Odd "circle" in imagery when taking photos - Forums

Topic starter

1 hour 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).

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Fabrice Lamidey

1 hour 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.

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Dunk

1 hour 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.

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texasastrophotography

Topic starter

1 hour 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.

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texasastrophotography

Topic starter

40 minutes 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.

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