Issue with circle in middle of my Image

[Knmurphy1]

Hi Guys,

Any idea why I'm getting this circle in the middle of my image? I used a Televue 76 with an 0.8x reducer / flattener . Camera was a ZWO 183 MC Pro. The scope and camera are a good match apparently. Is it possible it was caused by dew. I was only testing the new camera and didn't use my dew heater.

Edited May 1, 2023 by Knmurphy1

[Elp]

Have you applied flat calibration images?

[Knmurphy1]

No, there are no calibration files applied. I have never seen this before in a un-calibrated image

[Elp]

It could be dew, but from my experience dew reduces contrast of stars significantly as if you were imaging through cloud. Your stars look fairly uniform across the image.

Have you imaged with your setup described above previously without issue? What was the exposure time? Was it a single image only that was affected? Was you using a filter or a filter drawer?

Edited May 1, 2023 by Elp

[Knmurphy1]

The exposure time was 60 seconds - about 20 images stacked. The issue is on all the images. This was my first time using the set-up and I was just experimenting.  I have used a similar camera before (QHY 183C) with same scope / reducer without an issue.

[Elp]

I also wouldn't rule out some sort of light leak (doubtful), or stray light entering the front objective at an angle.

Edited May 2, 2023 by Elp

[Knmurphy1]

I’ll check that out. Thanks for the input it’s very helpful 

[carastro]

I haven't seen anything like this.  It doesn't look typical for lack of flat frames.  I hope you resolve the problem and would be interested to know what caused it.

Carole 

Edited June 2, 2023 by carastro

[Craney]

I would go with dew as well, especially if you take the scope or reducer from a warm house and take it into coolish Spring night.

The camera would be the first port of call to check  ( the chip 'window')   but the reducer might have at least 4 surfaces that could mist.

[vlaiv]

14 hours ago, Knmurphy1 said:

I have used a similar camera before (QHY 183C) with same scope / reducer without an issue.

Was it exactly the same setup? Connectors / extension rings and all?

I did some calculations for faster 3" class scopes and 2" focusers - for field illumination. They are very short and have short FL and that causes vignetting on anything but focusers with very short draw tube.

If you add different extension - or even use different connection as QHY camera has fixed 2" part and it is removable with ZWO and that means different extension tubes to get the same thing with reducer, this can lead to different focuser position.

Different focuser position means that drawtube can be inserted further down into the OTA - and that will create vignetting with rather small central illuminated field - much like in image above.

If you want to rule out the dew and other things - just shoot some flats in daytime when it is warm and there is no chance of dew forming - but be careful to have focuser at the same position it was during the night. Examine flats - and if they have similar bright spot - then try to make your setup in such way that focuser is racked further out when you are in focus.

This can be achieved by pushing reducer / flattener in focuser tube instead of attaching it at the end of the tube (if it can be done).

In any case - above looks to be flats issue and should be solved with flats. I quickly ran synthetic flats on red channel and got this:

(so this is not background removal - not subtraction, but actually division with synthetic flat - as flats are supposed to work).

 

 

[Knmurphy1]

9 hours ago, vlaiv said:

Was it exactly the same setup? Connectors / extension rings and all?

I did some calculations for faster 3" class scopes and 2" focusers - for field illumination. They are very short and have short FL and that causes vignetting on anything but focusers with very short draw tube.

If you add different extension - or even use different connection as QHY camera has fixed 2" part and it is removable with ZWO and that means different extension tubes to get the same thing with reducer, this can lead to different focuser position.

Different focuser position means that drawtube can be inserted further down into the OTA - and that will create vignetting with rather small central illuminated field - much like in image above.

If you want to rule out the dew and other things - just shoot some flats in daytime when it is warm and there is no chance of dew forming - but be careful to have focuser at the same position it was during the night. Examine flats - and if they have similar bright spot - then try to make your setup in such way that focuser is racked further out when you are in focus.

This can be achieved by pushing reducer / flattener in focuser tube instead of attaching it at the end of the tube (if it can be done).

In any case - above looks to be flats issue and should be solved with flats. I quickly ran synthetic flats on red channel and got this:

(so this is not background removal - not subtraction, but actually division with synthetic flat - as flats are supposed to work).

 

 

Thank you for that very detailed input. I think the only difference with the previous setup was that I used a 1.25" connection on the QHY 183 camera and a 2" for the above image. Not sure if this would make a difference?

[Knmurphy1]

10 hours ago, carastro said:

I haven't seen anything like this.  It doesn't look typical for lack of flat frames.  I hope you reolve the problem and would be interested to know what caused it.

Carole 

I will do, thanks Carole. I have never seen this before. I'm a relative novice, only recently switched from DSLR cameras.

[tomato]

Were you using active cooling on the camera?  It does look like moisture forming on the sensor to me although the symmetry of the light area is unusual. Do a I see a bright area on the LH edge of the image also?

[vlaiv]

9 minutes ago, tomato said:

Do a I see a bright area on the LH edge of the image also?

That is some amp glow, if I'm not mistaken.

Here it is additionally stretched

[Knmurphy1]

On 04/05/2023 at 10:14, vlaiv said:

That is some amp glow, if I'm not mistaken.

Here it is additionally stretched

Yes its a ZWO 183 MC Pro. Lots of amp glow with this particular camera unfortunately.

By the way, I removed the 0.8x reducer and the "circle" issue seems to be resolved. Here's an image of M3 from 08.05.23. There are dark's added (or subtracted?) so there should be no amp glow. Not a perfect image by any means. Lots of cloud and bad seeing (in addition to my menial abilities).

Edited May 9, 2023 by Knmurphy1

[Knmurphy1]

On 04/05/2023 at 10:03, tomato said:

Were you using active cooling on the camera?  It does look like moisture forming on the sensor to me although the symmetry of the light area is unusual. Do a I see a bright area on the LH edge of the image also?

Yes I was using cooling.  -10C. There is probably all sorts of noise and artifacts in the image as it wasn't processed. I was just testing the camera

[Elp]

14 hours ago, Knmurphy1 said:

I removed the 0.8x reducer

Have you got another camera to test with the reducer? The reducer shouldn't introduce an unevenly lit imaging circle.

[Knmurphy1]

6 hours ago, Elp said:

Have you got another camera to test with the reducer? The reducer shouldn't introduce an unevenly lit imaging circle.

Yes, I'll try that with a DSLR, thanks

Edited May 10, 2023 by Knmurphy1

[AstroMuni]

I suspect the circle is as a result of a combination of focuser tube being too far inside (as vlaiv said) and the fact that you are now capturing the image thro a 2". In the 1.25" image this circle would still have been there but closer to the edges.