How to remove this very strong gradient?

[Tom Oskar Ortleb]

Hello Han,

I was curious what ASTAP could achieve with an image set I took 2020 for testing purposes with the nice Samyang F135mm f/2.0 lens: 180x8s lightframes. The stacking result looks better than I expected, with nice star images and good nebula detail (24 mins integration time only).

Besides the more prominent objects (M42/43 + NGC2024 + B33/IC434) I discovered also the faint shape of NGC1909 aka "Witch Head Nebula" when playing with massive histogram stretching – just as a side node.

Due to the image field shift on the sensor (I guess), there is unfortunately a very strong and somewhat stepped gradient in the lower right corner, see screenshot.

Besides the more prominent objects (M42/43 + NGC2024 + B33/IC434) I discovered also the faint shape of NGC1909 aka "Witch Head Nebula" when playing with massive histogram stretching – just as a side note.

I've tried several things but nothing satisfactory. I'm wondering if it's even possible to get rid of that background and work out the details in it. I therefore ask for your support.

NB: I send you a download link of the stacking result from ASTAP via PM.

Thanks,
too

[Grimstod]

A gradient like that is pretty serious. I think this is a post processing thing. If you right click on the image there is a gradient tool. I have tried it before but my gradients were alwasy weird so it did not work for me but yours looks pretty straight. The gradient tool you click and drag like you would be creating a gradient in photoshop.

[han.k]

Tom,

I missed this somehow. (already the second).

Looking to your screenshot I would try it with tab pixelmath 1, Equalise background tool or the viewer gradient tool.

Equalise background tool
First crop it (in the viewer) to remove the red borders.
Use option mono after step 1
Rmove then deepsky with the popup menu of the viewer with the red icon

The result is attached in jpeg. The FITS file is here
https://ufile.io/xyo9ln2o

There are still some rings visible. Not perfect.

Will try one other tool....

[han.k]

The other tools are less successful. You can play a little with factor in step 3) which is default. I gets a little better at 15.

Aftercare can be done with background colour removal in tab pxielmath1. Maybe Photoshop or Gimp can do better.

[Tom Oskar Ortleb]

Hello Han,
I was sure that you must be the one to get the most out of this difficult (at best) data - your result looks way better than anything I've tried before with the gradient tool. I was stumped, but thanks to your pointers, I'll try the image again this week.

Big thanks for you support,
too