Fringes caused by the CCD chip cover plate....
- Merlin66
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Fringes caused by the CCD chip cover plate....
This article may be of interest to those members interested in the development of "fringes" in their images..
Comparison study of three different cameras...
http://www.astrosurf.com/aras/fringing/ ... ripple.htm
Comparison study of three different cameras...
http://www.astrosurf.com/aras/fringing/ ... ripple.htm
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
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"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer
https://groups.io/g/astronomicalspectroscopy
http://astronomicalspectroscopy.com
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer
- swisswalter
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Re: Fringes caused by the CCD chip cover plate....
Hi Ken
thank you very much for the article. Well tilting seems to be the solution to get rid of the NR, something which we have reported from the field experience
thank you very much for the article. Well tilting seems to be the solution to get rid of the NR, something which we have reported from the field experience
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Re: Fringes caused by the CCD chip cover plate....
Thanks for the info and link, Ken. Very timely (for me)!
I've just begun some field testing with my new SHG and was pondering these fringes. Here are sample video frames taken near H-alpha and H-beta. The fringes are especially evident on the H-beta spectrum.
At the moment, these fringes are the least of my worries! I have a terribly uneven field illumination which, I believe, is being caused mostly by the central obstruction in my catadioptric telescope lens at the high f# of my device. Here's a sample H-alpha image.
It shows promise with the spectral resolution (around 0.1 Angstrom fwhm bandwidth) but I have to overcome the uneven field. I'm setting up a new telescope for that purpose.
Cheers.
Peter.
I've just begun some field testing with my new SHG and was pondering these fringes. Here are sample video frames taken near H-alpha and H-beta. The fringes are especially evident on the H-beta spectrum.
At the moment, these fringes are the least of my worries! I have a terribly uneven field illumination which, I believe, is being caused mostly by the central obstruction in my catadioptric telescope lens at the high f# of my device. Here's a sample H-alpha image.
It shows promise with the spectral resolution (around 0.1 Angstrom fwhm bandwidth) but I have to overcome the uneven field. I'm setting up a new telescope for that purpose.
Cheers.
Peter.
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Re: Fringes caused by the CCD chip cover plate....
Peter, can the uneven brightness be caused by uneven slit width? Can you show us your SHG image before post processing?
I noticed the NR before, and I think it on SHG images is not as important as on the H-a filter images.
It will be removed by flat field process anyway.
I noticed the NR before, and I think it on SHG images is not as important as on the H-a filter images.
It will be removed by flat field process anyway.
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Re: Fringes caused by the CCD chip cover plate....
Wah:
The slit width is quite uniform, apart from the roughness and dust giving rise to the horizontal "transversalium" lines. You can tell by the uniformity of the spectral lines in the attached video frames. The instrument behaves quite well "behind" the slit, but, when I use the mirror-lens telephoto, I run into problems. I never noticed this previously at lower system f# but it is quite apparent now at around f30. When I attach the mirror-lens to a SLR camera the effect of the obstruction is strongly visible in the viewfinder of my old film Pentax but not so visible in my Canon 5D DSLR. I'm a little puzzled and disheartened by it. I'm hoping a simpler refractor telescope will cure the problem.
I agree that the fringes should not be too problematic for SHG image generation. They run in the same direction you "slice" out of the spectrum. There does appear to be another fringe pattern running at an angle to the spectral lines. I'll have to investigate further. Flat fielding should get rid of these.
The slit width is quite uniform, apart from the roughness and dust giving rise to the horizontal "transversalium" lines. You can tell by the uniformity of the spectral lines in the attached video frames. The instrument behaves quite well "behind" the slit, but, when I use the mirror-lens telephoto, I run into problems. I never noticed this previously at lower system f# but it is quite apparent now at around f30. When I attach the mirror-lens to a SLR camera the effect of the obstruction is strongly visible in the viewfinder of my old film Pentax but not so visible in my Canon 5D DSLR. I'm a little puzzled and disheartened by it. I'm hoping a simpler refractor telescope will cure the problem.
I agree that the fringes should not be too problematic for SHG image generation. They run in the same direction you "slice" out of the spectrum. There does appear to be another fringe pattern running at an angle to the spectral lines. I'll have to investigate further. Flat fielding should get rid of these.
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Re: Fringes caused by the CCD chip cover plate....
What kind of slit are you using? Mechanical or glass plate kind?
Can the source of problem be found by using a mask with a hole moving around the front of telescope/lens?
Can the source of problem be found by using a mask with a hole moving around the front of telescope/lens?
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Re: Fringes caused by the CCD chip cover plate....
Hi all
that tunrns out to be a very interesting thread
that tunrns out to be a very interesting thread
Only stardust in the wind, some fine and some less fine scopes, filters and adapters as well. Switzerland 47 N, 9 E, in the heart of EUROPE
from 7 am - 7 pm http://www.nanosys.ch
from 7.01 pm - 6.59 am http://www.wastronomiko.com some times vice versa
from 7 am - 7 pm http://www.nanosys.ch
from 7.01 pm - 6.59 am http://www.wastronomiko.com some times vice versa
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Re: Fringes caused by the CCD chip cover plate....
Peter,
Your problem is a classic one, it appears you don't have a field lens at the slit. If the objective (entrance pupil) is not imaged onto the collimator or Littrow lens (depending on the design) then the grating will be fully illuminated only at the center of the field. Rays originating off axis will fail to intersect the grating and be lost. A simple lens located at (or very close to) the slit will image the objective onto the grating if its focal length is correct. The focal length of field lenses is not super critical, so a stock lens can usually be used.
Joe
Your problem is a classic one, it appears you don't have a field lens at the slit. If the objective (entrance pupil) is not imaged onto the collimator or Littrow lens (depending on the design) then the grating will be fully illuminated only at the center of the field. Rays originating off axis will fail to intersect the grating and be lost. A simple lens located at (or very close to) the slit will image the objective onto the grating if its focal length is correct. The focal length of field lenses is not super critical, so a stock lens can usually be used.
Joe
Observing the Sun with complex optical systems since 1966, and still haven't burned, melted or damaged anything.
Not blind yet, either!
Light pollution? I only observe the Sun, magnitude -26.74. Pollute that!
Not blind yet, either!
Light pollution? I only observe the Sun, magnitude -26.74. Pollute that!
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Re: Fringes caused by the CCD chip cover plate....
Hi Joe.
Thanks for the advice. In fact, based on your earlier recommendations of a field lens, I actually did configure an arrangement to incorporate one with inconclusive results, so far.
Although it's not apparent from the image I posted, my problem is actually the "inverse" of what you describe! The centre of the field is dark (which I attribute to the central obstruction), getting brighter as you move to the periphery. I tried to alleviate this a little by twisting the alignment of the optics but that solution is far from acceptable and gives the vignetted result in the image above. As I mentioned previously in the thread, the obstruction effect is also strongly evident in the viewfinder when I simply use the "mirror-lens" by itself on a SLR camera.
I'm going to try different telescope optics to see what happens.
I think I'll play with the field lens idea as well. At worst, I can gain some light throughput.
Peter.
Thanks for the advice. In fact, based on your earlier recommendations of a field lens, I actually did configure an arrangement to incorporate one with inconclusive results, so far.
Although it's not apparent from the image I posted, my problem is actually the "inverse" of what you describe! The centre of the field is dark (which I attribute to the central obstruction), getting brighter as you move to the periphery. I tried to alleviate this a little by twisting the alignment of the optics but that solution is far from acceptable and gives the vignetted result in the image above. As I mentioned previously in the thread, the obstruction effect is also strongly evident in the viewfinder when I simply use the "mirror-lens" by itself on a SLR camera.
I'm going to try different telescope optics to see what happens.
I think I'll play with the field lens idea as well. At worst, I can gain some light throughput.
Peter.
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Re: Fringes caused by the CCD chip cover plate....
Hi Peter, how about my previous suggestion of using a mask to find out light from which parts of the scope won't go through the 2 sides of the slit?
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Re: Fringes caused by the CCD chip cover plate....
Hi Wah.
It's a good suggestion but I think I'll try simpler telescope optics first. (I've already dismantled!)
I want to get back to the catadioptric ultimately. We'll see what happens.
Peter.
It's a good suggestion but I think I'll try simpler telescope optics first. (I've already dismantled!)
I want to get back to the catadioptric ultimately. We'll see what happens.
Peter.
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Re: Fringes caused by the CCD chip cover plate....
I'm confused, this image:
Shows the classic missing or wrong field lens.
This one, already processed:
is harder to interpret, but shows less detail at the top and bottom edge, as though there is poorer signal to noise there, like if the signal is low. A raw image would be revealing. If the image is actually dark in the middle and you think it is an artifact of the central obstruction in the objective, then the objective pupil is being imaged at or near the final image plane. It should be at the grating. Wah's suggestion will yield some information, you should also check to see if the objective's pupil is imaged at the grating, the catadioptric's secondary should be apparent there, or an object placed near the aperture will do.
Shows the classic missing or wrong field lens.
This one, already processed:
is harder to interpret, but shows less detail at the top and bottom edge, as though there is poorer signal to noise there, like if the signal is low. A raw image would be revealing. If the image is actually dark in the middle and you think it is an artifact of the central obstruction in the objective, then the objective pupil is being imaged at or near the final image plane. It should be at the grating. Wah's suggestion will yield some information, you should also check to see if the objective's pupil is imaged at the grating, the catadioptric's secondary should be apparent there, or an object placed near the aperture will do.
Observing the Sun with complex optical systems since 1966, and still haven't burned, melted or damaged anything.
Not blind yet, either!
Light pollution? I only observe the Sun, magnitude -26.74. Pollute that!
Not blind yet, either!
Light pollution? I only observe the Sun, magnitude -26.74. Pollute that!
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Re: Fringes caused by the CCD chip cover plate....
Hi Joe.
The images I posted were the result of a "skewed" optical alignment in which I tried to circumvent the problem with the obstruction. The dark at the upper edge of the spectrum is the obstruction becoming visible. Unfortunately, I didn't save any of the images where the effect of the obstruction is obvious. Your suggestion that the objective pupil is being imaged at the final image plane is probably correct but I haven't come up with a remedy yet - possibly the right field lens would solve this problem.
Peter.
The images I posted were the result of a "skewed" optical alignment in which I tried to circumvent the problem with the obstruction. The dark at the upper edge of the spectrum is the obstruction becoming visible. Unfortunately, I didn't save any of the images where the effect of the obstruction is obvious. Your suggestion that the objective pupil is being imaged at the final image plane is probably correct but I haven't come up with a remedy yet - possibly the right field lens would solve this problem.
Peter.