fixing coma with the offset filter and darken mode. Help please!

[StargeezerTim]

Hi all,

I need to fix up coma in the corners of my images (try as I might, I cant get the optics spot on). I have been trying the offset filter with darken mode (Photoshop) as described here and elsewhere with mixed success. I find I can get round stars with, say, a 2 and 1 pixel offset, but that the background is darkened slightly. If I apply levels or adjust the brightness of the the corner layer, I can equalise the background but lose the star changes. here is an example to show the change in background:-

 

Does anyone know of a way of adjusting the background without losing the changes?

Thanks in advance... Tim. 

[FunkyKoval35]

The simplest way is to work on mask with stars only. 

[StargeezerTim]

10 hours ago, FunkyKoval35 said:

The simplest way is to work on mask with stars only. 

Thanks... I had tried this but all I got was like little crescents on the stars themselves. I have just tried increasing the selection expansion to 10 pixels instead of four and that seems to have done the trick!

 

[ollypenrice]

The technique works best if you are using the offset in only one direction, which would mean rotating the image to the correct angle for each corner in turn before running the routine. This would be long winded but it might just work if recorded once from beginning to end as an action. Since the coma will diminish away from the corners a feathered eraser might, with experiment, fade the routine by the right amount. Personally I think you'd be lucky to make this workable though. (You know that you don't have to work in integer offsets BTW? Edit - Fade Offset will give you sub pixel offsets.)

Do you know the star rounding routine? It works only on one star at a time but is quite quick if saved as an action.

1) Use magic wand to select star core.

2) Record action saved to a shortcut key like F1 etc.

3) Select, modify, expand by (experiment needed...)

4) Select, feather by (about 2/3 of the 'expand by' value.)

5) Filter blur, radial blur set to Spin and Best.

6) Repeat step 5. (Press Ctrl F)

7) Deslect.

8) Stop recording.

Now you just select a star with magic wand and hit the function key. If you like this method you can do a large star and a small star version using different expand-feather values, smaller for smaller stars. I used to do this when imaging without a flattener on the TEC 140. When I moved to a full frame CCD the game was up and I needed to spend the thousand dollars on Mr Petrunin's excellent flattener!!!

Olly