Help with Imaging

[Leti Theobald]

Hi everyone. Would someone be able to help me with an issue I’m having with my subs - I had a friend also stack it for me in case it was my stacking but had the same issue. 
This is what my image looks like - there is a colour ring around it. I’m using the below gear:

Skyways 80050 telescope - 80 mm aperture and 500 mm focal length

Canon EOS 600d Astromodified 

Skywatcher HEQ5 Mount

Optolong L-eNhance Tri Band Deep Sky Filter 1.25

T Ring adaptor 

ISO 800 and 100” exposure (around 60 frames)  

we stacked both using bias, darks and flats and also with just lights and the result is the same

Thank you so much!

 

[happy-kat]

Do you get the same ring if you don't use a filter?

[Leti Theobald]

This is a histogram super stretched picture without the filter . So still there. This is just the Astro modified camera with loads of light pollution 

[City9Town0]

Is it visible in the single un-corrected frames? Does it move slightly over time such as light from a street lamp entering?

[Leti Theobald]

This is a single frame stretched 

[Leti Theobald]

Stacked Tif image attachedStacker Horsehead.tif

[Leti Theobald]

r_bkg_pp_lights_stacked.fitAnd Siril stacked file also attached

[Leti Theobald]

23 minutes ago, City9Town0 said:

Is it visible in the single un-corrected frames? Does it move slightly over time such as light from a street lamp entering?

it could be light from somewhere, I can't quite think where - might trial another from a different side of the garden and see if it changes

[ONIKKINEN]

8 minutes ago, Leti Theobald said:

Stacked Tif image attachedStacker Horsehead.tif

2 main reasons for the ring. The first is that you are using a 1.25'' filter with an APS-C sized camera, so there is severe vignetting. You need bigger filters for APS-C sized chips to get less vignetting.

The second, and what is the bigger issue here, is that your flats have not worked. Looks like dust donuts are still visible so flats are not working, and the ring artifact would be also gone had flats worked.

How did you take the flats? Did you take them immediately after imaging, or better yet during imaging without first moving the camera or touching anything on the scope in any way. And also, how is the focuser on the scope, if it cant keep the camera sturdily in all orientations you will never have proper flats.

[Leti Theobald]

31 minutes ago, ONIKKINEN said:

2 main reasons for the ring. The first is that you are using a 1.25'' filter with an APS-C sized camera, so there is severe vignetting. You need bigger filters for APS-C sized chips to get less vignetting.

The second, and what is the bigger issue here, is that your flats have not worked. Looks like dust donuts are still visible so flats are not working, and the ring artifact would be also gone had flats worked.

How did you take the flats? Did you take them immediately after imaging, or better yet during imaging without first moving the camera or touching anything on the scope in any way. And also, how is the focuser on the scope, if it cant keep the camera sturdily in all orientations you will never have proper flats.

Thank you!

My telescope only fits 1.25" lens so I'm a bit stuck with that for now.

For the flats, I took just after imaging - my current technique is white shirt with an elastic band to hold it tight and a laptop screen on max brightness white screen. Camera on AV at the same ISO.

The camera feels quite sturdy but it's not an amazing telescope, I know I will have to upgrade soon

[ONIKKINEN]

14 minutes ago, Leti Theobald said:

Thank you!

My telescope only fits 1.25" lens so I'm a bit stuck with that for now.

For the flats, I took just after imaging - my current technique is white shirt with an elastic band to hold it tight and a laptop screen on max brightness white screen. Camera on AV at the same ISO.

The camera feels quite sturdy but it's not an amazing telescope, I know I will have to upgrade soon

No flats could work better than badly working flats, try without the flats?

Also you will need to crop the image to a size that is inside the ring this way but you may have needed to do that anyway because of the field curvature (also the image is slightly out of focus on the red channel and completely out of focus on the green/blue channel because of chromatic aberration but not much you could do about this)

[Leti Theobald]

30 minutes ago, ONIKKINEN said:

No flats could work better than badly working flats, try without the flats?

Also you will need to crop the image to a size that is inside the ring this way but you may have needed to do that anyway because of the field curvature (also the image is slightly out of focus on the red channel and completely out of focus on the green/blue channel because of chromatic aberration but not much you could do about this)

I’m sure I can still do some work on my focus, still learning how to get it spot on! 
Without flats didn’t look that great either. Do you have any tips on how to take them?

[ONIKKINEN]

24 minutes ago, Leti Theobald said:

I’m sure I can still do some work on my focus, still learning how to get it spot on! 
Without flats didn’t look that great either. Do you have any tips on how to take them?

The method of taking the flats you mentioned sounds ok to me. Only thing to make sure is that there is no screen flicker with the laptop (probably not a problem) and if there is the exposure needs to be long enough to even the flicker out but you can just add more interlayers with t-shirts or printer paper or something like that in that case (if the screen flickers try to make the automatic exposure at least 1/2s, that should even everything out).

The mechanical accuracy and stability of everything between the lens cell and the camera sensor are what make or break flats, and just looking at the scope i would assume the focuser to be the weak link here. If the focuser can wobble from one side of the sky to the next it also means the vignetting profile changes, so flats become impossible to match to lights. Its a problem even with good scopes sometimes, the focuser really needs to be much better than just ok to be reliable for astrophotography so this might not be something you can fix easily. Looks like it might have a tension adjuster? If it does, tension it so that you can only barely move the focuser in and out to make it more stable.

[Leti Theobald]

21 minutes ago, ONIKKINEN said:

The method of taking the flats you mentioned sounds ok to me. Only thing to make sure is that there is no screen flicker with the laptop (probably not a problem) and if there is the exposure needs to be long enough to even the flicker out but you can just add more interlayers with t-shirts or printer paper or something like that in that case (if the screen flickers try to make the automatic exposure at least 1/2s, that should even everything out).

The mechanical accuracy and stability of everything between the lens cell and the camera sensor are what make or break flats, and just looking at the scope i would assume the focuser to be the weak link here. If the focuser can wobble from one side of the sky to the next it also means the vignetting profile changes, so flats become impossible to match to lights. Its a problem even with good scopes sometimes, the focuser really needs to be much better than just ok to be reliable for astrophotography so this might not be something you can fix easily. Looks like it might have a tension adjuster? If it does, tension it so that you can only barely move the focuser in and out to make it more stable.

Thank you, I will try again using your tips!

[Elp]

In the absence of applied flats I've done this in around 30 minutes manually (the flats issue is still present but reduced), no painting as sometimes you need to paint out dust motes and stuff like the streak on the top LHS, image would help with a crop (i've left it so you can see the full frame) and more integration time to reduce average noise level and improve signal:

 

Edited February 10, 2023 by Elp

[Leti Theobald]

11 hours ago, Elp said:

In the absence of applied flats I've done this in around 30 minutes manually (the flats issue is still present but reduced), no painting as sometimes you need to paint out dust motes and stuff like the streak on the top LHS, image would help with a crop (i've left it so you can see the full frame) and more integration time to reduce average noise level and improve signal:

 

Thank you, maybe my flats are making it worse