HELP! Horrible chaotic gradients in widefield mosaic

[FenlandPaul]

Hi all

I'm hoping someone can point me in the right direction, or tell me to stop pursuing a lost cause!

Recently I attempted a widefield nightscape mosaic at 85mm of the Orion region.  I wanted to create something super detailed, but I've ended up with a bit of a monster and now wish I'd just gone with fewer panes and a wider lens.  But the night has personal significance and if I'd love it if I could salvage something.

I ended up with 18 panes in the mosaic, each of which was 3x120s exposures at ISO800 and f/4, using a modded Canon 6D.  I also shot flat frames "just in case" - I hadn't really expected to need them at f/4 with the Samyang 85mm (widest aperture is f/1.4).  Having stacked each pane in DSS, after a bit of persuasion in PTGui the whole thing stitched, which I was delighted about.  

But I seem to have lots of "micro-gradients" that resemble a vignette in each frame, despite taking flats.  I've never seen this before, but also I've never attempted a mosaic of this complexity before.

The underlying data seems reasonable (nice stars, good detail in the Rosette etc), so I'm after suggestions around how I can clean up these horrible gradients / vignettes (obviously I'm not concerned about the bright horizon).  I use PhotoShop for editing, and have tried using AstroFlat Pro, but the result is fairly ugly.  

Any suggestions would be very gratefully received (as I say, even if the suggestion is "don't bother, Paul") - thank you in advance.

I've copied below the lightly stretched output from PTGui, together with the image map from PTGui showing how the image stitched.

Paul

 

[Elp]

Learn Siril. It has a background extraction function built in which will transform your images. It's free, there is no excuse not to use it.

You'll have to perform it on every panel whilst the data is in a linear state, do your stretches, colour correction etc. Then stitch in PS and colour match again between panels. Mosaics can be a pain.

Edited February 6 by Elp

[tomato]

Astro Pixel Processor is great on large, wide field mosaics, especially in removing individual gradients and smoothing the joins. It’s not free but there is a 30 day free trial if I remember correctly.

[windjammer]

Hi

I tried gradexpert, (my goto tool for this - it is free and vg normally), but was roundly defeated!  Fiddled with DBE in PI but also butt kicked.  So I tried old school:  take an image layer and gaussian blur so all stars are blurred out and gone (gaussian radius about 20-30 pixels on your posted jpeg image).  Subtract the gb image from the original. A bit of levels and repeat.  This is what came out... hope this is useful ?

Simon

original - quick and dirty crop for square, suggest you add a black background to your mosaic to square it up

 

first round of gaussian blur and subtract

 

second round  of gb sub

 

and some curves:

 

[Elp]

The issue with doing that method is unless you completely blur or clone stamp out any nebulosity before the subtraction, the subtraction will remove such details. I use a similar method when making synthetic flats when I have flat calibration issues, it doesn't always work very well.

[windjammer]

I think the trick is to choose the right gb radius.  The fov of this pic is so huge that nebulae of interest are tiny compared with the swathes of pollution! But overall, the method is indeed one of last resort!

[FenlandPaul]

15 hours ago, windjammer said:

Hi

I tried gradexpert, (my goto tool for this - it is free and vg normally), but was roundly defeated!  Fiddled with DBE in PI but also butt kicked.  So I tried old school:  take an image layer and gaussian blur so all stars are blurred out and gone (gaussian radius about 20-30 pixels on your posted jpeg image).  Subtract the gb image from the original. A bit of levels and repeat.  This is what came out... hope this is useful ?

Simon

original - quick and dirty crop for square, suggest you add a black background to your mosaic to square it up

 

first round of gaussian blur and subtract

 

second round  of gb sub

 

and some curves:

 

I really appreciate that. I’ll have a look at that method later today (along with hopefully revisiting the location to try again with a wider lens). Definitely something to work with - massively thankful.

The idea of learning another new application doesn’t fill me with excitement, so anything I can do in PS is a bonus!

[Clarkey]

I managed to get this by cropping to a rectangle and running through DBE in PI. I suspect it would give a better result without a the JPEG compression. I would also suggest Astro Pixel Processor. Unfortunately, my licence has expired so I could not try it.

Edited March 5 by Clarkey