Test/Problem with DS Lunt-Quark Chromosphere

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Test/Problem with DS Lunt-Quark Chromosphere

Post by H-Alpha »

Hi all,

I hope you are all well.

Before deciding to buy a second Lunt etalon for DS with my Lunt 130MT, a friend kindly let me a Quark (Chromosphere) to test if it could work for DS.

I made the test today and the result was quite disappointing as you will see below. First I tried with ASI1600 and had tremendous Newtonian rings apart the very poor quality of the image. I tilted the camera and the rings issue was partly solved, but apparently the sweetspot issue could not be resolved, so I tried with ASI183, which has a larger sensor and smaller pixels. The results were better, but still poor.

I have three questions in case any of you with much experience on etalons can have answers.

1) Is there any theoretical reason why a Quark could not work with a Lunt etalon?

2) My captures were very grainy. Is it just due to the high Gain that I had to use in DS? What are typical combinations of shutter speed/Gain you are using in DS for full disks and what for higher magnifications? Do you reach Gains above 300, as in the third image were despite 60 ms of shutter speed I had to be well above Gain 300?

3) Surprisingly, as you can see in the histogram of Firecapture in the third image attached, although the histogram bars were well below the maximum, the value was 100! It is the first time I see something like that and cannot explain it. Any idea?
1.jpg
1.jpg (423.54 KiB) Viewed 369 times
2.jpg
2.jpg (365.94 KiB) Viewed 369 times
3.jpg
3.jpg (423.41 KiB) Viewed 369 times
Thanks for any comment/answer you may have.

Best wishes,
Alexandros


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Re: Test/Problem with DS Lunt-Quark Chromosphere

Post by pupak »

1. It doesn't exist. I have been using a combination of LS80 and Quark combo etalons for several months.
The optical path must be set correctly. Lens preferably F7, then LS80, then TZ4x for F28 and then Quak combo. Ideal cameras are IMX174 and IMX432 or IMX533. The Lunt blocking filter will not be used. Only Quark itself with its blocking filter will be used.
2. DS will always reduce the luminous flux, so the exposure time will be stretched, but it is clear from the picture that you do not have the etalons set correctly. Quark does not have a homogeneous image, so flat is a must. It feels like you have two blocking filters in there at once.
I normally use exposure under 5ms and gain up to 200. For prominences I increase the gamma to 50-60.
3. The combination of these two etalons shows the surface and prominences well, so I like to use it, but it needs to be adjusted even better.
Notice:
For camera capture, I recommend removing the front blue filter from the LUNT blocking filter. The exposure time will be significantly reduced. ATTENTION, for camera recording only.
Last edited by pupak on Tue Nov 22, 2022 1:58 pm, edited 1 time in total.


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Re: Test/Problem with DS Lunt-Quark Chromosphere

Post by MAURITS »

Alexandros, I think that you have potential with the Lunt - Quark double stack, it's only a matter of tuning the etalons.

The images are certainly not bad.

Ps
About the histogram I never go above the 70%.


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Re: Test/Problem with DS Lunt-Quark Chromosphere

Post by Bob Yoesle »

Is there any theoretical reason why a Quark could not work with a Lunt etalon?
It depends on what you mean by “work.” It can work, the question is will it work well. This depends on the quality of the Quark etalon. These are generally going to be of more variable and likely of lesser uniformity than the LS130MT Lunt etalon you are double stacking. Double stacking reduces the filter system FWHM, and reveals any bandpass variations. The result is what you are seeing with poor bandpass uniformity, leading to variations in brightness.

A flat will correct the brightness issue, but not the bandpass uniformity variation. The only solution for that is a better DS etalon.
My captures were very grainy. Is it just due to the high Gain that I had to use in DS? What are typical combinations of shutter speed/Gain you are using in DS for full disks and what for higher magnifications? Do you reach Gains above 300, as in the third image were despite 60 ms of shutter speed I had to be well above Gain 300?
Double stacking reduces filter transmission based on the transmission levels of both etalons. If you double stack two 80% transmission etalons, the resultant transmission is 0.80 x 0.80 = 0.64 > 64%. The Quark (or any mica) filter will generally have a much lower transmission overall by itself since it blocks out the birefringence image with a circular polarizer (50%), and then adds its own blocking filters, so it is the binding constraint. To compensate you’re using more gain to brighten the image, and getting more grain.

If you are using the Quark in the Lunt blocking filter, you are adding insult to injury by further reducing transmission. In this case the Lunt blocking filter is unnecessary, as the Quark has its own blocking filter which makes the Lunt BF completely redundant.
Surprisingly, as you can see in the histogram of Firecapture in the third image attached, although the histogram bars were well below the maximum, the value was 100! It is the first time I see something like that and cannot explain it. Any idea?
Your exposure time is limiting the maximum brightness level you can achieve at your gain and gamma settings.
Ps - About the histogram I never go above the 70%.
Indeed it is better to underexpose an image and not have any oversaturated areas. Post-capture processing can then bring out prominence and other details.

Histogram adjustment:

Histogram processing.jpg
Histogram processing.jpg (121.34 KiB) Viewed 309 times

Curves adjustment:

Curves processing.jpg
Curves processing.jpg (119.86 KiB) Viewed 309 times

Create successively brighter images from the original and make a HDR image from these. Original image on far left ("Full Disc 1"), brightness adjusted copies follow; resultant HDR image on right:

HDR processing 2.jpg
HDR processing 2.jpg (484.57 KiB) Viewed 309 times
For camera capture, I recommend removing the front blue filter from the LUNT blocking filter. The exposure time will be significantly reduced. ATTENTION, for camera recording only.
While transmission will improve, you’re also letting significant levels IR impinge on the downstream order selection filter and/or camera, which may contribute to its eventual demise. I’d replace BG38 with a KG3, and this will render the desired better exposure times, remain safe for visual use, while at the same time better protecting the more expensive to replace OSF and your camera.
Solar-spectrum-at-the-top-of-the-atmosphere-and-at-sea-level-1.png
Solar-spectrum-at-the-top-of-the-atmosphere-and-at-sea-level-1.png (55.38 KiB) Viewed 309 times
BG38 vs KG3.jpg
BG38 vs KG3.jpg (90.5 KiB) Viewed 309 times
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Re: Test/Problem with DS Lunt-Quark Chromosphere

Post by H-Alpha »

Dear pupak, Maurits and Bob,

Thank you all very much for your answers! This kind of precious knowledge and advise on both the theoretical and practical questions is what I needed. :-)

I have one more overall question for Bob and two regarding the KG3 filter advice.

Bob, you wrote:
Bob Yoesle wrote: Tue Nov 22, 2022 3:22 pm
A flat will correct the brightness issue, but not the bandpass uniformity variation. The only solution for that is a better DS etalon.
Bob, this is an important view and given that I was planning a Lunt DS and I fully respect your experience and knowledge, can you please confirm if the right way to achieve the best possible DS with my Lunt130MT is a second Lunt etalon, or other options may reach even better results?

Bob Yoesle wrote: Tue Nov 22, 2022 3:22 pm
If you are using the Quark in the Lunt blocking filter, you are adding insult to injury by further reducing transmission. In this case the Lunt blocking filter is unnecessary, as the Quark has its own blocking filter which makes the Lunt BF completely redundant.
For camera capture, I recommend removing the front blue filter from the LUNT blocking filter. The exposure time will be significantly reduced. ATTENTION, for camera recording only.
While transmission will improve, you’re also letting significant levels IR impinge on the downstream order selection filter and/or camera, which may contribute to its eventual demise. I’d replace BG38 with a KG3, and this will render the desired better exposure times, remain safe for visual use, while at the same time better protecting the more expensive to replace OSF and your camera.

Bob
I understood that keeping both blocking filters was a mistake. By removing the Lunt BF, the BG38, which is incorporated in the BG38 (= front blue filter, right?), if I got it right, I need nothing more, isn't it?

You proposed to change BG38 to KG3, and I don't understand why, since I don't need any of them, if the above is correct. Or what you meant is that if I remove the Lunt BF I have to add a KG3 in front of the Quark?

If this is the case, any KG3 will do the job? For example the Optolong KG3 is fine?

Thanks once more,
Alexandros


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Re: Test/Problem with DS Lunt-Quark Chromosphere

Post by Bob Yoesle »

Hi Alexandros,

The best way to double stack would be a front etalon, but these are generally not available above 100mm diameter. So the next best is a second internal DS collimator/etalon from Lunt. Next would be an optimized telecentric/mica etalon system - with the Quark being the last option.

As far as the BG38, which is the front blue filter in the Lunt diagonal, you're right that you needn't remove it for use with the Quark, since you wouldn't use the Lunt BF is that situation to begin with.

But for the LS130MT in single stack mode or double stacked with a second air-spaced etalon, you would use the Lunt BF, and I would advise the KG3 for the brightness increase it would provide.* The Optolong KG3 should be fine. The BelOptik filter's UV/IR thin film coatings might help reduce any possible substrate aging and a bit of thermal stress from the <400 and ~ 700-1200nm wavelengths that could be passed through to the narrow bandpass filter.

* if the image too bright when used for a single etalon, add some ND filtering to knock down the brightness.

Bob


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Re: Test/Problem with DS Lunt-Quark Chromosphere

Post by H-Alpha »

Thank you very much indeed Bob!
Really precious and kind help. :-)
Best wishes,
Alexandros


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