Strange round pattern

[javaruba]

Hi folks,

I think my vision is failing me, or I'm just losing my mind. A few nights ago, I did a session on the Jellyfish Nebula. I took about 51 subframes of 120 seconds and added Flats, Dark-Flats and Darks as required. After stacking and calibrating the images with AstroPixelProcessor (APP), there seems to be a big circle at the lower edge of the frame. I thought this might be a dust bunny on my L-Pro filter, and the flats should have picked it up. Taking a look at the flats, which I took right after completing the session, shows no indication of the circle. I then started looking at each light frame, and surely I can see the circle but really faint when significantly stretched.

The strange thing is that some light frames have this circle pattern, and some don't. I'm scratching my head now. It's the first time I've seen such a pattern. Does anyone have a clue what's going on here?

Below is the stacked image fully stretched to make the circle more pronounced and a flat frame taken on the same night. The third image is just one frame stretched (note the circle pattern isn't there).

Equipment used:

• Scope: Sky-Watcher Esprit 100ED Triplet APO
• Camera: ZWO ASI294MC Pro
• Mount: Sky-Watcher EQ6-R Pro
• Guide System: ZWO ASI120MM (mini) & ZWO OAG

Thanks in advance for any help.

Kind regards,

Jairo

Edited January 27, 2021 by javaruba

[JamesF]

I suspect that you're seeing patterns that aren't really there, in that there's no ring in any of the images.  It's just a pattern of darker areas of the image that your brain is turning into a ring.

As to why it's visible in some frames and not others, perhaps that's down to variation in the sky quality throughout the imaging period.

James

[wimvb]

My gues is that it's a reflection from the brightest star in your image. It's too weak to be visible in single frames, but above the noise floor in the stacked image. As @JamesF noted, variations in sky quality during the session can make it more prominent in some subs.

[javaruba]

11 hours ago, wimvb said:

My gues is that it's a reflection from the brightest star in your image. It's too weak to be visible in single frames, but above the noise floor in the stacked image. As @JamesF noted, variations in sky quality during the session can make it more prominent in some subs.

Hi Wim,

Thanks for the reply. Do you think it might be some reflection on the L-Pro I'm currently using? Below is a picture on how I currently have it setup.

[newbie alert]

Sorry but don't really know of what it could possibly be, I've just checked my image from the last clear skies.. can't see anything like it on my HA image

[javaruba]

6 minutes ago, newbie alert said:

Sorry but don't really know of what it could possibly be, I've just checked my image from the last clear skies.. can't see anything like it on my HA image

What's the equipment you're using here? Is it an Esprit 100ED as well?

[newbie alert]

Just now, javaruba said:

What's the equipment you're using here? Is it an Esprit 100ED as well?

It's with a esprit 80, with a atik 460ex,  astronomik 6nm HA filter 

[wimvb]

1 hour ago, javaruba said:

Hi Wim,

Thanks for the reply. Do you think it might be some reflection on the L-Pro I'm currently using? Below is a picture on how I currently have it setup.

That could very well be. Do you have the possibility to take subs without the L-pro?

[CCD-Freak]

Did you use a UV-IR filter or a filter that cuts IR? 

 

 

John
CCD-Freak
WD5KX

[javaruba]

13 minutes ago, CCD-Freak said:

Did you use a UV-IR filter or a filter that cuts IR? 

 

 

John
CCD-Freak
WD5KX

Yes UV/IR, it's an Optolong L-Pro broadband filter. Below the emission graph.

[javaruba]

53 minutes ago, wimvb said:

That could very well be. Do you have the possibility to take subs without the L-pro?

I can probably do that when it the clouds go away. Not sure if light pollution might cause another artifact(s) to appear.

[wimvb]

Most light blocking and nb filters have a highly reflective coating on one side and ar coating on the other side. The ar coated side should face the camera otherwise light can reflect off the camera window and then off the coated filter back into the camera. If you test for this, and whether the reflection is caused by the filter, just target a very bright star. Even single subs should then show the reflection.