3D Printed High Resolution Spectrograph

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Julien Lecomte

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Jun 10, 2022, 12:05:30 PM6/10/22
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Hi everyone,

I am lucky to speak French somewhat fluently because it's allowed me to notice that there is a really astonishing project that has been blossoming in the French astronomy community, and that project is named Sol'Ex/Star'Ex. In this project, the author (the very famous Christian Buil, who was a pioneer of the use of CCD cameras by amateur astronomers in the late 80s/early 90s in France) proposes to build a high resolution spectrograph out of 3D printed parts. If, like me, this makes you raise your eyebrows, check out the web site:


There is also a YouTube channel named "Astro Spectro" (however, it's all in French) that covers how to build the spectrograph and how to use it.

This is not a toy at all. It is a true high resolution spectrograph (spectral resolution > 20,000, which means that you can see details as low as 0.3Å or 0.03 nm in H-alpha for example) and it allows you to see phenomena that evolve in the matter of hours. With visual observing or imaging, a star is basically just a dot (sometimes with some color, and sometimes with a brightness that evolves slightly over time) But with a spectrograph, it becomes much more interesting. For example, you can actually see the period of rotation of some stars, with a tiny telescope, e.g., a 4" refractor, and a 3D printed spectrograph from a light polluted suburban area #mindblown

It is also a little complicated to use. The calibration is a little daunting, and it requires a fairly decent understanding of spectroscopy (although there are lots of resources to guide you, alas most of them are in French)

In the Sol'Ex configuration, you can use it as a spectroheliograph and capture images of the sun in any part of the spectrum you want (H-alpha, Calcium K, even Helium...) that will be better than those taken by people who invested many thousands of $$$ in their solar scope...

In the Star'Ex configuration, you can use the spectrograph in high resolution mode (using a 2,400 lines/mm holographic grating) to image stars up to 7th or 8th magnitude iirc. You can also swap the grating to one that has 300 to 600 lines/mm and use the spectrograph in low resolution mode, which allows you to image objects as dim as mag ~ 12 (depending on the size of your telescope)

I ordered a kit (~ $500 USD) and am planning to build this thing over the next few months. I think this is really exciting, and there are many ongoing research projects I am planning to participate in, by collecting and submitting spectra of specific targets that are of particular interest to professional researchers (example: Be stars. micro quasars, etc.)

I am also thinking of using the spectrograph to measure the real bandpass of common narrowband filters used in deep sky imaging. Manufacturers don't always tell the truth in their spec sheet, so that will be a sure way to measure those very precisely at almost no cost.

Anyway, I'll occasionally post here to share my progress, and of course to my YouTube channel (shameless plug :)

- Julien (DarkSkyGeek on YouTube) -

Philippe Fossier

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Jun 10, 2022, 3:45:09 PM6/10/22
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This is very interesting.
Do you have access to a 3D printer or are you planning to use a commercial service ?
I am using an 8" SCT scope with a Baader AstroSolar filter mounted directly on the OTA.
Am I correct to assume that it won't work with the Sol'Ex ?
I would be interested to follow how your building and using this progresses.
thanks,

~ philippe




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Tony Hurtado

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Jun 10, 2022, 3:49:29 PM6/10/22
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I just read the blurb on the front page and a bit of the theory page of this project's web site. Impressive! It does, however, mention that it is intended to be used with a refracting telescope.  I understand what spectroscopy accomplishes and a little about how it does it but is there a requirement that a refracting system be used as opposed to using a reflecting setup (e.g. dob, SCas) for this purpose? 

I'm thinking RaspPi+a camera board.

-Tony

Matthew Buynoski

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Jun 10, 2022, 3:55:12 PM6/10/22
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Any telescope can work with a spectroscope. All the major observatories (4m to 10m mirrors) use reflective optics and they all have (enormously expensive and sophisticated) spectroscopes.  However, the design of any specific spectroscope may not work well with low f/number instruments, with their rapidly converging light cones.

Julien Lecomte

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Jun 10, 2022, 5:17:34 PM6/10/22
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I'll answer multiple questions at once.

Do you have access to a 3D printer?

Yes, I have one at home that's capable of printing PETG fairly well. See my latest project as an example:

> It does, however, mention that it is intended to be used with a refracting telescope.

Maybe something got lost in translation? You can use Sol'Ex/Star'Ex with any kind of telescope. For low-resolution spectroscopy, a reflector can be better because refractors (even those of very high quality) have some residual chromatism that reduces the quality of the spectrum in the blue.

> the design of any specific spectroscope may not work well with low f/number instruments

This is absolutely correct!!! Sol'Ex/Star'Ex is optimized for F/10 to F/12. But it will work at lower F/ ratios. My telescope is a 130mm refractor at F/7. Due to the vignetting on the grating, this will be equivalent to the amount of flux received from a 100mm telescope at F/10. No big deal...

Surprisingly enough, large telescopes are at a disadvantage when it comes to resolution in spectroscopy. The actual reason why is a little complicated to explain in an email, so if you happen to see me at a dark site, remind me and I'll explain it to you :)

Thanks,
Julien


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Pawan Singh

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Jun 11, 2022, 3:05:26 PM6/11/22
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Thanks for posting this Julien. 

I was at AIC this year and saw this which is much more basic than what your link:


Pawan

Jeff Crilly

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Jun 11, 2022, 3:24:42 PM6/11/22
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>
> https://www.rspec-astro.com/

This was my curiosity… how this compares the the Star Analyser.

Probably compares only to the Star’Ex.

I’m a bit ignorant on how the Sol’Ex and Star’Ex works — haven’t read the site.

The Sol’Ex looks it has some sort of tunable filter for Ha and CaK… but I presume not for visible use — I see cameras attached.
When I think of Solar imaging, I don’t really think of spectroscopy, but very narrow filters. So I find it the mention of spectroscopy confusing in that context… but I’m probably missing something.


Julien Lecomte

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Jun 11, 2022, 3:37:28 PM6/11/22
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Not at all!!!

Sol’Ex/Star’Ex is a spectrograph! The default configuration is for a high resolution spectrograph, and you can turn the grating to study various parts of the spectrum. You can swap the grating to have it work in low resolution mode, which is interesting for some studies, and also because it allows you to go much deeper in magnitude.

In high-res mode, on the sun, you can use it as a spectroheliograph, i.e. you let the disk of the sun drift. You can take a video of the spectrum around Halpha (or whatever line you want) and software like INTI or spec INTI can reconstruct the image of the sun in that wavelength.

There are no filters in this device.

- Julien


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Julien Lecomte

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Jun 11, 2022, 4:01:29 PM6/11/22
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RSpec and star analyzer have been around for a while. It’s a good way to get started in spectroscopy, but it won’t get you very far if you get the bug. It really depends on what you want to do. Sol’Ex/Star’Ex is more expensive, much more complicated (you have to build it, you have to calibrate it carefully) but it can take you very far. Both are valid approaches.

- Julien

Matthew Buynoski

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Jun 11, 2022, 6:17:29 PM6/11/22
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I have a LHIRES Lite from Shelyak Instruments in France.  This fellow is no trivial instrument with a resolution of over 10,000, costs something like 1,340 Euros, and will show you hundreds to thousands of absorption lines in the solar spectrum. I used it today for public outreach (at Gamble Garden in Palo Alto) and it was well received. Shelyak also makes a much less expensive handheld spectroscope (98 Euros) that shows, when aimed at white paper in bright sunlight, a few of the more prominent Fraunhofer lines. Shelyak’s spectroscopes useable with a telescope for stars at night are the Alpy, a modular instrument, so price depends on how many modules you buy for it, or the LHIRES3 (cousin to the LHIRES  Lite, with resolution about 17,000) at (eek!) 3,700 Euros. Baader Planetarium (in Germany) has the DADOS spectroscope which is similar in performance to the LHIRES3. To get detailed spectra from all of those, of course, will require a fair amount of post-processing sophistication and a mount (probably more expensive than the spectroscope :-) that can be guided at least as well as for astrophotography because the target star’s image must be kept on the spectroscope’s entrance slit, and those are on the order of 10-25 microns wide if you want high resolution results.

My take on this is: if you want to see a whole lot of detail in one star's spectrum, the LHIRES Lite is all you need. Getting the same level of detail on stars at night is a LOT of work and is NOT a visual observation…I started into it and quickly found it to be, for me, too much work for a hobby.  Here I’d recommend something like the star analyzer (180 Euros), which can easily show the relative amounts of red, green, blue in a spectrum (for example, compare Rigel to Betelgeuse) and that directly relates to the stars' surface temperatures. You can also see (at least an 8” scope is required) some of the hydrogen lines in Vega, the molecular bands in Antares and Betelgeuse, and (in a large dob) even a few metallic lines in Capella.  This kind of presentation is colorful at the eyepiece, is enough science to wow most of the public (occasionally you get students from an astronomy class :-) and is very popular with kids when told what they are looking at is the “rainbow” (technically it isn’t, of course, there being no refraction by water drops involved :-) of another star.

FYI.

microflite

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Jun 11, 2022, 7:24:48 PM6/11/22
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Hi Matthew, I would've loved to see the demo a gamble but didnt know you were doing an outreach. Let me know where it was announced so I don't miss the next one.

Ashok

Julien Lecomte

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Jun 11, 2022, 8:39:55 PM6/11/22
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The Sol’Ex kit costs less than $500, and (with the addition of a low-res grating and a shorter focal length objective lens) will be able to do a lot more than LHIRES (high-res, low-res, spectroheliograph, 3 configurations in a single device) By the way, the Sol’Ex kit is sold by … Shelyak instruments! The only “downside” of Sol’Ex is that … well … you have to build it from scratch, and it can be a good bit of work. But that’s also part of the fun.

Yes, the calibration and the processing software can be a bit of a challenge to learn how to use, but I’ve done a ton of research, and it does not look insurmountable at all. Actually, spectroscopy is very popular in France, and a lot of people there are making it an integral part of the hobby.

- Julien


Paolo Barettoni

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Jun 12, 2022, 2:37:49 PM6/12/22
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Very interesting project! I will follow your progress with attention, let keep us informed

(and thanks for the remembrance of my first VeloSolex....)

Paolo

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