Problem, or this is the famous 'sweet spot'?

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Problem, or this is the famous 'sweet spot'?

Post by H-Alpha »

Hi all,

I hope you are all well.

Those late autumn-winter days are not favorable for solar photo by me, not only because of clouds, rain, wind and bad seeing, but also because from my veranda I have only two hours of sun in the middle of the day... :-(

Therefore, it's time to come back to something that I observed while capturing with my H-alpha and worries me a bit...

One month ago, I made some FDs in H-alpha with my LUNT130MT. It wasn't the first time since I received the ASI1600 (which has a large 4656x3520 sensor), bought precisely for this purpose. It seems weird to me that I did not notice the following problem during the several FDs I had made before that day, so please have a look at the following photo...
2021-10-30-1336_7, potential sweetspot issue.jpg
2021-10-30-1336_7, potential sweetspot issue.jpg (819.77 KiB) Viewed 526 times
These are different successive captures made with the sun in different places in the frame (sensor) of ASI1600, and obviously the H-alpha doesn't work well or at least not homogeneously for the entire field. Nevertheless, I succeeded to make a relatively descent photo in only one particular position.

Is this a problem with my etalon (something that got untuned, or moved inside the filter) or is it normal and it is the famous 'sweet spot issue'?
How is it possible that it wasn't so apparent before?
And finally, can I do something about it apart trying to find the best position for the sun's disk?
Should I contact Lunt? (who have an extremely slow and not always effective answering process).

Thanks in advance for any help or advice you may provide,
Alexandros
Last edited by H-Alpha on Sun Dec 05, 2021 12:55 am, edited 1 time in total.


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Re: Problem, or this is the famous 'sweetspot'?

Post by rigel123 »

Nothing wrong, you are seeing the area the “sweet spot” covers on a large format chip. With that much acreage you can move the image around on the chip to see the best position so the entire disk is on band. The upper right image looks very close to having the entire disk on band. Adding a Barlow will typically bring the entire FOV to be on band if your scope is tuned properly.


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Re: Problem, or this is the famous 'sweetspot'?

Post by marktownley »

Yes, that's the sweet spot. There is nothing wrong with the scope, the larger scopes are designed for medium and hi res work rather than full disks. If you want to image full disks you should get a 60mm Lunt, it will be much more fit for purpose.


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Re: Problem, or this is the famous 'sweetspot'?

Post by DeepSolar64 »

marktownley wrote: Thu Dec 02, 2021 6:36 am Yes, that's the sweet spot. There is nothing wrong with the scope, the larger scopes are designed for medium and hi res work rather than full disks. If you want to image full disks you should get a 60mm Lunt, it will be much more fit for purpose.

Would the same apply to the SMII60 Vs the SMII90 as well? But the 130 Lunt IS significantly larger than the 60 and that's a BIG amout of difference.


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Re: Problem, or this is the famous 'sweetspot'?

Post by minhlead »

Hi. Disclaimer: I have never ever used anything other than a Quark and a few peeks at a PST several years ago so I am NOT the authority on this. But here's my thoughts:
First, make sure the problem is not due to passing clouds that make some part of the frame brighter/darker than the other, through stacking, a cloudy sun can give a very similiar result to what we are seeing above. I am using a 130mm F/7 scope with a QHY163 before (similiar chip to asi1600) and I must say the sun/moon will take up about half of the frame which make the result above very weird: none of the images above shows any kind of relative position to each other. If it's the "sweet spot" then the pattern should be fixed, when you slew the sun in different places in the chip you should be able to see the relative position of these sweet spots to each other and map them physically on the chip (assuming you are not rotating your camera relative to the etalon for the whole imaging session) so perhaps a few full fov of the above photo without cropping would shed some light on this. If it's cloud, on the other hand then it perfectly makes sense. Even explain why you did not notice the un uniformity ealier.
Secondly, if you can confirm it's not cloud then you can simply map the bad spot on the etalon, rotate the camera so that it's not falling on the chip (the bad spot falls on the short side of the sensor). And finally, try cropping out the bad spot and using flat frame to achieve a flatter field.
At least that is what I did with my Quark and it's working very well so far.


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Re: Problem, or this is the famous 'sweetspot'?

Post by marktownley »

minhlead wrote: Thu Dec 02, 2021 8:03 am Hi. Disclaimer: I have never ever used anything other than a Quark and a few peeks at a PST several years ago so I am NOT the authority on this. But here's my thoughts:
First, make sure the problem is not due to passing clouds that make some part of the frame brighter/darker than the other, through stacking, a cloudy sun can give a very similiar result to what we are seeing above. I am using a 130mm F/7 scope with a QHY163 before (similiar chip to asi1600) and I must say the sun/moon will take up about half of the frame which make the result above very weird: none of the images above shows any kind of relative position to each other. If it's the "sweet spot" then the pattern should be fixed, when you slew the sun in different places in the chip you should be able to see the relative position of these sweet spots to each other and map them physically on the chip (assuming you are not rotating your camera relative to the etalon for the whole imaging session) so perhaps a few full fov of the above photo without cropping would shed some light on this. If it's cloud, on the other hand then it perfectly makes sense. Even explain why you did not notice the un uniformity ealier.
Secondly, if you can confirm it's not cloud then you can simply map the bad spot on the etalon, rotate the camera so that it's not falling on the chip (the bad spot falls on the short side of the sensor). And finally, try cropping out the bad spot and using flat frame to achieve a flatter field.
At least that is what I did with my Quark and it's working very well so far.
It's not cloud in the images above, you can see the solar disk going off band, this is what happens out of the sweetspot. The sweet spot is the area where it is all on band.

In the ideal world, the sweetspot is larger than the solar disk itself, and in this instance the whole disk will be on band.

Sweet spot is a function of aperture vs etalon size. The bigger the etalon relative to the aperture the bigger the on band area. Moving the image around on the chip might find a 'best' spot, but this won't be the whole disk.

Making flats will compensate for changes in illumination across the field of the etalon but will not compensate for the change in bandpass or centre wavelength caused by the sweet spot.


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Re: Problem, or this is the famous 'sweetspot'?

Post by minhlead »

marktownley wrote: Thu Dec 02, 2021 3:27 pm
minhlead wrote: Thu Dec 02, 2021 8:03 am Hi. Disclaimer: I have never ever used anything other than a Quark and a few peeks at a PST several years ago so I am NOT the authority on this. But here's my thoughts:
First, make sure the problem is not due to passing clouds that make some part of the frame brighter/darker than the other, through stacking, a cloudy sun can give a very similiar result to what we are seeing above. I am using a 130mm F/7 scope with a QHY163 before (similiar chip to asi1600) and I must say the sun/moon will take up about half of the frame which make the result above very weird: none of the images above shows any kind of relative position to each other. If it's the "sweet spot" then the pattern should be fixed, when you slew the sun in different places in the chip you should be able to see the relative position of these sweet spots to each other and map them physically on the chip (assuming you are not rotating your camera relative to the etalon for the whole imaging session) so perhaps a few full fov of the above photo without cropping would shed some light on this. If it's cloud, on the other hand then it perfectly makes sense. Even explain why you did not notice the un uniformity ealier.
Secondly, if you can confirm it's not cloud then you can simply map the bad spot on the etalon, rotate the camera so that it's not falling on the chip (the bad spot falls on the short side of the sensor). And finally, try cropping out the bad spot and using flat frame to achieve a flatter field.
At least that is what I did with my Quark and it's working very well so far.
It's not cloud in the images above, you can see the solar disk going off band, this is what happens out of the sweetspot. The sweet spot is the area where it is all on band.

In the ideal world, the sweetspot is larger than the solar disk itself, and in this instance the whole disk will be on band.

Sweet spot is a function of aperture vs etalon size. The bigger the etalon relative to the aperture the bigger the on band area. Moving the image around on the chip might find a 'best' spot, but this won't be the whole disk.

Making flats will compensate for changes in illumination across the field of the etalon but will not compensate for the change in bandpass or centre wavelength caused by the sweet spot.
Yeah frame with the out of band spot was obvious. But the rest could be cloud since the gradient does not seems to have fixed pattern although the sun supposed to take up a significant portion of the sensor at this imaging scale. I expect to see some relation of the pattern between shots but I am not.
And yes, the detail lost because some part of the frame going offband can never be recover with flats. I have a trick to fix this: take picture on 1 side and then rotate the camera 180* in relation with the etalon and take another. Combine the 2 in post to have better details over the bad spot.


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Re: Problem, or this is the famous 'sweetspot'?

Post by Bob Yoesle »

Yeah frame with the out of band spot was obvious. But the rest could be cloud since the gradient does not seems to have fixed pattern although the sun supposed to take up a significant portion of the sensor at this imaging scale. I expect to see some relation of the pattern between shots but I am not.
There is more at work with these images than the "sweet spot," and a good deal if not most of the brightness and band-pass asymmetry is likely due to inaccurate tuning, which produces a "sweet ring." See Christian Viladrich's treatise here. This will exacerbate off-band behavior of the filter system and makes it appear asymmetrical (which it is not), and which otherwise may be excellent.

An identical issue recently has come up on Cloudy Nights, and some interesting video's were made by Chuck Young. It appears that unless you precisely tune your image to be on-band at the center of the Jacquinot spot, the Jacquinot spot becomes a "sweet ring" donut. To realize the best Jacquinot spot behavior it is essential to tune your filter precisely to 656.28 nm. A Hydrogen spectrum tube might be a useful tool if you find this difficult to do by eyeball.


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Re: Problem, or this is the famous 'sweetspot'?

Post by H-Alpha »

Dear Warren, Mark, James, Minh and Bob, thank you all for your thoughts, comments and useful info.

The first time I observed the above behaviour of my etalon I also though about clouds because some were passing that day, although my impression was that the 'strange' results were independent from the clouds. I had also some doubts about the sweet spot, because it was not a clear situation where at one place the sun disk was tuned and all around it was not. Therefore, I repeated the 'experiment' the next day with no clouds, so I think it cannot be due to them. After reading the detailed explanation by Bob and the "sweet ring", I think that this was my problem.
Bob Yoesle wrote: Thu Dec 02, 2021 10:39 pm It appears that unless you precisely tune your image to be on-band at the center of the Jacquinot spot, the Jacquinot spot becomes a "sweet ring" donut. To realize the best Jacquinot spot behavior it is essential to tune your filter precisely to 656.28 nm. A Hydrogen spectrum tube might be a useful tool if you find this difficult to do by eyeball.
Very interesting and precious to learn about this Bob and many thanks for the links and advice. I am not familiar with the "Jacquinot spot" and will try to learn more (EDIT--> Just realized from your post in Cloudy nights that "Jacquinot spot" is the scientific term for "sweet spot :-) ), but with my first search I could not find something more about Hydrogen Spectrum Tubes for astronomy. Given that my etalon is a Lunt one with the pressure knob and can never be sure if I am exactly at 656.8 nm (I only stop where I think I have the darkest image on my screen), I would like to try the H spectrum tube, if not very expensive, to experiment with.

Can you please indicate where I can find one to use as you suggested?

Many thanks once more,
Alexandros


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Re: Problem, or this is the famous 'sweet spot'?

Post by Bob Yoesle »

Hi Alexandros,

The spectrum tube itself is not very expensive, but the transformer can be, unless you are handy at electronics and very careful and cautious with high voltages. These are typically sourced from science supply vendors for schools, etc.

Here's one possible source among many.

Alternatively, a local school which teaches physics or chemistry would likely have one and a selection of tubes of various elements, and Hydrogen is typically among them.


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Re: Problem, or this is the famous 'sweet spot'?

Post by H-Alpha »

Bob Yoesle wrote: Sun Dec 05, 2021 2:39 pm Here's one possible source among many.
Thank you very much Bob!

Now that you showed me what this is, I found one at good price (tube and transformer) in the UK and will try to import it smoothly, hopefully, to the EU. Here are the links of the eshop in ebay if someone else interested:
https://www.ebay.com/itm/144290793498?_ ... 7024262518

https://www.ebay.com/itm/362541409324?e ... BMyva_07Ff

Will try to follow your instructions here and in the Cloudy Nights post.

Thanks once more,
Alexandros


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