Odd "circle" in imagery when taking photos - Forums

Topic starter

23 days ago

Hi all,

Starting with: This is a 12” newtonian (1200mm), with a Poseidon-M connected to 50mm (round) filters and an OAG-MAX (with a coma corrector) - 56mm back focus

So I’ve been banging my head against a wall trying to figure out what this is… I first noticed it when stacking some frames over a few nights worth of one of my targets. When finishing with integration and stacking, I got this really odd ‘circle’ around the center of my image (200 flats + dark flats with each filter):

📷 1.pngQuite confused with this, I decided to analyze my flats. The flats looked pretty normal and nothing I wouldn’t have expected, so I ruled out the idea that the flats were at fault. To further confirm this, I decided to integrate without the flats anyway to see what I would get… Here’s what I found (stretching it to emphasize the circle):

📷 2.pngMy thought process went to the fact that a few nights prior, there was some terrible dew and it managed to basically envelope my secondary. I thought “Okay it must be super dirty”. I brought the scope back in, cleaned the mirrors (which actually were not that dirty btw), and put it back together.

I also read online some people talking about “light leaking” in the back of the telescopes or through the focuser.

Well the focuser itself is flushed w/ the scope so there’s no light leak there (and these circles do not appear in darks at all).
I created a custom cover for the back of my telescope, which made sure to let the fan still have its opening to blow against the back of the primary, and holes surrounding the screws to collimate - but otherwise, there’s virtually no light leakage on the back.
I even got a new dew shield to help against trailing light around the edges, in case there were lights from around the house leaking in and reflecting off the interior walls or something

None of these solutions worked and I’m sort of out of ideas on what’s going on here. Tonight, I tried again with a single frame, and observed the exact same results (after stretching to look for it):

📷 3.pngI decided to inspect my old telescope setup (old 8” newtonian with a 1600mm, smaller imaging circle) and saw this ‘circle’ did not exist whatsoever in any of the images. I almost wonder if it’s actually the secondary mirror, but the imaging circle for this scope is advertised from Skywatcher as 28mm and the diagonal of the Poseidon-m is 28.3mm, which means to me there should theoretically be minimal vignetting (definitely not to this level anyway).

So with that - does anyone have any ideas what this could be and what’s causing it? I am totally blocked on taking photos with this setup because once I stack images, the circle cannot be removed even with flats apparently (although it DOES show up in the flats).

Well Written Helpful

Fabrice Lamidey

23 days ago

Hi,

I have had and seen that on RGB images from high light pollution areas.

It is usually reflections from stray lights.

A couple things to check :

If you use a coma corrector it may be protruding inside the tube and catch some stray light.
Your dew shield may be too short to prevent stray light from entering the front of the tube.

Helpful Concise Engaging Supportive

Dunk

23 days ago

It doesn’t quite look like it, but I could be frost on the camera sensor? This is quite common with cooled camera’s and dew… (I had it on my ASI294MM).

If that is what it is, then you (a) need to dry the camera out fully and (b) may need to replace or refresh the desiccant tablets.

Helpful Concise Engaging Supportive

texasastrophotography

Topic starter

23 days ago

Fabrice Lamidey · Mar 13, 2026, 02:18 AM
Hi,
I have had and seen that on RGB images from high light pollution areas.
It is usually reflections from stray lights.
A couple things to check :
If you use a coma corrector it may be protruding inside the tube and catch some stray light.
Your dew shield may be too short to prevent stray light from entering the front of the tube.

Hi Fabrice,

Thanks for the mention. I’m actually out in the country with not a lot of light pollution (once upon a time it was bortel 2-3, but now it’s bortel 4). My dew shield was primarily to see if it’d help w/ dew issues, but decided to use it as a helper for when the moon came out (no moon out right now). That being said, it’s 22” in length and it’d be really surprising if any stray light managed to get in there at all.

Dunk · Mar 13, 2026, 02:23 AM
It doesn’t quite look like it, but I could be frost on the camera sensor? This is quite common with cooled camera’s and dew… (I had it on my ASI294MM).
If that is what it is, then you (a) need to dry the camera out fully and (b) may need to replace or refresh the desiccant tablets.

The latest/bottom image from tonight is actually uncooled (no fan running at all).

Thought I’d also share real quick that I pulled out my old asi 1600mm to see how it’d look with this… The circle’s still there but takes up a larger part of the frame, which makes sense since the diagonal is smaller on the 1600mm (again, uncooled and stretched):

📷 image.pngSo I can at least confirm the imaging train itself is probably not at fault… Which begs the question of “what” would do that for this scope (especially as it’s supposedly an imaging newt).

The only "modification” I have to it between the camera and mirrors otherwise is a mirror mask, but the mirror mask is not only non-reflective but only covers the mirror clips - something I had on my old telescope which did not have the same issue.

Well Written Helpful Respectful Engaging

texasastrophotography

Topic starter

23 days ago

So I’m not quite sure I understand 100% why, but…. I added some ‘empty spacers’ at the end of my coma corrector. Basically extending the length of the coma corrector a whole bunch (going to have to make sure this won’t hit the secondary when the focuser’s at the 0 position).

Image references: Top left is no spacers → top right is one long spacer → bottom left is twice as many spacers. This appears to have somewhat helped.

📷 image.png

So maybe the new question on this is: Does this mean I should get a different coma corrector (using the skywatcher coma corrector for up to f/4, which this telescope, the quattro 300p, is exactly f/4), or is this just a limitation of this telescope when using my current imaging train? Hopefully not the latter, but starting to have my doubts with Skywatcher lol.

Well Written Concise Engaging

andrea tasselli

Frequent contributor

22 days ago

Take a long dark during the day and see there is no light leak then. If it isn't then you have a reflection within the image train.

TiffsAndAstro

22 days ago

Does you coma corrector / focus tube need flocking?

Does it have any shiny bits?

Brian Puhl

IOTD Staff

22 days ago

I’ve had this exact thing happen with one of my newts, it plagued me a bit. I thought it was light leak. Ended up being improper calibration. It doesn’t math out in my head, but I ended up making sure my bias frames were around 0.5 seconds, (not the absolute shortest exposure), and I ensured all my flat frames were at least 1 second, preferably 2 or 3. From there on out, I never had the issue again. I know you said you shoot dark flats, and I suppose if you took them when the camera was disconnected and completely blacked out, they’re probably safe.

I would strongly suggest trying this route. My master stacks looked EXACTLY like yours, to a tee.

Well Written Helpful Concise Engaging Supportive

Haakon Rasmussen

22 days ago

I had this problem, and it was caused by the coma corrector. It showed up when the background became to bright. I think i had exactly that CC. Now i am using the TS-Maxfield, and when the sky is very bright (Full moon) i can sometimes see a hint of this ring. Daniel Nimmervoll on youtube has a video on this problem.

https://www.youtube.com/watch?v=wdrkpRF-_Ss

I hope this can help.

CS Haakon

Helpful Respectful Concise Engaging Supportive

texasastrophotography

Topic starter

22 days ago

Hey everyone, thanks a ton for all the responses and help! I’ve got some of my own updates, I’ll share after replying to messages below:

andrea tasselli · Mar 13, 2026, 07:36 AM
Take a long dark during the day and see there is no light leak then. If it isn't then you have a reflection within the image train.

Unfortunately I think light would leak through during the day at a small amount because of the primary mirror on the back of my telescope, even with my makeshift cover to try and block light as much as possible. While light would not leak through the front or focuser from what I can tell, I know sunlight would make its way through the back of the newt. So unfortunately not sure if that would be the best way or not to confirm this situation. If it helps at all, I have to always bring a flashlight out or turn on my porch light when going outside to work on it when the moon’s not out because it’s pitch black.

TiffsAndAstro · Mar 13, 2026, 08:30 AM
Does you coma corrector / focus tube need flocking?

Does it have any shiny bits?

I’m not too familiar with flocking, but from what I understand of it, I’m not sure where it’d be applied in my case. That being said, it’s all pretty “flat” (not shiny) outside the glass itself.

Brian Puhl · Mar 13, 2026, 11:34 AM
I’ve had this exact thing happen with one of my newts, it plagued me a bit. I thought it was light leak. Ended up being improper calibration. It doesn’t math out in my head, but I ended up making sure my bias frames were around 0.5 seconds, (not the absolute shortest exposure), and I ensured all my flat frames were at least 1 second, preferably 2 or 3. From there on out, I never had the issue again. I know you said you shoot dark flats, and I suppose if you took them when the camera was disconnected and completely blacked out, they’re probably safe.
I would strongly suggest trying this route. My master stacks looked EXACTLY like yours, to a tee.

This was kind of what I had thought at the time as well, but considering I see the problem even in single frames that haven’t gone through any kind of calibration is what leads me to believe it’s not related to that. I compared images from my old telescope/old camera setup to my new telescope/camera setup and new telescope/ old camera setup. The old scope images were perfectly flat all the way across, but the new scope for both cameras had this circle issue in single frame images.

Haakon Rasmussen · Mar 13, 2026, 05:11 PM
I had this problem, and it was caused by the coma corrector. It showed up when the background became to bright. I think i had exactly that CC. Now i am using the TS-Maxfield, and when the sky is very bright (Full moon) i can sometimes see a hint of this ring. Daniel Nimmervoll on youtube has a video on this problem.
https://www.youtube.com/watch?v=wdrkpRF-_Ss
I hope this can help.
CS Haakon

I’ll have to check this video out later tonight to see what he says - but this is basically what it’s sounding like to me.

So for my updates:

I decided to stack my current images from last night with the little makeshift ‘empty extensions’ on my coma corrector, without flats (haven’t taken those quite yet), and the circle is virtually gone.. I can still see the overall shape of it with the vignetting, but have slight hopes that it’ll go away when I take the remaining frames tonight + the flats for the image and do a more official stack/calibration of the images. Here’s a preview of the L filter stacked and stretched, no edits made:

📷 image.pngAs can be seen, the vignetting is there, but the primary circle that blows up in the dead center isn’t. When I lower my stretch values, I can see the circle forming in the noise, but it’s no where near as powerful as my previous images without the extensions.

So overall, this is really pointing to the coma corrector and view that it gives to the secondary mirror from the camera. If I had to guess, is it’s extending beyond the secondary mirror in a wider field of view, causing this weird effect of sorts. If anything, the extension rings I’m attaching just make it so the background becomes black instead of the wall of my telescope behind the secondary mirror, making it “less reflective”, if that makes sense. This is just a theory of mine at the moment though. If the flats at least knock this vignetting out to a place where you couldn’t even tell, I might just stick with my setup - if it’s too hard to work with even with flats, I’ll likely consider the alternative CC (but going to watch that video too to make sure it’s 1:1!)

Well Written Respectful

andrea tasselli

Frequent contributor

22 days ago

Unfortunately I think light would leak through during the day at a small amount because of the primary mirror on the back of my telescope, even with my makeshift cover to try and block light as much as possible. While light would not leak through the front or focuser from what I can tell, I know sunlight would make its way through the back of the newt. So unfortunately not sure if that would be the best way or not to confirm this situation. If it helps at all, I have to always bring a flashlight out or turn on my porch light when going outside to work on it when the moon’s not out because it’s pitch black.

*Mine one even with two (!) layers of blackout caps on the back end still left some light to creep out and it showed in the flats and the same applies to the focuser (that's when neighbour porch light interfered). Now I have a sock over the focuser too to prevent light leaks though in all honesty your case seems more likely due to the CC. Flocking or painting with deep black matt paint should help there.

Helpful

texasastrophotography

Topic starter

21 days ago

Hey all, just wanted to share my latest update. I think I’ve basically confirmed it was the CC - and the video shared above by @Haakon Rasmussen was spot on to what I think it is. It looks like because of how the CC protrudes differently with the new telescope + focuser, light is hitting the glass at the front (the one part I mentioned was reflective lol) and causes this effect. Me adding on the lil extension tubes at the end look to have resolved the ring effect altogether, and vignetting virtually vanished completely once I applied the flats with my image. I think this was the solution.

Here’s an image of one of my integrated images stretched out to right before it goes all black, you can see it’s pretty clean.

📷 image.png

I appreciate all the ideas and suggestions from everyone - I hope this thread and solutions all help others in the future who may run into the same issue!

Well Written Helpful Respectful Concise Engaging Supportive