Littrow SHG prototype

[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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Any questions just let me know.

[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?

[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.....

[solarscope180]

How do you plan to drive the nodding mirror?

[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.

[marktownley]

All very cunning

[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

[marktownley]

Try the cut out feature instead of ROI Ken, this centralises it rather than putting (the ROI) up in the corner.

[taratasy]

Hi Merlin, I have a question : why don't you use a 1800 or 2400 grooves/mm grating, too much light loss ?
thanks.

[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.

[highfnum]

nice carpentry work

[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.

[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???

[Merlin66]

Hmmm
Looks like the V2.5 Beta may have better ROI controls......
I'll download a copy for trials.

[Wah]

You may try ROI fucnction in SharpCap rather than Firecapture.

[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.

[highfnum]

do you own or used any other SHG types

[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.

[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

[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.

Housing primed

shg build 007s.jpg (134.47 KiB) Viewed 4489 times

final assembly in testing

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final assembly in testing

shg build 018s.jpg (95.83 KiB) Viewed 4489 times

[highfnum]

looks good
it reminds me of a SHS
but it not -- correct?

[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.

[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

Resonance bands at Ha

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Still showing bands at Na

ASI174_Na_banding.JPG (48.49 KiB) Viewed 4370 times

Bands less visible at CaK

ASI174_CaK_banding.JPG (33.91 KiB) Viewed 4370 times

[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

[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 ?