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Re: Littrow SHG prototype

Posted: Sun Sep 25, 2016 1:50 am
by Merlin66
Similar to NR but different....
The resonance pattern extends across the whole spectrum, NR are within the specific Ha band region.

Re: Littrow SHG prototype

Posted: Mon Sep 26, 2016 10:49 pm
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.

Re: Littrow SHG prototype

Posted: Tue Sep 27, 2016 3:25 am
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.

Joe

Re: Littrow SHG prototype

Posted: Tue Sep 27, 2016 8:14 am
by Merlin66
Joe,
Thanks for that.
I'll give it a go and report back.

Re: Littrow SHG prototype

Posted: Mon Oct 03, 2016 5:35 am
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 2874 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

Re: Littrow SHG prototype

Posted: Tue Oct 04, 2016 12:56 am
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.

Re: Littrow SHG prototype

Posted: Tue Oct 04, 2016 1:40 am
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.

Re: Littrow SHG prototype

Posted: Tue Oct 04, 2016 4:25 am
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.

Re: Littrow SHG prototype

Posted: Tue Oct 04, 2016 5:06 am
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.

Re: Littrow SHG prototype

Posted: Tue Oct 04, 2016 5:29 am
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 2817 times

Re: Littrow SHG prototype

Posted: Sat Oct 22, 2016 4:05 pm
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.

Re: Littrow SHG prototype

Posted: Sat Oct 22, 2016 10:01 pm
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.
102408_frame_depth.JPG
Ha single frame profile
102408_frame_depth.JPG (101.2 KiB) Viewed 2717 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.

Re: Littrow SHG prototype

Posted: Sun Oct 23, 2016 10:58 am
by highfnum
basically you are getting near end of construction and testing - correct?

Re: Littrow SHG prototype

Posted: Mon Oct 24, 2016 8:16 pm
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......

Re: Littrow SHG prototype

Posted: Sat Nov 05, 2016 12:12 am
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.

Re: Littrow SHG prototype

Posted: Mon Jun 18, 2018 7:33 pm
by highfnum
what did you use for slit?

Re: Littrow SHG prototype

Posted: Mon Jun 18, 2018 11:00 pm
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.