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Identifying defects in photo image

Astrophotography
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#1 Peter Schmitz

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Posted 14 May 2022 - 08:14 PM

Hi, I'm experiencing defects in my photos and cannot identify the origin to fix the issue.  I have 5-7 rings and 2 streaks.  I have a Celestron EdgeHD 9.25, 0.7 reducer lens and ZWO533 camera.  I have attached a photo to help with troubleshooting.  Any ideas where to start?  I have cleaned the corrector plate, reducer lens and the camera but remains the same.  Many thanks for any help.

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  • anomalies.jpg


#2 ram812

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Posted 14 May 2022 - 08:20 PM

Hello! Don't know about the streaks but those donuts look like dust motes or internal reflections hitting multiple places inside the OTA. Are you correcting with flat and dark frames? I assume you are...nice capture, though.

CS, RAM
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#3 idclimber

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Posted 14 May 2022 - 08:26 PM

Those are donuts and are totally normal. They are corrected by using flat frames and calibration. This includes darks and bias frames for your 533. Once you start calibrating the images those will disappear. I am guessing the streaks are part of processing uncalibrated data, but the key is too go back in the history and find out when they first appeared. If they are not in the original subs then something added them. 


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#4 Peter Schmitz

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Posted 14 May 2022 - 08:31 PM

I only started this week and they have been there from the start.  I did stack in DSS with darks but not flats so I'll try that next.  Thanks, good to hear that the donuts are normal.


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#5 idclimber

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Posted 14 May 2022 - 08:36 PM

Just taking a second look and I am convinced those streaks are a series of donuts. Cleaning will not fix this. You will only destroy the optics on your nice scope. I have cleaned the corrector plate on my SCT once in 10 years. Yea it is full of dust and probably needs to be done again. 

 

I only use a blower on my optics routinely and only plan to do more aggressive cleaning when flats no longer work. Which so far is never. 


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#6 idclimber

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Posted 14 May 2022 - 08:39 PM

I only started this week and they have been there from the start.  I did stack in DSS with darks but not flats so I'll try that next.  Thanks, good to hear that the donuts are normal.

Very nice start, once you add full calibration you will be quite please. You have nice details in that image and stars look pretty good for one week. 


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#7 Peter Schmitz

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Posted 14 May 2022 - 08:42 PM

Just taking a second look and I am convinced those streaks are a series of donuts. Cleaning will not fix this. You will only destroy the optics on your nice scope. I have cleaned the corrector plate on my SCT once in 10 years. Yea it is full of dust and probably needs to be done again. 

 

I only use a blower on my optics routinely and only plan to do more aggressive cleaning when flats no longer work. Which so far is never. 

I'll make sure to make some flats on my next opportunity.  The clouds just decided to roll in after a totally clear day .  Many thanks for the advice from you experienced folks!



#8 NMCN

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Posted 14 May 2022 - 09:55 PM

You could try using synthetic flats to see if they will correct the issue while you wait for a chance to take regular flats.  I followed this tutorial to create synthetic flats to fix an issue that regular flats weren't taking care of.  Worked like a champ.


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#9 Peter Schmitz

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Posted 15 May 2022 - 07:22 PM

You could try using synthetic flats to see if they will correct the issue while you wait for a chance to take regular flats.  I followed this tutorial to create synthetic flats to fix an issue that regular flats weren't taking care of.  Worked like a champ.

Hi, thanks.  I tried this on a portion of the picture with only stars as I didn't see a tutorial on doing this with a galaxy.  It did work like a champ.

 

I did try to make flats today but it didn't do anything to clean up the donuts.  It may be because I had removed the camera since the session on Wed and when I put it on today it was probably not in the same position.  I tried to view the master flat in PI but was not able to detect any defects in the picture when I stretched it.  Is there a way to view the flat with defects?  Then I might be able to orient the stacked master file to work with my lights. 

 

Thanks again.



#10 Alex McConahay

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Posted 15 May 2022 - 07:51 PM

>>>>>It may be because I had removed the camera since the session on Wed and when I put it on today it was probably not in the same position.

 

Could be.....but then where you had dark dust donuts before, you would not be getting light dust donuts instead. 

 

As far as seeing them, a good extreme stretch should show where the dust bunnies are. But I you would have to go to Photoshop/layers/transform/rotate to get the things to match.....and that may be real difficult. And then, you are probably matching the bunnies but not doing much for the other pixels the flat is supposed to be correcting. 

 

Some other techniques---reverse the image That is, make the blacks white and vice versa. It is called different things in different programs. But is it is a good way to spot aberrations.

 

You can also use threshold (or binarize). YOu display the image, and then move a slider (or otherwise specify a breakpoint) and everything above that point is white, everything below is black. And move the breakpoint back and forth until the dust spots show on your flats. 

 

Good luck.

 

Alex




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