Off-Centre Flat Frames/vignette

[AstroRuz]

Hey all,

Here's something I hadn't noticed before. Whilst exploring my calibration frames recently with my DSLR camera I noticed that the vignetting is off centre.

I decided to double check this with the flat frames. Whilst a flat frame shows vignetting in the same area - as in its off centre - I would think this would still calibrate and correct properly then. As it's still in the same area of the light frame, correct? Though the flat frame does need moving about a bit. So I would guess it isn't actually correcting 100% and I may have just not noticed it.

My question is: is the vignetting a natural artifact of using a DSLR and telescope. Or could it be down to droop/tilt, or even focuser collimation? Or am I just worrying about nought.

To answer questions that may pop up:

• Camera wasn't removed or rotated between shooting and flats
• The final image is corrected
• The scope is a Sky-Watcher Evostar 80ED Refractor
• The Camera is a stock Canon 450D

Attached are a stretched flat frame showing the vignette, a stretched raw, a calibrated raw and a gif showing incrementing stretches. Is it just me, or does it look like the gif is showing uneven illumination?

I think maybe my optics are off centre?

Any insight would be interesting and appreciated.

Thanks,

Ruzeen

[knobby]

No expert but I'd say, yes it will calibrate out ok and it's caused buy slight tilt.

edit - love your youtube channel !

Edited April 30, 2020 by knobby

[AstroRuz]

Hmm if that's the case then I wonder where the tilt might be coming from. Will have to investigate that.

[ollypenrice]

12 minutes ago, AstroRuz said:

Hmm if that's the case then I wonder where the tilt might be coming from. Will have to investigate that.

Drawtube or camera chip? Try rotating the camera 180 and see if the offset follows that.

Your offset vignetting is pretty typical of many systems (even better than some I've used) and wouldn't keep me awake as long as your stars are good across the chip. 

Olly

[RayD]

Yup agree with Olly.  That amount is not an issue and will calibrate out easily.  I certainly wouldn't chase trying to reduce or straighten it as you could make matters worse, especially if your stars are good shapes across the FOV.

The GIF to me shows things very slightly brighter on the right side, but this could be skyglow or even local lighting source causing this.  Running a gradient remover or DBE (in PI) will easily deal with this and again, not unusual at all.

APS-C chip is a reasonable size, so what you have here is actually pretty good.  Providing you have decent flats taken before moving the camera this will all calibrate out with no issues.  I've seen far worse.

[AstroRuz]

1 hour ago, RayD said:

Yup agree with Olly.  That amount is not an issue and will calibrate out easily.  I certainly wouldn't chase trying to reduce or straighten it as you could make matters worse, especially if your stars are good shapes across the FOV.

The GIF to me shows things very slightly brighter on the right side, but this could be skyglow or even local lighting source causing this.  Running a gradient remover or DBE (in PI) will easily deal with this and again, not unusual at all.

APS-C chip is a reasonable size, so what you have here is actually pretty good.  Providing you have decent flats taken before moving the camera this will all calibrate out with no issues.  I've seen far worse.

Sounds good then. I had changed the focus tube on my telescope from the stock one to the Crayford one so maybe that could be causing issues. I can't recall seeing that in my ZWO ASI 533mc Pro flats. But I'll need to recheck them really. 

The localized lighting on the right hand side of the image could be from external lighting. I know there are a few street lights laying just out of view of my garden. Also Markarian's Chain was right over the top of my head. So maybe could be. I'm close to town. I'll try a different light pollution filter some day then and see if that helps anything.

I won't go chasing any faults nah. Was just wondering what peoples' thoughts could be. I don't have gradient xTerminator, for example, but I've been using the subtract method that AstroBackyard shown. Works quite nicely.