LOSPEC 3D printed spectrograph

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LOSPEC 3D printed spectrograph

Postby Terry Bohlsen » Tue Aug 28, 2018 5:29 am

Hi all
I have made a LOSPEC spectrograph designed by Paul Gerlach that is available from here https://www.thingiverse.com/thing:2455390.
My first attempt was not great as I found lots of problems that made it difficult to use and underwhelming but I have iron out some of the problems and successfully took some very good spectra last night.

The first problem was a very poor guider image that had terrible astigmatism. The problem was that it was hard to align the guide mirror but I found a way to do it.
First I removed the lens to the guider. I then placed a laser collimator in the front of the spectrograph and aimed it at the slit mirror. The guider mirror holder is a press fit into the slit assembly and can be adjusted a little. I placed a mirror where the guide camera should be and reflected the image back to the laser. By slightly moving the guider mirror I was able to centre the laser. When I tested it with a star the image is now very good.

The next problem was focusing the star image. I focused the guider on the slit but when I took a spectra the image was split and clearly not in focus. I could focus the scope to create a clean spectra but then the guider image was out of focus. I suspected that this was due to the slit that is an OVIO wheel. I had placed it with the mirror surface closest to the scope. I tried it the other way us and this problem seems to have resolved. I do have a ghost image in the guider for very bright stars but this is OK. My LISA and LHIRES also have similar ghost images.

The other very big problem I initially had was very poor sensitivity. I could take a 5 min exposure of a bright star and not saturate the image. A similar star through my LISA would saturate with 10sec. There was obviously a problem. I was using an old 600l/mm grating. I also have a 1200l/mm grating that was in my L200 so decided to compare them. I realised that the 600l/mm grating was not reflecting anywhere as much light as the 1200. The mirror surface was dulled. I put the 1200mm grating in and the difference is amazing.
I took some images last night.
tet1sgr raw.PNG

This is a single exposure 60 sec exposure of tet1 Sgr (a spectroscopic binary).
Processing in ISIS gives ~R=5000
filly.PNG
filly.PNG (278 KiB) Viewed 479 times

This is a filly lamp calibration exposure.
I had taken a spectra of the same star with my LHIRES earlier in the night and have compared the 2 images below. The LOSPEC image is 300s. The LHIRES is 1200s
tet1sgr comparison1.png


Nova Lup 2018 at about mag V=12 900s exposure.
_novalup2018_20180827_487_TBohlsen.png
_novalup2018_20180827_487_TBohlsen.png (6.31 KiB) Viewed 479 times

This is now a useful functioning spectrograph giving me another choice between my LISA and the LHIRES. The resolution could be improved with a smaller slit. All these images were through a 29um slit with a C11 at F/10. I will try using the scope with a focus reducer and see if it still gives an adequate image. I use the focal reducer with the LISA and this improves the sensitivity.
Terry
Terry Bohlsen
Armidale NSW
Australia
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Re: LOSPEC 3D printed spectrograph

Postby Olivier GARDE » Tue Aug 28, 2018 7:19 am

Hi Terry,

An extraordinary job you did on this spectrograph to make a real measuring tool.

Congratulations !!!!
LHIRES III #5, LISA, e-Shel, C14, RC400 Astrosib, AP1600
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Re: LOSPEC 3D printed spectrograph

Postby Robin Leadbeater » Tue Aug 28, 2018 11:08 am

Hi Terry,

Very interesting. It is good to see some objective comparisons between spectrographs. Are you planning to get a replacement low resolution grating? A comparison with the LISA at low resolution and fast focal ratio will be interesting. I think other LOWSPEC users have seen significant loss of resolution at the blue end with fast telescopes due to chromatic aberration, though of course the LHIRES suffers the same and the LISA is not immune to this either.

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
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Re: LOSPEC 3D printed spectrograph

Postby Berthold Stober » Tue Oct 09, 2018 3:38 pm

Hi Robin

I have the low spec too. With a 300 L grating. Do I understand it right that it would be better to use one with 600 or 1200l / mm because the problem with chromatic aberration?

friendly
Berthold
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Re: LOSPEC 3D printed spectrograph

Postby Terry Bohlsen » Wed Oct 10, 2018 11:51 am

Berthold.
I don't think the problem is the grating rather the speed of the telescope. Mine works well at F11 but not F6.4.
The other issue is blurring in the edge of the fields but as long as you don't use a big chip on your camera then this isn't a problem. I use a Atik314+ and it seems fine with it. I tried a bigger chipped camera and wider field obtainedwsn't much use due to blurring.
Terry
Terry Bohlsen
Armidale NSW
Australia
Terry Bohlsen
 
Posts: 131
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Re: LOSPEC 3D printed spectrograph

Postby Robin Leadbeater » Wed Oct 10, 2018 12:55 pm

Hello Berthold,

As Bernard (EDIT I meant Terry of course :oops: ) says, the chromatic aberration does not depend on the grating. It is just more obvious at low resolution with the 300 grating because of the wide wavelength range. With the high resolution grating you can refocus for the wavelength you are looking at and the difference in focus across the smaller wavelength range is less. (like the LHIRES)

Cheers
Robin
Last edited by Robin Leadbeater on Thu Oct 11, 2018 11:36 pm, edited 1 time in total.
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
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Re: LOSPEC 3D printed spectrograph

Postby Berthold Stober » Thu Oct 11, 2018 7:47 pm

Thanks for the answers!

Cheers
Berthold
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Re: LOSPEC 3D printed spectrograph

Postby Terry Bohlsen » Fri Oct 12, 2018 11:23 am

I have made a comparison between my 3 spectrographs that I found quite interesting. I have taken spectra of V777 Sgr (a very slow eclipsing binary) through all of them. The difference in resolution is very obvious as is the spectral range. They are taken with different cameras but all display the usable extent of each without moving the grating and re-exposing.
V777Sgr comparison_1.png

V777Sgr comparison_2.png
V777Sgr comparison_2.png (61.18 KiB) Viewed 138 times

Terry
Terry Bohlsen
Armidale NSW
Australia
Terry Bohlsen
 
Posts: 131
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Re: LOSPEC 3D printed spectrograph

Postby Berthold Stober » Sun Oct 14, 2018 7:33 pm

Hi Robin and others,

now I used my C14 without shapley lense. Now the spectrum is rather sharp over the hole wavelength range, but it is not so easy to detect the slit in the darkness. Additionally the big focus length is not so easy to handle.
How do you detect the position of the slit in the guiding camera in the darkness?

cheers Berthold
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Re: LOSPEC 3D printed spectrograph

Postby Ken Harrison » Sun Oct 14, 2018 8:21 pm

I use a Lodestar camera for guiding on the C11 @ f10 with PHD2 or AstroArt.
I find it easier to set up the guider during the day and record the position of the slit in X and Y then use these coordinates to position the star.
You can set up a virtual slit in PHD2 which accurately mirrors the real slit.
Hope that helps.
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before....
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