Weird Artifacts when shooting with SCT

bigCatAstro · Sep 22, 2025, 04:48 PM

Zhaoyu Xu · Sep 22, 2025, 04:33 PM

Hi, I’ve noticed I get a weird border artifacts on all sides of the picture when shooting with my SCT. I did not have this problem when I use the same camera/filter on a refractor. Anyone know what is causing this and how to fix it? I’ve shot multiple targets with the SCT and all seems to have this problem

The telescope is Celestron 925 Edge HD and the camera is ASI2600MC

📷 NGC7331.jpg

What do your flats look like? It appears, to me at least, that there’s something going on with the camera glass like dewing or fogging.

Here is one of the flats. I’ve used NINA‘s flat wizard to take it. I do also have a dew heater strap running at around 50% on the front of SCT. Do I also need one at the mirror at the back? Should I increase it to 100%? I also had the anti dew running on the ASI2600 as well

📷 FlatWizard_2025-09-15_20-59-31_S_-9.80_4.70s_0003.jpg

Zhaoyu Xu · Sep 22, 2025, 05:01 PM

bigCatAstro · Sep 22, 2025, 04:48 PM

Zhaoyu Xu · Sep 22, 2025, 04:33 PM

Hi, I’ve noticed I get a weird border artifacts on all sides of the picture when shooting with my SCT. I did not have this problem when I use the same camera/filter on a refractor. Anyone know what is causing this and how to fix it? I’ve shot multiple targets with the SCT and all seems to have this problem

The telescope is Celestron 925 Edge HD and the camera is ASI2600MC

📷 NGC7331.jpg

What do your flats look like? It appears, to me at least, that there’s something going on with the camera glass like dewing or fogging.

Here is one of the flats. I’ve used NINA‘s flat wizard to take it. I do also have a dew heater strap running at around 50% on the front of SCT. Do I also need one at the mirror at the back? Should I increase it to 100%? I also had the anti dew running on the ASI2600 as well

📷 FlatWizard_2025-09-15_20-59-31_S_-9.80_4.70s_0003.jpg

This flat doesn’t look right, too bright and not capturing the problems with the imaging train. It should be just bright enough to illuminate the dust motes and other imperfections.

bigCatAstro · Sep 22, 2025, 05:13 PM

Zhaoyu Xu · Sep 22, 2025, 05:01 PM

bigCatAstro · Sep 22, 2025, 04:48 PM

Zhaoyu Xu · Sep 22, 2025, 04:33 PM

Hi, I’ve noticed I get a weird border artifacts on all sides of the picture when shooting with my SCT. I did not have this problem when I use the same camera/filter on a refractor. Anyone know what is causing this and how to fix it? I’ve shot multiple targets with the SCT and all seems to have this problem

The telescope is Celestron 925 Edge HD and the camera is ASI2600MC

📷 NGC7331.jpg

What do your flats look like? It appears, to me at least, that there’s something going on with the camera glass like dewing or fogging.

Here is one of the flats. I’ve used NINA‘s flat wizard to take it. I do also have a dew heater strap running at around 50% on the front of SCT. Do I also need one at the mirror at the back? Should I increase it to 100%? I also had the anti dew running on the ASI2600 as well

📷 FlatWizard_2025-09-15_20-59-31_S_-9.80_4.70s_0003.jpg

This flat doesn’t look right, too bright and not capturing the problems with the imaging train. It should be just bright enough to illuminate the dust motes and other imperfections.

In this case, I need to decrease the power of the light pad then? But wouldn’t the Flat wizard from NINA auto calculate the exposure time so that it would not be as illuminated?

bigCatAstro · Sep 22, 2025, 05:18 PM

Zhaoyu Xu · Sep 22, 2025, 05:15 PM

bigCatAstro · Sep 22, 2025, 05:13 PM

Zhaoyu Xu · Sep 22, 2025, 05:01 PM

bigCatAstro · Sep 22, 2025, 04:48 PM

Zhaoyu Xu · Sep 22, 2025, 04:33 PM

Hi, I’ve noticed I get a weird border artifacts on all sides of the picture when shooting with my SCT. I did not have this problem when I use the same camera/filter on a refractor. Anyone know what is causing this and how to fix it? I’ve shot multiple targets with the SCT and all seems to have this problem

The telescope is Celestron 925 Edge HD and the camera is ASI2600MC

📷 NGC7331.jpg

What do your flats look like? It appears, to me at least, that there’s something going on with the camera glass like dewing or fogging.

Here is one of the flats. I’ve used NINA‘s flat wizard to take it. I do also have a dew heater strap running at around 50% on the front of SCT. Do I also need one at the mirror at the back? Should I increase it to 100%? I also had the anti dew running on the ASI2600 as well

📷 FlatWizard_2025-09-15_20-59-31_S_-9.80_4.70s_0003.jpg

This flat doesn’t look right, too bright and not capturing the problems with the imaging train. It should be just bright enough to illuminate the dust motes and other imperfections.

In this case, I need to decrease the power of the light pad then? But wouldn’t the Flat wizard from NINA auto calculate the exposure time so that it would not be as illuminated?

I would start there, lowering the brightness, since the flat looks basically white on my processing laptop. I never use NINA auto calculate for flats since it gives me really odd numbers. I take an iterative approach to finding the right brightness—granted I am using a DSD lid that I can control the brightness steps.

So the border artifact is related the flats then not really issue with dew on the corrector lens or the mirror?

Zhaoyu Xu · Sep 22, 2025, 05:21 PM

bigCatAstro · Sep 22, 2025, 05:18 PM

Zhaoyu Xu · Sep 22, 2025, 05:15 PM

bigCatAstro · Sep 22, 2025, 05:13 PM

Zhaoyu Xu · Sep 22, 2025, 05:01 PM

bigCatAstro · Sep 22, 2025, 04:48 PM

Zhaoyu Xu · Sep 22, 2025, 04:33 PM

Hi, I’ve noticed I get a weird border artifacts on all sides of the picture when shooting with my SCT. I did not have this problem when I use the same camera/filter on a refractor. Anyone know what is causing this and how to fix it? I’ve shot multiple targets with the SCT and all seems to have this problem

The telescope is Celestron 925 Edge HD and the camera is ASI2600MC

📷 NGC7331.jpg

What do your flats look like? It appears, to me at least, that there’s something going on with the camera glass like dewing or fogging.

Here is one of the flats. I’ve used NINA‘s flat wizard to take it. I do also have a dew heater strap running at around 50% on the front of SCT. Do I also need one at the mirror at the back? Should I increase it to 100%? I also had the anti dew running on the ASI2600 as well

📷 FlatWizard_2025-09-15_20-59-31_S_-9.80_4.70s_0003.jpg

This flat doesn’t look right, too bright and not capturing the problems with the imaging train. It should be just bright enough to illuminate the dust motes and other imperfections.

In this case, I need to decrease the power of the light pad then? But wouldn’t the Flat wizard from NINA auto calculate the exposure time so that it would not be as illuminated?

I would start there, lowering the brightness, since the flat looks basically white on my processing laptop. I never use NINA auto calculate for flats since it gives me really odd numbers. I take an iterative approach to finding the right brightness—granted I am using a DSD lid that I can control the brightness steps.

So the border artifact is related the flats then not really issue with dew on the corrector lens or the mirror?

I would start with squaring away the flats first and if that doesn’t solve it, go to the next step. There may have been some fogging/dewing at some point in time and now it’s just a part of the imaging train. If proper flats are taken it should correct them out of the image as @andrea tasselli also suggested.

Derek Mellen · Sep 22, 2025, 07:17 PM

A few people in my astronomy club have that telescope. I don’t know if you are using a reducer. They get dew on the reducer and need to use a dew heater strip on the reducer. It does look like your flats could be off.

yes I do have the reducer, but I get the same issue without the reducer. So it might not be the issue for now

John Tucker · Sep 23, 2025, 10:39 AM

Agree with the comments above that your flats are overexposed. The underlying hardware issue is likely something to do with condensation on your camera sensor, as the fogging effect creates a rectangular border around the edges of the image. Everything else in your imaging train is round and would produce a typical vignetting pattern with a circular “hole” of higher exposure in the middle of the image.

You might check to be sure that your sensor isn’t exposed to ambient air and try cooling the sensor less or more slowly.

but if its the sensor being fogged up, would that also fog up in a different telescope? I used the same camera on my refractors and it was fine. I do have the ASI camera anti dew on in both cases. Since I use asi air, I don’t think there is a way to control the cooling speed and I set the temp to -10C.

Zhaoyu Xu · Sep 23, 2025, 01:49 PM

John Tucker · Sep 23, 2025, 10:39 AM

Agree with the comments above that your flats are overexposed. The underlying hardware issue is likely something to do with condensation on your camera sensor, as the fogging effect creates a rectangular border around the edges of the image. Everything else in your imaging train is round and would produce a typical vignetting pattern with a circular “hole” of higher exposure in the middle of the image.

You might check to be sure that your sensor isn’t exposed to ambient air and try cooling the sensor less or more slowly.

but if its the sensor being fogged up, would that also fog up in a different telescope? I used the same camera on my refractors and it was fine. I do have the ASI camera anti dew on in both cases. Since I use asi air, I don’t think there is a way to control the cooling speed and I set the temp to -10C.

I see your point but its hard to come up with an alternative explanation for the rectangular pattern. Perhaps different ambient humidity on this occasion? Or connection of the camera through a plastic connector that conducts heat less well than a metal one used with your other telescopes?

bigCatAstro · Sep 23, 2025, 02:30 PM

Zhaoyu Xu · Sep 23, 2025, 01:49 PM

John Tucker · Sep 23, 2025, 10:39 AM

Agree with the comments above that your flats are overexposed. The underlying hardware issue is likely something to do with condensation on your camera sensor, as the fogging effect creates a rectangular border around the edges of the image. Everything else in your imaging train is round and would produce a typical vignetting pattern with a circular “hole” of higher exposure in the middle of the image.

You might check to be sure that your sensor isn’t exposed to ambient air and try cooling the sensor less or more slowly.

but if its the sensor being fogged up, would that also fog up in a different telescope? I used the same camera on my refractors and it was fine. I do have the ASI camera anti dew on in both cases. Since I use asi air, I don’t think there is a way to control the cooling speed and I set the temp to -10C.

I think the answer really has to do with your flats. The flats should be exposed uniquely for each differing imaging train and filters. I’ve never found NINA’s flats wizard to work exactly as needed for exposing flats. For my set-up, it produces a flat that is either over or under exposed. I’ve had to iteratively increase or decrease brightness on my lid until I reached the desired peak range.

A side note, an easy visual check would be to disconnect the camera and look at the senor window under light, I’m going to guess that you’ll see that “fogging” pattern under the glass. If you’re flats are calibrated right, they should get rid of the artifacts.

Ok, I checked the sensor and it was all looking normal. The question now I have is that if I were to take my flats not right after the imaging session, but say the next day when my scope is indoors, would the flat be able to solve the issue?

Zhaoyu Xu · Sep 23, 2025, 04:29 PM

bigCatAstro · Sep 23, 2025, 02:30 PM

Zhaoyu Xu · Sep 23, 2025, 01:49 PM

John Tucker · Sep 23, 2025, 10:39 AM

Agree with the comments above that your flats are overexposed. The underlying hardware issue is likely something to do with condensation on your camera sensor, as the fogging effect creates a rectangular border around the edges of the image. Everything else in your imaging train is round and would produce a typical vignetting pattern with a circular “hole” of higher exposure in the middle of the image.

You might check to be sure that your sensor isn’t exposed to ambient air and try cooling the sensor less or more slowly.

but if its the sensor being fogged up, would that also fog up in a different telescope? I used the same camera on my refractors and it was fine. I do have the ASI camera anti dew on in both cases. Since I use asi air, I don’t think there is a way to control the cooling speed and I set the temp to -10C.

I think the answer really has to do with your flats. The flats should be exposed uniquely for each differing imaging train and filters. I’ve never found NINA’s flats wizard to work exactly as needed for exposing flats. For my set-up, it produces a flat that is either over or under exposed. I’ve had to iteratively increase or decrease brightness on my lid until I reached the desired peak range.

A side note, an easy visual check would be to disconnect the camera and look at the senor window under light, I’m going to guess that you’ll see that “fogging” pattern under the glass. If you’re flats are calibrated right, they should get rid of the artifacts.

Ok, I checked the sensor and it was all looking normal. The question now I have is that if I were to take my flats not right after the imaging session, but say the next day when my scope is indoors, would the flat be able to solve the issue?

Probably not, but it might work. I take flats after each imaging target session, so others can weigh in if trying your idea of taking flats indoors after the session will work.

Typically you should take your flats during the session (before or after imaging) or in the morning with a diffused sky flat. If you’re going to use a wall for flats, really they should be done either before or after the imaging session. Likewise, you don’t want to move the imaging train (or as little as possible) before taking the flats. Again, I use a Deep Sky Dad automated flat panel lid, so I have NINA take my flats at the end of each target session.

John Tucker · Sep 23, 2025 at 05:47 PM

bigCatAstro · Sep 23, 2025, 05:30 PM

Zhaoyu Xu · Sep 23, 2025, 04:29 PM

bigCatAstro · Sep 23, 2025, 02:30 PM

Zhaoyu Xu · Sep 23, 2025, 01:49 PM

John Tucker · Sep 23, 2025, 10:39 AM

Agree with the comments above that your flats are overexposed. The underlying hardware issue is likely something to do with condensation on your camera sensor, as the fogging effect creates a rectangular border around the edges of the image. Everything else in your imaging train is round and would produce a typical vignetting pattern with a circular “hole” of higher exposure in the middle of the image.

You might check to be sure that your sensor isn’t exposed to ambient air and try cooling the sensor less or more slowly.

but if its the sensor being fogged up, would that also fog up in a different telescope? I used the same camera on my refractors and it was fine. I do have the ASI camera anti dew on in both cases. Since I use asi air, I don’t think there is a way to control the cooling speed and I set the temp to -10C.

I think the answer really has to do with your flats. The flats should be exposed uniquely for each differing imaging train and filters. I’ve never found NINA’s flats wizard to work exactly as needed for exposing flats. For my set-up, it produces a flat that is either over or under exposed. I’ve had to iteratively increase or decrease brightness on my lid until I reached the desired peak range.

A side note, an easy visual check would be to disconnect the camera and look at the senor window under light, I’m going to guess that you’ll see that “fogging” pattern under the glass. If you’re flats are calibrated right, they should get rid of the artifacts.

Ok, I checked the sensor and it was all looking normal. The question now I have is that if I were to take my flats not right after the imaging session, but say the next day when my scope is indoors, would the flat be able to solve the issue?

Probably not, but it might work. I take flats after each imaging target session, so others can weigh in if trying your idea of taking flats indoors after the session will work.

Typically you should take your flats during the session (before or after imaging) or in the morning with a diffused sky flat. If you’re going to use a wall for flats, really they should be done either before or after the imaging session. Likewise, you don’t want to move the imaging train (or as little as possible) before taking the flats. Again, I use a Deep Sky Dad automated flat panel lid, so I have NINA take my flats at the end of each target session.

I mean, if the “fogged” area is rectangular, it pretty much has to be these sensor, right? Because there is no other rectangular orifice in the light path. I don’t think checking it the next day rules out the possibility of condensation on the sensor the previous night, as it will evaporate in warmer temperatures.

The flats are “the issue” in the sense that properly exposed flats should correct for the underlying problem, which is fogging of the sensor.

I apologize if this comment strayed into the “who is right” territory, the forums here are mostly blissfully free of that sort of thing. I’m just thinking that understanding the problem is the best way to avoid it in the future. There has been some discussion of this issue on that other, unspeakable astronomy forum…. https://www.cloudynights.com/topic/843215-do-cooled-cameras-dew-up/

Sure, these points all make sense and probably get closer to a root cause.