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Tuning or bandwidth?

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Astrophil
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Tuning or bandwidth?

Post by Astrophil » Thu Jan 16, 2020 5:47 pm

We all strive to produce the best images from the equipment we have available to us. However, some images appear better than others. Putting resolution aside, my rudimentary knowledge of solar imaging leads me to the question of whether proper etalon tuning or system bandwidth is of primarily importance. Obviously both play an important role but how can it be determined which is primarily responsible for less than ideal images.

I am currently using a Lunt 80 pressure tuned etalon solar scope with an additional pressure tuned secondary. Placed between the etalons is a circular polarizer to minimize internal reflections. Instead of a blue filter before the blocking filter, I use a Lunt supplied filter that is more transparent, for imaging only. The advertised bandwidth is 0.5 angstroms and by double stacking the continuum leakage should be minimized.

My tuning ritual consists of the following. Beginning with the primary etalon, I adjust to obtain best contrast between filaments and the surface while minimizing the histogram. These usually coincide. At the same time the secondary is adjusted to match the primary by watching the histogram and brightness increase. This usually takes a few iterations before achieving what I think is optimal. This may or may not be the correct way to tune. Opinions welcome.

The Lunt 80 is a nice portable scope that delivers in my opinion a good image. I just wonder if or how they could be improved. Thanks for any suggestions.

Below are some images taken on the 15th of January.

Phil

20200115_125713_fd_cc.jpg
20200115_125713_fd_cc.jpg (871.63 KiB) Viewed 135 times
20200115_125713_fd_cm.jpg
20200115_125713_fd_cm.jpg (648.47 KiB) Viewed 135 times
20200115_130449_ar1.jpg
20200115_130449_ar1.jpg (232.07 KiB) Viewed 135 times
20200115_131445_filaments.jpg
20200115_131445_filaments.jpg (294.99 KiB) Viewed 135 times
20200115_130855_ar.jpg
20200115_130855_ar.jpg (349.71 KiB) Viewed 135 times
20200115_130728_proms_invc.jpg
20200115_130728_proms_invc.jpg (318.84 KiB) Viewed 135 times

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Re: Tuning or bandwidth?

Post by rsfoto » Thu Jan 16, 2020 6:47 pm

Hi Phil,

Looking at your images and my limited knowledge i would say you are very close to both best parameters. Not being able to look by myself into that due to now having the scope present it is also difficult ot judge.

As you said with a few iterations one tries tog et closer to that and as I can not see how it changes it is difficult to say if there is a better point then the one you are presenting.

Comparing you images to mine they are very close up to 95 equal ...

This tuning and Bandwidth-ing is subjective too. One day it will be better and the next one worse due atmospheric conditions.

All in all they look very good and BTW I stopped colouring my discs as I had the impressión that with colour somehow they looked less sharp the in B&W ... OK, that is also subjective and is a matter of Taste ...

Rainer
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Re: Tuning or bandwidth?

Post by rsfoto » Thu Jan 16, 2020 7:02 pm

Hi Phil,

I hope you do not mind but I downloaded your B&W image and processed it a bit according to my taste.

You have some residuals there in the histogram which makes your image a bit opaque as well as you can stretch it on the white side a bit more. OK, I have to be careful as monitor settings do playt a big role how others appreciate the images. Also if one converted to colour sRGB or not ... In this respect I calibrate my monitors every two weeks.


Below full image retouched a bit by me according to my taste

20200115_125713_fd_cm_et.jpg
20200115_125713_fd_cm_et.jpg (840.19 KiB) Viewed 123 times

and below half and half. Left your original, right the retouched image

20200115_125713_fd_cm_Phil_Rainer.jpg
20200115_125713_fd_cm_Phil_Rainer.jpg (792.22 KiB) Viewed 123 times
regards Rainer

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Re: Tuning or bandwidth?

Post by rsfoto » Thu Jan 16, 2020 7:24 pm

Hi Phil,

Look at the image below where I took your histogram and how I changed it. Red lines is your original image histogram and green is how I stretched it a bit.

Just as an idea how stretching the histogram can also influence what we see as sharpness = contrast.

20200115_125713_fd_cm_et_histogram.jpg
20200115_125713_fd_cm_et_histogram.jpg (300.85 KiB) Viewed 120 times
regards Rainer

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Re: Tuning or bandwidth?

Post by Astrophil » Thu Jan 16, 2020 8:10 pm

Thank you so much, Rainer. I like your results of reprocessing the image. Much better, even using the compressed jpeg.

I have not calibrated my monitors in a long time. That will be next on my to do list. I tend to agree that grey scale images show the beat detail. Someone once said that if the image is not the best, colorize it. If that doesn't help then invert it. If still not satisfactory, do both. I think in my case I must tend to make it dark and dull to mask inadequacies, subconsciously of course.

The multiple iterations is to insure that the primary etalon tuning is not overly influenced by the secondary etalon's initial setting. I agree that once both are set, no further tuning is necessary.

I appreciate your help and advice. It was your excellent images that prompted the concern over my scope's performance and imaging abilities. ;-)

Phil

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Re: Tuning or bandwidth?

Post by Valery » Fri Jan 17, 2020 4:39 am

In my opinion Phil's images are better balanced and they better represent what is seen on the sun.
They also consist more shades between the spiculaes (dark bushes on the disk).
Rainer's images looks too much contrasty - just like more wide etalons show in a telescope. The space between disk spiculaes are more washed out (too bright less details).

This is seen on my monitors on Dell Inspiron lap top with its native 17" and external 24" and the same seen on the screen of my MacBook Pro 13".

Usually, the narrower the bandwidth the more grayish the solar disk looks - more delicate shadows in between dark filaments and spiculae bushes and to the "surface" of the disk. And I need to change the contrast somewhat and to change the profile of the gamma curve to make these delicate shadows more visible.

Again, this is to my own taste and according to my understanding of the solar features nature and how do we see them in our telescopes.

CS,

Valery
Last edited by Valery on Fri Jan 17, 2020 7:25 pm, edited 3 times in total.
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Re: Tuning or bandwidth?

Post by rsfoto » Fri Jan 17, 2020 3:53 pm

Valery wrote:
Fri Jan 17, 2020 4:39 am
In my opinion Phil's images are better balanced and they better represent what is seen on the sun.
They also consist more shades between the spiculaes (darkes bushes on the disk).
Rainer's images looks too much contrasty - just like more wide etalons show in a telescope. The space between disk spiculaes are more washed out (too bright less details).

This is on my monotors on Dell Inspiron lap top with its native 17" and external 24" monitors and the same on the screen of my MacBook Pro 13".

Usually, the narrower the bandwidth the more grayish the solar disk looks - more delicate shadows in between dark filaments and spiculae bushes and to the "surface" of the disk. And I need to change the contrast somewhat and to change the profile of the gamma curve to make these delicate shadows more visible.

Again, this is to my own taste and according to my understanding of the solar features nature and how do we see them in out telescopes.
Hi Valery,

You are right ... Maybe I exaggerated a bit in order to show how processing can influence what we see.

As you say it resumes all to Taste ... Have a nice day.

Rainer
regards Rainer

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Re: Tuning or bandwidth?

Post by Astrophil » Fri Jan 17, 2020 5:24 pm

Taste aside for a moment. In order to image bright plage, dark filaments, and to easily visualize filaproms, is a certain minimum bandwidth required or is it only camera settings or selective post processing?

My reference library is lacking and knowledge of this subject inadequate so any thoughts would be most welcome.

Thanks,

Phil

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Re: Tuning or bandwidth?

Post by MalVeauX » Fri Jan 17, 2020 5:49 pm

There's a bit more to the tuning, since it's not just an issue of being on-band, it's about the transmission profile. The same reason a 0.3A single etalon will not eliminate the photosphere leaking through showing the double limb. Mean while, two 1A filters can stack and shave off the skirt of the transmission profile removing the photosphere from leaking through rather well, at a cost of lower transmission of course. So tuning is very critical of course. Sometimes we're close to on-band, sometime's a little off-band, when we're on-band and the filaments are super dark and plages very bright, the high contrast is pleasing (looks sharp without sharpening, a function of contrast). If the two are not tuned together of course, then there's issues. Bandpass is not as critical. 1A, 0.7A, 0.5A, 0.3A, etc, whatever number you want, the single etalons leak photosphere light. The bandpass number is largely no that useful even. Something that is 0.7A and 0.5A really doesn't matter, the transmission skirt is what matters, and that's where double stacking comes in, not to just tighten bandpass, but to effectively suppress the out of band wavelengths that are leaking through on the sides of the transmission curve. I would take two 1A~0.7A filters over a single 0.3A filter for limb shots for this reason. The alternative is a very narrow blocking filter, instead of the 6A blocking filters, a 1A blocking filter would do it (but that's a unicorn).

Lovely discs!

Very best,

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Re: Tuning or bandwidth?

Post by Merlin66 » Sun Jan 19, 2020 6:55 am

I am aware that there are 3A blocking/ sort filters being tested........
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