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Littrow SHG prototype

Posted: Sat Aug 06, 2016 12:26 am
by Merlin66
Just some images of the components and housing.

Telescope: 100/1300 Carton objective, 2" 10:1 focuser

Spectrograph: 60/660 achromat (Littrow), adjustable slit - 8mm long

Grating: 50x50mm 1200 l/mm, Optometrics blazed.

Camera: ZWO ASI 174MM at 400fps (1960x32 pixels), Borg helical focuser.

Initial build will be to verify design. The nodding mirror will be added later (optics and motor in hand)
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P1040143s.jpg
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P1040144s.jpg
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P1040145s.jpg
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Any questions just let me know.

Re: Littrow SHG prototype

Posted: Sat Aug 06, 2016 7:38 pm
by marktownley
This looks fantastic Ken! Keep us updated as you go along with it.

I do have a question, are there any commercially available SHGs that can be bought 'off the shelf' that can be used for solar?

Re: Littrow SHG prototype

Posted: Sat Aug 06, 2016 10:31 pm
by Merlin66
Thanks Mark.
There's no commercial digital SHG's available, as yet.
I describe Mete's prototype of the "SolarScan" in Chapter 12.3, but this is still some way off.....

Re: Littrow SHG prototype

Posted: Mon Aug 08, 2016 3:35 pm
by solarscope180
How do you plan to drive the nodding mirror?

Re: Littrow SHG prototype

Posted: Mon Aug 08, 2016 9:22 pm
by Merlin66
The nodding mirror, a 25mm square front surface mirror will be mounted on a small plate with a central axis.
The plate will in turn be lightly spring loaded against a plastic cam which gives a total image displacement of 8mm across the entrance slit (the size of the solar disk image being used).
The cam is driven by a small 10 rpm micro motor, giving a 6 sec cycle.

Re: Littrow SHG prototype

Posted: Tue Aug 09, 2016 8:40 am
by marktownley
All very cunning :)

Re: Littrow SHG prototype

Posted: Wed Aug 10, 2016 6:27 am
by Merlin66
OK the USB3 SSD drive MSI computer arrived today...
Downloaded FireCapture and set up the ASI 174MM on the bench...
With an ROI of 1920 x 48 (I've still to figure out how to move the ROI around the image for final positioning...) I was achieving over 800fps!!!
A great start but more work needed.
Onwards and Upwards

Re: Littrow SHG prototype

Posted: Wed Aug 10, 2016 10:46 am
by marktownley
Try the cut out feature instead of ROI Ken, this centralises it rather than putting (the ROI) up in the corner.

Re: Littrow SHG prototype

Posted: Fri Aug 12, 2016 9:27 pm
by taratasy
Hi Merlin, I have a question : why don't you use a 1800 or 2400 grooves/mm grating, too much light loss ?
thanks.

Re: Littrow SHG prototype

Posted: Fri Aug 12, 2016 10:52 pm
by Merlin66
A couple of reasons..
I already had a 50x50mm 1200 grating available ;-)
Also trying to find larger gratings - needed when you consider >1800 l/mm (due to the rotational angle of the gratings in a Littrow design) were beyond my finances.

Re: Littrow SHG prototype

Posted: Sat Aug 13, 2016 1:00 am
by highfnum
nice carpentry work

Re: Littrow SHG prototype

Posted: Sat Aug 13, 2016 3:35 am
by Merlin66
Thanks John.
I hope to have the dry fit up this week and optical axises checked with the laser....
(Collimating two sets of optics to match each other is interesting!)
The Littrow pick off mirror will work with the slit gap 90 degree from the usual "night-time" configuration - this is to maximise the imaging spectral height but also handle a narrower wavelength coverage.

Re: Littrow SHG prototype

Posted: Mon Aug 15, 2016 11:56 pm
by Merlin66
Mark, et al,
I'm still having "problems" with the ROI and Cut-out features of FireCapture.
The frame rate is much slower when the Cut-Out feature is selected....and using the commands given (Ctrl+drag to move) I can't get it to work!
The reduced ROI option gives fantastic frame rates, but the "Shift+Arrows to move" doesn't seem to work - it increases the size of the ROI!!
Can someone/ anyone with more experience with FireCapture help me???

Re: Littrow SHG prototype

Posted: Tue Aug 16, 2016 12:21 am
by Merlin66
Hmmm
Looks like the V2.5 Beta may have better ROI controls......
I'll download a copy for trials.

Re: Littrow SHG prototype

Posted: Tue Aug 16, 2016 3:56 pm
by Wah
You may try ROI fucnction in SharpCap rather than Firecapture.

Re: Littrow SHG prototype

Posted: Thu Sep 01, 2016 12:34 am
by Merlin66
Just an update - photos to follow.....
Housing is now complete and almost finished painted.
The telescope section has been baffled to allow an un-vignetted FOV just under one degree.
The Littrow baffles/ stops (As per "Imaging Sunlight", figure 9.64, p215) will be added after optical assembly.
The laser collimator shows the alignment of the telescope axis to be spot on, but the Littrow slit holder was found to be on axis but slightly tilted...
I'll correct this during final fit-up.
So far so good. All I have is basic hand-tools - measure once, measure twice - and still get it wrong!!!
More to follow.

Re: Littrow SHG prototype

Posted: Thu Sep 01, 2016 12:12 pm
by highfnum
do you own or used any other SHG types

Re: Littrow SHG prototype

Posted: Thu Sep 01, 2016 3:11 pm
by Merlin66
John,
The SHG illustrated in figure 6.1, p86 was assembled to allow further testing.
It's a combination of a ST80 scope and an ol' Classical spectrograph I built back in 2005.

Re: Littrow SHG prototype

Posted: Sun Sep 04, 2016 9:55 am
by marktownley
Looking forward to the update pics Ken! I've been reading your book this holiday and have caught myself thinking of the practical mechanics of how I could do a SHG, so, there may be another to join the club soon, I might make it my winter project ;)

Re: Littrow SHG prototype

Posted: Mon Sep 19, 2016 7:11 am
by Merlin66
Some images of the primed housing, later painted Matt Black interior and Gloss white exterior.
Weather has not allowed final testing but things are coming together.
All up weight is just over 8Kg...sits nicely on the HEQ5 mount.
More to follow.
shg build 007s.jpg
Housing primed
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shg build 017s.jpg
final assembly in testing
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shg build 018s.jpg
final assembly in testing
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Re: Littrow SHG prototype

Posted: Mon Sep 19, 2016 8:45 pm
by highfnum
looks good
it reminds me of a SHS
but it not -- correct?

Re: Littrow SHG prototype

Posted: Mon Sep 19, 2016 10:20 pm
by Merlin66
John,
No, not an SHS just a long focal length digital SHG....
I suppose it's a bit like Rondi's design (Imaging Sunlight - p171)- telescope sitting on a spectrograph but in this case a Littrow design; or similar to Mete's VHIRSS (p200) but folded back on itself.

Re: Littrow SHG prototype

Posted: Sat Sep 24, 2016 9:44 pm
by Merlin66
Testing continues....
Using the new ASI 174MM under FireCapture....
I'm seeing issues of resonance banding...worse at Ha and gradually disappears towards the CaK.
Not happy at the moment :cry: :cry:
ASI174_ha_banding.JPG
Resonance bands at Ha
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ASI174_Na_banding.JPG
Still showing bands at Na
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ASI174_CaK_banding.JPG
Bands less visible at CaK
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Re: Littrow SHG prototype

Posted: Sun Sep 25, 2016 12:23 am
by Merlin66
Hmmm
Just set up the ol' DMK41 - guess what??? NO resonance banding, but limited to 15fps.
Also tried SharpCap with the ASI174 - still shows banding, so not the software, it's in the camera.
Onwards and maybe a little Upwards

Re: Littrow SHG prototype

Posted: Sun Sep 25, 2016 1:35 am
by taratasy
I had the same thing with the Zwo ASI 120mms.
The webcam worked perfectly well... It really seems it depend on the camera.

Is this effect related to newtons rings ?