

 

Total cost for the four doublets, fold mirror, diffraction grating, and pinhole was about $250. I didn't keep track of the amount of material used by the printer, but probably less than $10 worth. $25 for the 1.25" nosepiece plus a few bucks in screws. Under $300 for a fully functional guided spectrograph. Not too shabby!

 

The images show the optical diagram, a cutaway model, the spectrograph being assembled, some stellar spectra, and the assembled spectrograph with cameras an a Stellarvue SV60 because it's funny.

 

Scott

Attached Thumbnails

  • 3D Printed Spectrograph Internal.jpg
  • Optical Layout.jpg
  • Print with Optics.jpg
  • Spectrograph on SV60.jpg
  • Vega.jpg
  • Aldebaran.jpg
  • Zeta_Per.jpg
  • HPS.jpg


#2 RAC

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Posted 19 September 2015 - 04:18 PM

Nicely done :waytogo:



#3 xiando

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Posted 19 September 2015 - 04:26 PM

Another darn good reason for me to finally build the prusa kit I bought (yikes! 2 years ago!). Cool idea!



#4 raal

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Posted 19 September 2015 - 05:47 PM

:bow:



#5 jabil

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Posted 19 September 2015 - 09:49 PM

@ Tucker512

Nice design. I think you have spent some time for this. Post your zemax file here. so we can see the spec.

Jabil



#6 CanaryMax

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Posted 20 September 2015 - 06:46 AM

Wonderful job, now you put a fancy name and produce them in series but not desorbites price, please



#7 Oberon

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Posted 20 September 2015 - 08:31 AM

Sorry but I'm curious and so I have to ask. What is for?

 

I know perfectly well what professionals do with spectrographs, but what does an amateur do? Is this just about satisfying curiosity or is there a real science project to be involved in?



#8 xiando

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Posted 20 September 2015 - 08:36 AM

For characterizing and normalizing one's image color content?



#9 Oberon

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Posted 20 September 2015 - 08:38 AM

One star at a time?



#10 xiando

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Posted 20 September 2015 - 08:39 AM

One star, properly chosen, is all it takes.

 

EDIT: PS the "poor man's"reference is Vega. A nice, almost white star, (yes, I know, please no one jump on me for that glaring reduction) one can use it as the white point if it happens to be in the frame or is shot during the evening as a reference.


Edited by xiando, 20 September 2015 - 08:48 AM.


#11 Oberon

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Posted 20 September 2015 - 08:47 AM

Keep going...



#12 xiando

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Posted 20 September 2015 - 08:50 AM

Well, it's quite simple (in my mind anyway). Suppose I can characterize the star color of a star in a frame. I can then apply that knowledge to adjust the white point for the entire photo. 

 

Edit: grok?


Edited by xiando, 20 September 2015 - 08:53 AM.


#13 Oberon

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Posted 20 September 2015 - 09:11 AM

Sort of. I'm thinking you're really trying to characterize the chip, which only has to be done once and is usually done with a reference calibration source. But maybe I'm getting rusty. Certainly astronomers doing spectroscopy will utilize standard spectrophotometric stars to calibrate their spectroscopic and photometric data, but I'm not used to stellar spectroscopy being done to calibrate color balance for imaging. But I suppose you could if you're comparing spectroscopic profiles with professionally produced standard profiles.



#14 Oberon

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Posted 20 September 2015 - 09:17 AM

PS. Obviously I don't do astrophotography. Not seriously.



#15 xiando

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Posted 20 September 2015 - 10:19 AM

Apparently not. Sky noise varies depending on environment and locale. I tend to shoot unfiltered one shot color, EDIT: most often from a white zone. For me, session calibration is a must.


Edited by xiando, 20 September 2015 - 10:53 AM.


#16 Oberon

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Posted 20 September 2015 - 06:50 PM

OK I'll buy that. But the OP mentioned determining the radial velocity of a star, aka identifying spectral shift, which in turn relies highly on physical stability of the instrument plus the ability to calibrate without moving etc. I'm just wondering whether people really do this, and now seriously, because if that's what you want then you'll want a spectrograph made from something more stable than printed thermoplastic.

 

I love the idea, I'm just wondering how serious it is.



#17 xiando

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Posted 20 September 2015 - 07:10 PM

idk. I use a printed bahtinov mask. printed parts can be very precise with correct adjustments of a printer and printing cycle, along with careful cleanup and assembly afterwards. After all, he's not printing the lenses or grating, just the shell into which they fit... (yes, I realize you know that. not being a snark) 

 

Anyway, it seems a cool idea to me. I wouldn't mind being able to characterize a star. Or for that matter, as you rightly suggested, characterizing my sensor periodically.

 

 

 

Tucker512, will you be releasing the files to the general public at some point?



#18 Oberon

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Posted 21 September 2015 - 01:29 PM

It's not the precision of the printer I'm thinking about, it's the mechanical aka thermal expansion properties of thermoplastic v keeping every aligned.


Edited by Oberon, 21 September 2015 - 01:48 PM.


#19 xiando

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Posted 21 September 2015 - 01:41 PM

It's not the precision of the printer I'm thinking about, it's the mechanical and thermal expansion properties of thermoplastic v keeping every aligned.

Hmm... good point about the thermal props. However, on the other, if printed properly, mechanically it should be fine for the loads it handles..

 

PS, but like you, I'm not the op or designer. it's a valid question.


Edited by xiando, 21 September 2015 - 01:41 PM.


#20 Tucker512

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Posted 21 September 2015 - 04:53 PM

Thanks! Obviously the plastic construction limits the applications.  It was mostly done because I can.  But I've been fascinated by spectrographs and this seemed a way to play around with one.  The radial velocity determinations are limited by the (in)stability of the spectrograph, and by its inherent spectral resolution (about 75 km/sec/pixel) and things like the accuracy of the wavelength solution (I get about 10 km/sec RMS using about 15 lines from argon and neon bulbs).  That said, I measured the radial velocity of Vega and Aldebaran with reasonably close to actual values.  Of course, these stars have RVs of tens of km per second.  You won't find planets with a 3D printed spectrograph!

 

If anything else this is a good way to see what works and what could be improved before building a real machined version, for example.  I plan to do one more refinement to the design, then I could post the 3D files.  Still a work in progress!

 

Zemax prescriptions below for the spectrograph itself and the guiding module.  Numbers in the comment column are Edmund part numbers.  Wavelengths used for the spectrograph are 456, 570, and 679nm, which span the 490EX sensor.

 

Scott

Attached Thumbnails

  • SG Zemax.jpg
  • Guider Zemax.jpg


#21 xiando

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Posted 21 September 2015 - 05:58 PM

Thanks Scott. An inspiring topic and design.



#22 Oberon

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Posted 21 September 2015 - 08:44 PM

Thanks! Obviously the plastic construction limits the applications.  It was mostly done because I can.  But I've been fascinated by spectrographs and this seemed a way to play around with one.  The radial velocity determinations are limited by the (in)stability of the spectrograph, and by its inherent spectral resolution (about 75 km/sec/pixel) and things like the accuracy of the wavelength solution (I get about 10 km/sec RMS using about 15 lines from argon and neon bulbs).  That said, I measured the radial velocity of Vega and Aldebaran with reasonably close to actual values.  Of course, these stars have RVs of tens of km per second.  You won't find planets with a 3D printed spectrograph!

 

If anything else this is a good way to see what works and what could be improved before building a real machined version, for example.  I plan to do one more refinement to the design, then I could post the 3D files.  Still a work in progress!

 

Zemax prescriptions below for the spectrograph itself and the guiding module.  Numbers in the comment column are Edmund part numbers.  Wavelengths used for the spectrograph are 456, 570, and 679nm, which span the 490EX sensor.

 

Scott

Then perhaps in your next design see if you can incorporate some sort of standard lamp, such as your argon or neon bulbs, inserting the lamp or a small flip mirror into the beam, so that you can take a calibration exposure either side of your star exposure without moving the telescope.



#23 jabil

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Posted 21 September 2015 - 10:15 PM

@ Tucker512

Thanks a lot for the zemax file.

Jabil



#24 Tucker512

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Posted 22 September 2015 - 06:33 PM

In my original design I placed a hole in the guider part of the spectrograph so I could hold an incandescent or neon lamp in front of the pinhole.  The problem is the position of the flat then can wander on the CCD if the bulb is not exactly centered each time.  So it could possibly work if the bulbs always stayed in place.  Otherwise the flat and calibration light needs to come through the telescope.  I use the flat to extract the position of the spectrum in software, so it has to be at least within a few pixels of the position of the actual object spectrum.  I recall seeing somewhere online where someone had clipped a neon bulb to the front ring of their OTA, just sticking into the edge of the aperture, and could just switch it on when needed.

 

Scott



#25 Oberon

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Posted 23 September 2015 - 12:22 AM

In that case mount your lamp/s on top of your secondary mirror so that it shines onto a white cardboard flap or cover placed over the UTA aperture, illuminating it as evenly as possible.




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