Image brightening at edge of field - any ideas why

[billyharris72]

Hi all:

An attempt at the Eastern Veil from last night in Ha. Shot with an ED72, Skywatcher FF, ZWO ASI 1600MM. 23 x 5 minute subs at (alleged) unity gain.

Overall I thought it was okay until I looked at the whole image stretched and saw this bright ring pattern, almost like the opposite of vignetting (but then getting darker at the corners).

I've tried to experiement a bit and have ruled out overcorrection from the flats - it happens even if I don't use flat frames. Neither is it the darks (just stacking the images on their own shows the same effect).

Looking at the individual subs, I can't see this issue in any of them - yet picking subsamples of any 10 subs and stacking results in the same thing.

Have stacked in both AstroArt (my usual) using additive, median and sigma clipped mean, and SIRIL (mean only). Same result each time.

So, does anyone know what might be causing this?

My working hypothesis is that this might just be the kind of optical aberration you get in any system and that it is showing becasue the image is over stretched (and that the individual subs lack the dynamic range to allow enough of a stretch to make this onvious). Sampling the dark pixels in the centre of the FITS image gave me a mean value of 172, while the background in the lighter halo is around 228. 50 ADU does not seem like that much.

So, thoughts appreciated - do I have an optical issue, a camera issue, or am I just suffering lack of data and overstretching?

Cheers,

Billy.

 

 

[Brian28]

Hi Billy , this isn't going to be an answer I'm afraid , which by the way I am eagerly waiting being new to Astrophotography myself , but that is a great pic aside the reverse vignette. .

Brian  

[Zakalwe]

Are you using a mono ASI1600 and 1.25" filters by any chance?

[billyharris72]

1 hour ago, Zakalwe said:

Are you using a mono ASI1600 and 1.25" filters by any chance? 

It's a mono ASI 1600 and unmounted 31mm filters (so pretty much 1.25", maybe a touch larger).

Why do you ask - is this a known issue?

Billy.

[Oddsocks]

Check that all the couplers and adaptors from (and including) the focuser draw tube right back to the camera are properly blackened internally.

Plain black-anodised aluminium just doesn't cut the mustard, all internal surfaces in the optical chain should be blackened with a pigment based paint (or flocking paper).

Plain black anodised aluminium is highly reflective at selective wavelengths and is particularly bad in the IR region.

High temperature mat-black barbecue paint from B+Q is a good absorber of IR and low reflectivity in visual wavelengths, blackboard paint is another good absorber.

See this document for further info:

http://diffractionlimited.com/flat-fields-stray-light-amateur-telescopes/

HTH.

[Laurin Dave]

Are you using an unmounted ZWO Ha filter? I.e. 31 or 36mm , if they don't have blackened edges or have a mask over the edge they can cause strange effects. Also are you calibrating using matched darks, flats and dark flats.  I had a similar issue when I first got my camera and was calibrating using bias dark and flats until I discovered on this forum the error of my ways. 

Dave

[billyharris72]

8 minutes ago, Laurin Dave said:

Are you using an unmounted ZWO Ha filter? I.e. 31 or 36mm , if they don't have blackened edges or have a mask over the edge they can cause strange effects. Also are you calibrating using matched darks, flats and dark flats.  I had a similar issue when I first got my camera and was calibrating using bias dark and flats until I discovered on this forum the error of my ways.  

I'm calibrating with matched flats and flat-darks, but the issue appears whether I apply the flat frames or not, so it does not seem to be related to the flats. You're right about the filters - they are 31mm unmounted. I might try blackening the edges.

[ollypenrice]

Interesting. Today I've been working with a guest on ZWO mono data from his home. He doesn't have flat darks and gets inverse vignetting when uncalibrated flats are applied. We found that the best we could do with what we had was to stack with flats and again without, then average the two stacks in AstroArt.

Your result is indeed odd and I'd certainly see what blackening might do for you. 

Olly

[Laurin Dave]

Intriguing,  as an alternative/addition to edge blackening you might try and get hold of some of ZWO's filter masks as per the photo below.  (FLO may have them or  I've seen on CN that folk get them direct from ZWO).  The effect they had on my flats can be seen below.    

My other thought is whether the filter is in the right way round? Anti reflective side should be towards the camera, see ZWO website for pictures of how to tell.

Dave 

 

[billyharris72]

Interesting thought Dave, and there is a slightly embarassing story there. Having changed from my Newt to the frac, and from M42 tube for my CC to T2 for the field flattener, I discovered last night that the threads of the Baader adjustable t-tube were fouling the carousel.

That's when I reaslised I've been using the whole filter wheel back to front for about a year.

Turned it round (in the dark, with much fumbling and swearing) and it worked okay - but of course it does mean that all my filters are now back to front - so yes, that could definitely be a problem!

On the plus side, I reckon the data I've captured will be usable if cropped or blended into lumumance or red, so not a wasted night.

Billy.