Lhires III fine tuning

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Lhires III fine tuning

Postby Paul Luckas » Mon Nov 17, 2014 3:07 am

Hi all,

I'm in the process of commissioning a new Lhires III. The results have been very encouraging, and I'm at the stage where I'm fine tuning the system to improve my results even further.

I have observed a slight asymmetric broadening of the neon lines. The effect is greatly reduced with fine focusing of the collimator (becoming more of a 'shadow' than a 'doubling'), but still not entirely removed. In addition, a star's spectral image can, at times, display a small amount of 'doubling' even with telescope focus optimised in the guide view.

I've been checking some of the numbers and noticed that my version of the Shelyak Atik 314 adapter places my CCD some 3mm further out than the published optimised back focus distance. Discussions with others have confirmed my understanding that the slit to collimator / collimator to CCD distances should be exactly the same. If that is true, and the specifications of the Lhires III have not changed, then my collimator to CCD distance is slightly off (by 3mm).

Is anyone aware of any subtle changes to the design of the Lhires III that may have required a small increase in the collimator to CCD distance? (perhaps when the slit design changed?). Alternatively, is anyone aware of variations in Atik's back focus specification for the 314L+ that may have illicited a change in Shelyak's adapter? (i.e. do Shelyak know something we don't?).

Finally, any general comments on asymmetric neon line broadening, or spectral image 'doubling' would be useful, as I learn more about fine tuning the Lhires III.

Many thanks,

Paul
Paul Luckas
 
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Re: Lhires III fine tuning

Postby Ken Harrison » Mon Nov 17, 2014 8:05 pm

Paul,
The ATiK314L has a 12mm backfocus.
Re. "asymmetric broadening" - Check the neon is sitting central to the entrance slit gap
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before....
"Imaging Sunlight - Using a digital Spectroheliograph" - Springer
http://www.astronomicalspectroscopy.com
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Re: Lhires III fine tuning

Postby Paul Luckas » Mon Nov 17, 2014 11:58 pm

Thanks Ken,

According to the mechanical specifications on the Atik web site it's 13.5mm.

http://www.atik-cameras.com/external/We ... usmech.pdf

I've played a little with neon positioning, but will take another look at it.

Cheers,

Paul
Paul Luckas
 
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Location: Perth, Western Australia

Re: Lhires III fine tuning

Postby Ken Harrison » Tue Nov 18, 2014 2:46 am

"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before....
"Imaging Sunlight - Using a digital Spectroheliograph" - Springer
http://www.astronomicalspectroscopy.com
Ken Harrison
 
Posts: 160
Joined: Thu Oct 25, 2012 5:45 am
Location: St Leonards, Australia

Re: Lhires III fine tuning

Postby Robin Leadbeater » Tue Nov 18, 2014 2:54 pm

Except possibly slightly at the edges of the field you should not have significant asymetry in the neon lines. There was a discussion of a similar problem with the LISA recently. Not sure how it was resolved though so best to contact Olivier or Francois if they do not respond here.
viewtopic.php?f=8&t=881

Re the doubling, do you mean the star image in the guider? (a double image here is normal, reflection off the top surface of the back silvered mirror slit, normally really visible only when the star is over exposed in the guider camera.

If you are seeing a vertical doubling of the spectrum (two parallel lines) with the guider star in focus this suggests that your guide camera focus is probably not spot on ie the star and the slit are not at the same plane (the doubling is actually due to the obstruction in the reflector (the spectrum of an extended donut due to the star being out of focus)

You can check the collimation of your LHIRES and set the camera to the exact distance by removing the grating holder and pointing a camera with a telephoto lens in there (Having first carefully set the focus of the telephoto lens at infinity using a distant object) If the LHIRES is correctly collimated the slit image should be in sharp focus. If not, refocus the doublet until it is sharp, replace the grating and move the imaging camera until the neons are once again sharp, noting the
distance for that camera future reference

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: Lhires III fine tuning

Postby Robin Leadbeater » Wed Nov 19, 2014 12:06 pm

There are a couple of useful pages on Christian Buil's website too on optimising collimation
http://www.astrosurf.com/buil/lhires_eval/optique.htm
and focusing for maximum throughput
http://www.astrosurf.com/aras/slit/method.htm
though this was with the original metal reflective slit. I find there is a better correspondence between guide camera, spectrum focus and throughput with the mirror slit

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: Lhires III fine tuning

Postby Paul Luckas » Fri Nov 21, 2014 2:09 am

Hi Robin,

Robin Leadbeater wrote:Except possibly slightly at the edges of the field you should not have significant asymetry in the neon lines. There was a discussion of a similar problem with the LISA recently. Not sure how it was resolved though so best to contact Olivier or Francois if they do not respond here.
viewtopic.php?f=8&t=881


I've made some changes which have improved the full slit height quality of my neon lines, including adjusting the camera back focus to match exactly the Shelyak specifications, and flocking the area around the grating as per Christian Buil's paper (http://www.astrosurf.com/buil/lhires_eval/optique.htm). Together with some subtle main mirror and collimator mounting adjustments, the neon lines are now very good indeed. Working at high resolution is certainly more demanding, and even subtle adjustments can have quite dramatic effects it seems. For example, just a slight repositioning of the collimator via its two mounting screws causes shadowing to appear and disappear quite obviously.

I noted a 'loose-ish' collimator lens on arrival (when backing off the two nylon screws the collimator thread is quite sloppy). I've addressed this to a certain extent by applying some vaccuum grease to the thread which now almost alleviates the need for any support by the nylon screws and seems to work well (its easier to focus with one finger too).

If you are seeing a vertical doubling of the spectrum (two parallel lines) with the guider star in focus this suggests that your guide camera focus is probably not spot on ie the star and the slit are not at the same plane (the doubling is actually due to the obstruction in the reflector (the spectrum of an extended donut due to the star being out of focus)


This still needs some work. I have noticed that placing the star towards the edge of the slit (as it appears in the guider) versus directly on the slit can affect the appearance and magnitude of this vertical 'doubling' effect, as does of course telescope focus. However, telescope focus in the guide view and imaging view don't necessarily exactly correlate, so I think I'm going to have to check things out using the camera / telephoto lens technique to verify collimation distances.

Thanks again,

Paul
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Location: Perth, Western Australia

Re: Lhires III fine tuning

Postby AndyWilson » Tue Jan 12, 2016 10:30 pm

I am seeing a similar issue with my Lhires III. The best collimator focus for the neon lines appears to be different to the best focus for the stellar spectrum. I wonder if this is what I would expect to see if my imaging camera position was not at the correct distance?

I'm reasonably confident that the star is focused on the slit. The slit appears as a fairly narrow dark line in the guide camera and when the star appears focused this maximises the signal in the spectrum. I've also tried focusing the telescope while ignoring the guide camera but this does not solve the problem.

I attach 2 sets of images. The first are with the collimator lens at position 1, with the neon lines at best focus and you can see the star looks out of focus as it is a doubled line (this is with a Ritchey-Chretien telescope so it has a large central obstruction). The second pair of images are taken with the collimator lens in a position that gives the tightest spectrum but the neon lines are way out of focus and I think this is also true of lines in the stellar spectrum.

Neon lines collimator focus position 1
GamCas_Neon-focus1.jpg
Neon focus position 1
GamCas_Neon-focus1.jpg (24.33 KiB) Viewed 3269 times


Stellar spectrum collimator focus position 1
GamCas-focus1.jpg
Star spectrum focus position 1
GamCas-focus1.jpg (7.81 KiB) Viewed 3269 times


Neon lines collimator focus position 2
GamCas_Neon-focus2.jpg
Neon focus position 2
GamCas_Neon-focus2.jpg (10.64 KiB) Viewed 3269 times


Stellar spectrum collimator focus position 2
GamCas-focus2.jpg
Star spectrum focus position 2
GamCas-focus2.jpg (7.04 KiB) Viewed 3269 times


Any advice would be appreciated!

Thanks,
Andy
LHIRESIII L200 SXVR-H694 10" F8 RC AP1200
AndyWilson
 
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Re: Lhires III fine tuning

Postby Ken Harrison » Wed Jan 13, 2016 1:36 am

Andy,
My take on it....
Position #1 with the sharp neons is correct....
The star focus ON the slit is out - if you set the collimator to #1 and then re-focus the telescope to give the narrowest stellar image I think is the answer....
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before....
"Imaging Sunlight - Using a digital Spectroheliograph" - Springer
http://www.astronomicalspectroscopy.com
Ken Harrison
 
Posts: 160
Joined: Thu Oct 25, 2012 5:45 am
Location: St Leonards, Australia

Re: Lhires III fine tuning

Postby AndyWilson » Wed Jan 13, 2016 7:19 am

Hi Ken,

Thanks for your reply.

Trying to focus the telescope with sharp neon lines is actually what I have previously tried many many times, but without ever being able to achieve a nice sharp line for the stellar spectrum. The best I can get is what I've shown as position 1. I get nice results from the spectrum, but clearly the stellar spectrum is smeared. I've also tried re-collimating the telescope in case that was the problem, but this appeared to have no effect.

I've just given it a quick go again now as the sky is still clear this morning. I attach my results, this time showing the guide image and the stellar spectrum. In both cases I've focussed the spectrograph doublet to get nice sharp neon lines, and not touched it while changing the telescope focus between the 2 sets of images.

Guide image telescope focus position 1
Spica1_Guide.jpg
Guide image star focused in guide camera
Spica1_Guide.jpg (5.7 KiB) Viewed 3252 times


Stellar spectrum telescope focus position 1
Spica1_Spectrum.jpg
Star spectrum star focused in gude camera


Guide image telescope focus position 2
Spica2_Guide.jpg
Guide image star spectrum tight
Spica2_Guide.jpg (5.96 KiB) Viewed 3252 times


Stellar spectrum telescope focus position 2
Spica2_Spectrum.jpg
Star spectrum star spectrum tight


Thanks,
Andy
LHIRESIII L200 SXVR-H694 10" F8 RC AP1200
AndyWilson
 
Posts: 30
Joined: Wed Feb 18, 2015 9:02 am

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