Littrow SHG prototype

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Littrow SHG prototype

Post 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)
P1040141s.jpg
P1040141s.jpg (90.21 KiB) Viewed 8991 times
P1040143s.jpg
P1040143s.jpg (73.42 KiB) Viewed 8991 times
P1040144s.jpg
P1040144s.jpg (77.62 KiB) Viewed 8991 times
P1040145s.jpg
P1040145s.jpg (72.78 KiB) Viewed 8991 times
Any questions just let me know.


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Re: Littrow SHG prototype

Post 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?


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Re: Littrow SHG prototype

Post 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.....


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Re: Littrow SHG prototype

Post by solarscope180 »

How do you plan to drive the nodding mirror?


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Re: Littrow SHG prototype

Post 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.


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Re: Littrow SHG prototype

Post by marktownley »

All very cunning :)


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Re: Littrow SHG prototype

Post 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


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Re: Littrow SHG prototype

Post by marktownley »

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


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Re: Littrow SHG prototype

Post 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.


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Re: Littrow SHG prototype

Post 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.


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Re: Littrow SHG prototype

Post by highfnum »

nice carpentry work


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Re: Littrow SHG prototype

Post 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.


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Re: Littrow SHG prototype

Post 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???


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Re: Littrow SHG prototype

Post by Merlin66 »

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


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Re: Littrow SHG prototype

Post by Wah »

You may try ROI fucnction in SharpCap rather than Firecapture.


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Re: Littrow SHG prototype

Post 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.


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Re: Littrow SHG prototype

Post by highfnum »

do you own or used any other SHG types


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Re: Littrow SHG prototype

Post 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.


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Re: Littrow SHG prototype

Post 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 ;)


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Re: Littrow SHG prototype

Post 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.
Housing primed
Housing primed
shg build 007s.jpg (134.47 KiB) Viewed 8620 times
final assembly in testing
final assembly in testing
shg build 017s.jpg (73.28 KiB) Viewed 8620 times
final assembly in testing
final assembly in testing
shg build 018s.jpg (95.83 KiB) Viewed 8620 times


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Re: Littrow SHG prototype

Post by highfnum »

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


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Re: Littrow SHG prototype

Post 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.


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Re: Littrow SHG prototype

Post 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:
Resonance bands at Ha
Resonance bands at Ha
ASI174_ha_banding.JPG (45.91 KiB) Viewed 8501 times
Still showing bands at Na
Still showing bands at Na
ASI174_Na_banding.JPG (48.49 KiB) Viewed 8501 times
Bands less visible at CaK
Bands less visible at CaK
ASI174_CaK_banding.JPG (33.91 KiB) Viewed 8501 times


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Re: Littrow SHG prototype

Post 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


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Re: Littrow SHG prototype

Post 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 ?


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Re: Littrow SHG prototype

Post by Merlin66 »

Similar to NR but different....
The resonance pattern extends across the whole spectrum, NR are within the specific Ha band region.


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Re: Littrow SHG prototype

Post by Merlin66 »

Robin Leadbeater has made me aware of a technical paper on "ripples" generated within the CCD chip
http://www.aphesa.com/downloads/download2.php?id=1
If this logic also applies to the ASI 174MM CMOS construction, it looks like I'm screwed with the fast frame SHG concept.


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Re: Littrow SHG prototype

Post by Spectral Joe »

Ken

Try putting a short (<75mm) focal length lens immediately in front of the camera, as close to the chip as possible. That camera has a diagonal of around 13.5mm, so a 25mm diameter lens is big enough. If that helps, try other (longer) focal lengths.

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Re: Littrow SHG prototype

Post by Merlin66 »

Joe,
Thanks for that.
I'll give it a go and report back.


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Re: Littrow SHG prototype

Post by Merlin66 »

The attached image from the ASI 174 in the SHG and based on a white light illumination of the entrance slit (you would call it the zero image in conventional spectroscopy)

It was generated from an AVI of 5000 frames, extracted to a bandwidth of 0.2A

so, I think around R=25000
Spectro_112715c_small.jpg
Spectro_112715c_small.jpg (15.05 KiB) Viewed 7683 times
Further investigations....

The image...the ripples/ banding seems to be caused by fluctuations in the intensities of the frames.

You can see a slight "flicker" on the capture program screen as the AVI video is captured.

Changing the gain or exposure doesn't make any difference.

Ken


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Re: Littrow SHG prototype

Post by Spectral Joe »

My interpretation of the image is that the slit is horizontal and time is vertical, sampling just the zero order. If that's the case then these are either the result of ripple in the source intensity or some sort of hum pickup. Lamps run on AC have substantial intensity variation, I run all of my light sources (white light and spectral) on DC, just to get away from this. As for pickup, it can be from stray ground currents or magnetic, such as having a motor or transformer too close to the camera. Try illuminating the slit with a DC source, like a flashlight.


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Re: Littrow SHG prototype

Post by Merlin66 »

Joe,
Thanks for the comments.
Yeah I had reached the same conclusion that it was due to the Fluoro lamp I used for testing.....
I do have a different Incandescent lamp set-up I'll change over to.


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Re: Littrow SHG prototype

Post by Spectral Joe »

As long as the incandescent lamp is run on DC you should be OK, even incandescents show a lot of ripple, especially at 50 Hz.


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Re: Littrow SHG prototype

Post by Merlin66 »

Joe,
OK understood......
I've gone back to the SHG Ha data collected from the ASI 174 on the 24th Sept.
I've managed to extract a profile, calibrate and start analysing....
The ripples are almost exactly at 2A spacing....
The magnitude of the ripples are +/- 4.6 ADU on a recorded average 165 ADU part of the image giving +/- 2.8% variation.
(The recorded dispersion was 0.07 A/pixel - exactly as predicted by the SimSpec SHG spreadsheet!!)

Details to follow.


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Re: Littrow SHG prototype

Post by Merlin66 »

Here's the calibrated 1D profile of the Ha line with the ASI 174.
SHG Ha ASI 174.jpg
SHG Ha ASI 174.jpg (25.58 KiB) Viewed 7626 times


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Re: Littrow SHG prototype

Post by solarscope180 »

Ken
Have you resolved the banding issue with the ASI174 camera? I was planning to use this camera with a SHG that I am building but wondering if I should try a different camera.


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Re: Littrow SHG prototype

Post by Merlin66 »

The testing continues......
I have managed to obtain faster speeds using the ROI (1930 x 100 pixel) in FireCapture. Around 880 fps average.
The size of the AVI file generated with "normal" drift scanning can be 9000 frames, and up to 20Gb (full frame). None of the available processing software will handle these large files.
I have to crop in VDub to 1930 x 90 (when testing full frame) to get the file down around 2.8 Gb. Still too big for Wah's SpectralLineMerge (2 Gb limit) but seems to work with John's BASS Project.
Ha single frame profile
Ha single frame profile
102408_frame_depth.JPG (101.2 KiB) Viewed 7526 times
There's a small optical misalignment - telescope to slit to be resolved.... but the 8 bit image depth is a concern.
I used an exposure of 0.55ms @ 70fps for the above single frame image of the Ha absorption line extracted from the AVI.
With the continuum saturated (>256ADU) the bottom of the Ha "core" shows around 70 ADU; in a perfect world with good/ great exposure the bottom should be around 40-50 ADU (16-20% of the continuum)
I would say that the ASI 174MM could be a solution....the spectral ripples may affect the results but so far not an absolute show stopper.


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Re: Littrow SHG prototype

Post by highfnum »

basically you are getting near end of construction and testing - correct?


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Re: Littrow SHG prototype

Post by Merlin66 »

John,
Yes, getting close....
I can now see a possible solution to the frame rate - I can maintain fps >400 with a horizontal selected ROI of 1930 x 100 pixel and exposure of 0.55ms
Ideally I need some further trials to get the exposure up a notch, getting a useable 2ms would be good.
The final tweaking of the telescope/ slit alignment is straight forward.
Also, an improvement to the telescope/ slit focus (I have a 10:1 Crayford) fine control would be helpful.
Once the above is confirmed, I can then finalise the nodding mirror arrangement and head towards a full disk scan of 4 seconds......


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Re: Littrow SHG prototype

Post by Merlin66 »

OK, just an update....
The weather, - wind and rain has hampered any real testing.....
I've decided to replace the Vixen dovetail with a wider Losmandy type; to give (maybe) a bit more stability....
Also adding a SW electronic motor to the 10:1 focus knob on the Crayford. I'll use a 30 tooth pulley and 80 tooth belt (same as the set-up I use on the 102 PST mod)
In the background I'm finalising the nodding mirror assembly which, if the frame rate/ exposure issue can be resolved could be the ultimate step.
Onwards and Upwards.


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Re: Littrow SHG prototype

Post by highfnum »

what did you use for slit?


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Re: Littrow SHG prototype

Post by Merlin66 »

I used the adjustable slit assembly from Surplus shed.
I intend to further improve by using a pair of stainless steel slit blades which I have and mount them on a parallelogram frame similar to Rousselle, see "Imaging Sunlight", p178.


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