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Spectrograph / Spectroscope (LOWSPEC)

by PJHGerlach, published

Spectrograph / Spectroscope (LOWSPEC) by PJHGerlach Jul 27, 2017

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Fusion 360

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4833Views 712Downloads Found in Physics & Astronomy

Summary

A spectrograph lets you analyse light. Attaching it to a telescope enables you to discover all sorts of things about the stars. In fact, almost all we know about the universe has been obtained through spectroscopy!
This field of science is becoming more and more popular amongst amateur astronomers. A commercial spectrograph however will set you back € 2000 or more!
That’s why I decided to build one with a 3D printer. Of course, you’ll have to buy the optical components (slit, lenses, mirrors and grating) . But that will ‘only’ cost you about € 500. Which is a huge difference compared with a commercial spectrograph. This spectrograph is designed for telecopes which are f/8 or slower.

This is still a work-in-process project. I’ve included a STEP file of the whole design so you can change or improve upon it. Later on I will include more information on how to assemble the instrument. This will be done as a series of images that I will add in the coming days.

I do not have in-depth knowledge of optics. So if you can improve this design, please let me know. Even better, share it with the rest of the world!

IMPORTANT:
It is possible that I will make slite alterations to the design. So be sure to always download the latest STL files before you start printing.

Best regards,
Paul Gerlach

Print Settings

Printer:

Felix Pro 2

Rafts:

Doesn't Matter

Supports:

Yes

Resolution:

100-200 micron

Infill:

50-80%


Notes:

The main body can be printed at 200-250 micron (infill >= 50%).
Print smaller part a 100 micron with 80% infill.
Some parts require support.

Hardware list

Assembly

Step 1

Print the main body at 200 micron with >=50% infill. Use support (max overhang 60 degrees).
Remove support after printing.
Check that the two threads fit the main body before you glue them in place using two component Epoxy glue.

Step 2A

You can use a 3/8" nut so you can eventually attach the instrument to a photo tripod. Glue the plug in place.

Step 2B

Screw in the threaded M4 insert.

Step 3

Use a M4 threading tool to thread the four locations indicated.
Next screw Allen Grub Screws (M4) into these threaded holes.

Step 4A

Insert the round coupling nut M5 x 25mm (mine had a diameter of 7.9mm) into the center hole of the focus slider. It should be a tight fit and you may have to resort to some brute force...

Step 4B

Push the M3 nuts into the slots (two in both rail holders).
Cut two pieces of aluminum pipe (length 55mm, diameter 6mm) and slide them into the two other holes of the focus slider. Make sure that they can slide smoothly.

Step 4C

Cut two coilsprings to size (aprox. 22mm) and slide them over the aluminum tubes. Assemble this part by sliding both rail holders over the tube ends.

Step 4D

Make sure the whole focus part fits in the designated slots underneath the main body. If it does not then make sure that both sliding tubes are pressed far enough into the rail holders.
Still no fit? Than you’ll have to shorten both sliding tubes a bit.

Step 4E

Screw it in place with four M3 cylinder screws.
Check that the focus slider can travel the whole length!
If not, unscrew it and change the coil springs.

Step 4F

Take a M5 threaded rod (length 75mm) and screw a M5 locking nut on one end. Make sure it’s tight.
Screw the rod in place. Screw it all the way in.

Step 4G

Screw the Knurled nut onto the other end of the rod.
If every thing looks ok, place a drop of superglue into the top opening of the nut.
Test the focus-unit by turning the knurled nut.

Step 5A

Press the mirror into the main mirror holder. Wear gloves so you will not damage the mirror!
Also insert a M3 nut in the slot on the backside of the mirror holder.

Step 5B

Make sure the Allen Grub Screws do not protrude before placing the mirror holder.

Step 5C

Place the mirror holder and secure it with a M3x10mm Philips screw. Take care not to over tighten the screw. The three Allen Grub Screws will be used later to change the tilt of the mirror.

Step 6

Cut a aluminum pipe (dia. 6mm) to length (16mm) and secure it in place with some glue into the hole of the focus slider.

Step 7A

Use a M4 threading tool for the small hole and screw in a 6mm Allen Grub Screw.
Press the camera lens into the holder (should be a tight fit). Be aware of the correct orientation of the lens.

Step 7B

Slide the camera lens holder over the protruding piece of aluminum pipe and tighten the Grub Screw. Make sure the biggest bulging side is facing as illustrated.

Step 8A

Press the collimator lens into its holder. Wear gloves to protect the surface of the lens. Note the position of the parts. Insert a M3 nut all the way into its slot.

Step 8B

Place the collimator holder in the slot at the bottom of the main body. Take note of the orientation.

Step 8C

Use a M3x12mm philips cylinder screw to keep the collimator in place.
By loosening this screw a bit you are able to slide the collimator back and forth.

Step 9A

Use a 4M wire tap to tap the two holes on top of the rotary section of the grating holder.
Screw in two M4x6mm Allen Grub Screws.
Carefully slide in the reflective grating. Take great care not to touch the surface of the grating as it can easily be damaged!
Also, notice the arrow that is drawn on one of the sides of the grating (the 'blaze direction'). It should be pointing in the direction as is indicated in the illustration above.
Gently tighten the two Allen Grub Screws so that they hold the grating in place. Be careful! Over tightening them could break the glass grating!

Step 9B

Place the whole rotary section onto the base plate and put the cap on top.
Check that the rotary section can move freely. If needed you can glue the cap to the base.

Step 9C

Insert two M3x12mm Cylinder Philips screws and install the spring.
Take care not to damage the grating!

Step 9D

Insert two M3 nuts into the indicated positions above.

Step 9E

Gently screw the grating holder in position. Again, take great care not to damage the surface of the grating. To access the screw under the coil spring you may need to push it aside a bit with you screwdriver. Take care not to touch the grating!. The other screw can be accessed by rotating the grating.

Step 10A

Use a 4M wire tap to tap the hole of the guide platform and screw in a Allen Grub Screw.

Step 10B

Place the APLY slit and secure it by gently tightening the Allen Grub Screw.

Step 10C

Insert the 12.7mm Thorlabs mirror into it's socket. Use gloves and make sure it's pushed in all the way.
Next, insert the mirror holder into it's socket.

UPDATE 10/29/2017: the mirror holder has been redesigned a bit so it looks different than in the image above.

Step 10D

Slide the autoguide/slit platform into the main body.

Step 11A

Insert the autoguide lens into it's holder (08_guidelens_holder). Make sure that themore curved surface is facing the guide mirror...

Step 11B

...and push this into the socket at the back of the autoguide port.
Make sure it's firmly in place. If in doubt use some glue to keep it in place.

Step 12

Place the micrometer and secure it with the Allen Grub Screw.
The micrometer used has a mounting diameter of 9.5mm and a travel of 1-13mm.

Step 13A

Insert the five M4 locking nuts.

Step 13B

Place the lid and secure it with five hexagon socket head cap screws (M4 x60mm).
Also screw in the knurled Screw (M4 x 20mm).

The assembly is now complete!

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Planning any updates before I build this?

No major updates. Some STL files will be update today. But these are minor adaptations.

hey, what are the supported wavelenghts in the finished product. just visible and UV or could it go IR?

I got a LWIR sensor I want to use for astronomy and would love to do some hyperspectral imaging.

but I guess it relies on the optics I buy?

coated mirrors and germaium lenses are a lot more expensive then a visible setup I guess.

Well it's was constructed for the optical spectrum (400 - 700 nm). These are the specifications of the used lenses. Beyond 700 nm (or 7000 Angstrom) is possible. At the short wavelength part (beyond 380 nm) there is some blurring. Probably because these achromatic lenses were not designed for that particular part of the spectrum.
Also PLA seams to be transparent in the infrared part of the spectrum. So if you were to use PLA for this project, then I would suggest to cover the device in some aluminum foil.
I used ASA and have not experienced Infrared transparency with this material.

Hi Paul, finally, figured out the full spectrum focusing onto a Canon 650D chip using a 600l/mm grating. See the photo I attached. Regards percy

Hi Paul, thanks for the "congrates", but, all credit goes to you for designing this in the 1st place and then putting it out there for guys like me to copy and build. You did all the clever thinking. On the plastic, no idea, my mate printed for me. He's preference I suppose. He has some or other fancy hi-tech printer and has a hobbyist model printing setup. If I see a problem, I will paint black inside. Should be okay though as all is dark outside and unit is sealed. Thanks for the tip on the 100mm lens for back focus. Yes, was planning outside on the solar spectra this morning and saw the back focus issue. Of course, no problem for the ZWO, but of course spectrum is too large for the chip and one has to “page” through it. I’m off to the ATM class and will see if I can scratch out a 100mm lens from one of the boxes of scrap. On the guide mirror, still work in progress. Going to sort this out when the proper slit arrives. Thanks though for the tip – great idea. Looking forward to showing the guys at telescope class.

Thanks Paul, built fast due to night time prep work and we had a public holiday on Wednesday. My luck, everything arrived on Tuesday. I must say, it did take a few hrs and prep to put together. On the gluing of the T rings, feel a bit stupid here. Yes, you did say this in the instructions. Think I was too busy looking at the other steps and busy modifying where I needed to that I forgot to follow that step. I did read it though. Anyway, at least I figured it out before a disaster.
On DSLR, and star test – work in progress once I get the proper slit from Shelyak. In the mean time, I will try tonight with the temporary slit I have. The pic’s I took with the DSLR were without the slit and of a street light– was just eager to see what I would get. From the pic I took, things look promising. I loaded this iamge in "made" so vcan see Paul

The back focus may be an issue which I will find a solution for, or I will just use my ZWO ASI130MM camera. I have a similar issue with back focus on my homemade etalon H-Alpha solar scope. I have not yet got the DSLR to work in this system yet so I use the ZWO on my solar pic’s. So, will let you know how the actual star test goes.
For now, running around looking at different lights is fun. Clearly shows the high resolution and clarity of the setup.
On calibration, thanks, and yes, basically what I do now.
Thanks for all the guidance Paul
Regards

Percy, let me congratulate you on this build! It bears all the markings of a true ATM’er. Or should I say ASM’er (Amateur Spectroscope Maker)?

I did not have the chance to finish the assembly instructions due to a vacation. I hope to do soon. But I see that that did not stop you in successfully completing this build.

What material did you use to print? PLA? Why blue? Simply because that was available? You might consider painting the inside black as to minimise internal reflections. I used ASA filament to print mine version.

If you’re unable to focus the DSLR camera, you might consider changing the 80mm camera lens with a 100mm lens. That’ll give you 20mm extra back focus. It does also increase the dispersion and resolution of the spectrum but I think that the large sensor of the DSLR will be able to accept these changes.

I see that the auto guiding mirror is not completely inserted into its socked. It still sticks out a bit. If it does not fit correctly, you’ll have to remove it and enlarge it’s socket a bit. If you are unable to remove the mirror carefully drill a small hole in the backside of the mirror holder and gently push it out.

Did you collimate the collimator lens? I still have to include this procedure into the instructions but the steps are:

  1. remove the camera lens.
  2. point the spectrograph at a bright light (light bulb).
  3. take a small telescope (finder) that is focused at infinity and place it at the camera side of the spectrograph.
  4. turn the grating so you can see the zero order of the slit (the 'real' image).
  5. Loosen the philips screw of the collimator and slide the collimator until you see a sharp image of the slit true the finder.
  6. fasten the screw again.
  7. Place back the camera lens.

If you use an eyepiece and notice that the spectrum does not appear in the centre of the field of view, you can change the tilt of the main mirror to bring it back in the centre.
Loosen the centre screw of the mirror slightly and adjust the tilt with the three crub screws. Then thighten the centre screw again. Take care not to over tighten it.

Best regards,
Paul

photo's loaded Paul

How can I send you photo's Paul?

maybe it's a good idea to place some pictures of your build in the 'I Made One' section?

Hi Paul, ok, just waiting for the slit from Shelyak. Otherwise, all built and HAPPY. In the meantime, I am using a slit I "robbed" from an old lab spectrograph. Works great. Absolutely clear. In hand held position, I can clearly see the Fraunhofer lines. Looking at my fluorescent tube in the garage, absolutely clear with precise emission lines (even split the 2 sodium d lines). No, just need to get a chance and get the spectroscope onto my scope and on a star such as eta carina. Hopefully going to get a chance tomorrow night.

On improvements;

  1. Thanks for the updated drawings. Glad to see the correction on the “plug”
  2. Only one major thing that needs a change, and that is the screw threads. For me, 3D printing threads don’t work. Or, maybe my threads just came out bad. What I did, is fit std eyepiece reducer into the plastic ring and glued in the plastic ring to main body. For the nose piece, did the same. This was important to glue this ring piece to the main body because it kept on clipping out. This would be a disaster if it clipped out when on the scope with a camera connected. I had extra attachments lying around to be able to do this. If I did not, I would have just bought them. Not expensive. This all makes the fitment to the scope secure and allows me to change eyepieces or camera or adjust distances easily
  3. On the focus lens slider block, the movement is a bit coarse. Here, I am going to place a very then PPE plastic against the sides of the slider block and this should make it smoother. However, not necessary. The instrument works just fine.
  4. Basically, now that I know what you have pains takingly designed, I would have been able to source most of the pieces from scrape binocular items and other stuff that we have in our telescope making class – even the mirrors. Don’t regret buying everything, but found I could have scratched all the pieces together if I spent a little time trying. Just a ATM’er story – try keep the buying to a minimum and scratch in old – other peoples – junk. Anyway – next time.
  5. On the spectrum seen, I only get the FULL spectrum in the image - no reference star. This is going to make calibration a bit more difficult – no big deal though – just a little more effort. A trade off for the higher resolution. Nice to page through the spectrum though
  6. Fitted my DSLR, Canon 650D, to the spectroscope, and WOW.
  7. For me, with my own few mod’s, it cost me around €630. However, now that I know what to do and what is needed, I am sure I can get this cost to around €300. Either way, still half the cost vs an entry level commercial spectroscope.

As soon as I take a pic of my 1st star spectrum, I will send to you.

Thanks again for spending the time to design and then share this Paul. Really appreciated. I finally have a reasonable resolution spectroscope and looking forward to the various analysis I am busy with.

I will be showing this at SCOPEX with of course you as reference as the designer. I am sure to impress a few people.

Regards
Percy

Hi Percy,

WOW! You were quick in building this thing!
Nice to hear that the instrument appears to function correct.

Ad 2.
3D printing T2 threads is indeed not easy. The problem lies partly in the fact that when the material is extruded by the printer, it expanse a little bit. This expansion depends on material, extruder temperature, etc. No 3D printers are equal. The fact that the threads print on my printer, does not mean that it will print on every printer. Maybe i should include a warning concerning this issue. Maybe I should investigated the option of glueing 'standard' metal T2-threaded rings.
The printed T2 rings must ALWAYS be glued to the main body as stated in the assembly instructions.

Ad. 3
The holes of the focus slider block should fit just right. It is possible that the printed holes are a little bit smaller and that they must be increased a bit with a small round file.

Ad. 4
Using scrap optical elements is great! It'll cut cost even more.

Ad. 5
I don't undestand your comment on reference star.
I'm thinking of designing a separate calibration unit that will allow to feed a calibation light (Ne-Ar) onto the slit.
With this spectrum you will then be able to calibrate the wavelength scale of every other spectrum.
I can illustrate this with my workflow:

  1. point the telescope onto the star of interest.
  2. make a spectrum of that star.
  3. Make a calibration spectrum of a known light source (e.g. Ne-Ar). The known emission lines of this spectrum will enable you to calibrate the wavelength scale.
  4. make a spectrum of a know reference star (e.g. A0-star) and use this spectrum to create a correction curve (response file) of your optical train (telescope, spectrograph and camera). Use this correction curve to calibrate the intensity profile of the star of interest.

Ad.6
Did you get a focused spectrum using a DSLR? I thought that that would be impossible because the focus lies approx. 40mm outside the main body and the backfocus needed for a DSLR is 55mm. Or could I be wrong?
The fact that this seems to work is great news! I originaly designed this instrument for my ATIK 314L camera. The Canon has a mutch bigger sensor, so your choice of a 600 l/mm grating may be a good one. I'm curious if you can get a focused spectrum across the whole optical range.

Regards,
Paul

Oh, ok, thanks Paul. Thought i would be clever, but forget about the lens configuration and ccd size. Lets hope this works for me. No problem if i have to "page" through the spectrum.I already have a 200L blazed grating and therefore wanting higher resolution. Thanks for the advice - regards Percy

Also, just a note, on the grating, I have bought the 600 line version. Same as the Aply. Higher resolution than the 300

The LOWSPEC with the 300 l/mm grating has a higher resolution (R~800-900) than the ALPY.
Grating and lens combo has been chosen so that 4000 to 7000 Angstroms are covered on a ATIK314L CCD.
A 600l/mm grating will double the resolution but also half the spectral coverage.

Great, thanks Paul. On the micrometer, not to worry, I so this and have a plan to get one. This is much better. This afternoon, off to the shops. Then, hopefully, if everything arrives as planned, by Tuesday, I should be able to spend the Wednesday, a public holiday here, assembling. The guys at our ATM Class are also putting pressure on to have this ready for Scopex. I need to put up a display on your whole story

Hi Percy,

I've placed the hardware list in a separate section now.
Also I see that one piece is missing. The micrometer with a mounting diameter of 9.5mm and a travel of 1-13mm.

Paul

Hi Paul, Percy here, slowly but surely making progress. Hoping to start building soon. My mate with a 3D printing shop is busy printing for me, parts are all ordered and waiting for arrival. Is there any chance you could send me a better quality Hardware list please. There are one or 2 things I can't quite make out. Pretty sure I am 95% sure of what I see, with a magnifying glass. Would make it easier if you could. Thanks Percy

Thanks a ton Paul. Yes, of course, no commercial use. And yes, anyone who sees this or asks, credit absolutely passed on. I must say, this must have taken some designing. I myself have been trying to build a slit spectroscope from scrap stuff, have quite a collection already, from old analytical equipment, but thats how far I have got. Our SCOPEX event is on the 16th Sept, and hoping to have it complete by then. Take a look at our SCOPEX web site - https://www.scopex.co.za. From the file I downlaoded, the sti files, will my mate with a 3D printer be able to open them? Percy

Well South Africa is not quite in the neighbourhood (I live in The Netherlands).
The STL files can be used to print the parts. But not all 3D printers are equal so some holes could end up being to small due to different tolerances. That's one of the reasons I included the STEP file. So you can tinker a bit to make it fit.

Please let me know if you have any questions. Response will be quick as we live in the same time-zone ;-)

Paul

Hi Percy,
I hope this will help you to explore the universe in a different light.
Of course it's ok to print this! In fact it's ok to change the design and adapt to your needs. As long as you do it in a non-commercial manner and give appropriate credit.
Here's the link to the terms:
https://creativecommons.org/licenses/by-nc-sa/3.0/

If you make any improvements please let me know. Let the whole community know! That way we all benefit.

I've printed this spectrograph with ASA filament.

Regards,
Paul

Hi Paul, thanks for this. Really going to make. It's just what I have been looking for over the years. Even the cheapest commercial spectroscope is expensive. This really fits into my budget and capabilities. I am a ATMer (Amateur Telescope Maker) and a have a keen active interest in spectroscopy. I currently just use a simple 200 lines/mm blazed transmission diffraction grating. Just clarifying with a local buddy of mine who has a 3D printing shop to see if he can print for me - if this is ok with you of course. Not too sure how this all works with regards to your intellectual property on the design and drawings? All going well, I would like to have completed before our annual SCOPEX event to put on display and show others. Thanks again, glad I was sent the link to this by a friend. Regards Percy

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