Is there a way to remove field curvature with Photoshop?

[NightCal]

My present imaging set up produces a degree of field curvature. What I need is a field flattener. The trouble is that equals money and at the moment I don’t have as much I want for buying astro stuff. (do I ever?)

Looking at the effects of field curvature on the image it strikes me that this looks very similar to star trailing, except that the direction of trailing varies with angle and the amount of trailing varies with distances from the centre.

Now, I know that there are several different ways of removing star trails in Photoshop.
http://www.focusmagic.com/ (http://www.focusmagic.com/)
http://www.sightsabove.com/article.php?article=20 (http://www.sightsabove.com/article.php?article=20)
http://stargazerslounge.com/imaging-tips-tricks-techniques/113075-startrail-remover-ps-action.html (http://stargazerslounge.com/imaging-tips-tricks-techniques/113075-startrail-remover-ps-action.html)

and probably many more I don’t know about.

So, is there a way of applying any of any star trail removal techniques so that the degree of star trail removal varies with distance from the centre and the angle of trailing removal rotates around the image?

[spearo]

Michael,
Not sure if there is an automated way of doing it but one way may be along these lines:
copy image into a new layer
set to darken
select a quarter of it
filter offset
adjust by slight increment in different directions by 1 or 2 pixels to fix up the stars
repeat in 3 other quadrants

hope this helps
frank

[Steffen]

Are you sure it's field curvature you're up against? In the absence of other aberrations field curvature would manifest itself as the inability to bring both the centre and the edge of the field into focus at the same time, i.e. in order to obtain an image that's in sharp focus throughout it would have to be projected onto a curved screen.

What you describe sounds like coma to me. Mind you, I have no astro imaging experience, just speaking in general optical and photographic terms :)

Cheers
Steffen.

[Geoff45]

Field curvature will also cause elongated stars because as well as the lack of focus, the imaging sensor is effectively tilted in relation to the curved focal plane as soon as you move away from centre field.

You can fix up the elongation in PS. There is a chapter in the book "Photoshop Astronomy" by Scott Ireland which describes how to do it--basically like the procedure described by Frank
Geoff