Pseudo-Flat Fields

[Joel Shepherd]

As endless gray skies drift by above (which I would use to make a flat if only they weren't also dropping rain), I'm curious if others have tried something like the following for making pseudo-flat field images.

Essentially:

1. Take the (stacked?) image you have and open it (as a TIF say) in Photoshop.
2. Apply a Gaussian blur at 50 or so to it and save.
3. Re-apply that to the original image as a flat.

Apparently this is a fairly common technique for microscopy.

This most likely wouldn't do a thing for dust particles and such, but it seems like it'd have some benefit for general uneven lighting and vignetting.

Has anyone tried this ... with some success (or not)? Any tips or refinements you'd recommend?

Thanks! -- Joel.

[FunkyKoval35]

i am using artificial flat, but I am doing it a bit different:

1. Open stacked image, and stretch your object.

2. Duplicate layer

3. Using "Clone stamp tool" remove all most lighten objects - stars, galaxies, nebulas, etc.

4. Filter -> Noise -> Dust & Scratches, radius 20 - 35px

5. Filter -> Noise -> Median, radius 20 - 35px

6. Filter -> Blur -> Gaussian Blur... at 30 - 50%

7. Select -> All

8. File -> New...

9. Edit -> Paste

10. Come back to original image, and remove blurred layer from point "2."

11. Image -> Apply image... and there:

12. Source: file from point "8.", Blending: Subtract, Offset: 30

Works for me...

[swag72]

I have used an artificial flat a couple of times and I have found it works really well. I use this method http://www.corius.net/?page_id=395

[Christopher Davenport]

Hi

I use this all the time to flatten images. i find it also removes some of the noise. 

I have it in a PS5 action script, with some other techniques. (here)

This works well when you have some small and easy to Isolate objects.

(I tend to clone tool the objects out so as not to darken them.)

If you have a large image like the N.American nebula ... then it becomes a bit more difficult.

(I find that I can still use it if I do a luminance layer afterwards which pumps up the nebula afterwards.)

Hope this all helps.

[Joel Shepherd]

Wait ... Are you saying I'm not nuts to try it? ;-)  Thanks for the tips. After playing with it a bit last night and reading the tips folks have suggested, I think I need to upgrade PS: the elderly version I'm running can't do much with a 16-bit TIFF and the degradation in quality is noticeable.

FunkyKoval35 does it after stretching. Is that true for other folks as well? Curious mostly because a photographic flat would be applied before stretching, not after.

Thanks -- Joel.

[Fred_76]

These methods are quite OK to remove gradients and vignetting, but inefficient for dust, scratches and stray light patches.