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If your image has elongated stars in the corners - here is a tip on how to fix that issue.
Jun 9, 2017 10:56:12   #
nikonshooter Loc: Spartanburg, South Carolina
 
Last night, we had mostly cloudy skies but I decided to try again to fix the elongated stars that are driving me nutz when using the ASI1600mm-C and the 8inch Newt. It seems that I have run out of ideas on how to fix my optical train and mount to rid myself the dreaded football shaped stars - all pointing to the center - showing their worse in the corners. This has only been happening with my Astrograph, Newt 8 inch with the ASI1600MM-C camera - with coma corrector. The Newt has 69mm of back focus. From sensor to flange, the camera has 11.5mm of back focus, plus the filter wheel -> 20mm, plus the OAG -> 10mm - I have added a 20 mm extender to a 8-12mm variable extender - this gives me the wiggle room to nail the 69 inches needed to flatten the field to the point where my stars are acceptable....since the Baader Coma corrector is not a reducer - I am using the 69mm that Orion says is the backfocus needed and not the front of the Baader Corrector. No matter - I still can't get sharp tight stars from corner to corner. My polar alignments are exceptional - 1.5 arc peak to peak on a bad night and it is not field rotation as my footballs all point toward the center which is a field flattening issue. So, since I cannot fix on the capture side, I came up with a photoshop fix. If you are experiencing elongated stars and use Photoshop, here ya go,

1)First make a duplicate layer
2)Change the blending mode from NORMAL to DARKEN
3)From your tools, select the MARQUEE tool and draw a selection around the stars you need to fix - in my case I first drew a selection in the upper left corner - then the bottom right since those stars are all pointing toward the center of the image. When I have those right, I drew a selection at the bottom left.....and when I got those numbers right - I repeated the same settings in the upper right corner.
4)Now for the settings - At the top of PS, select FILTERS--->OTHER--->OFFSET.
5)Change the values for vertical and horizontal - warning - you should not go over 15 plus or minus in either vertical or horizontal. I just click on the place where you add values and use my up and down arrow keys to change the value ....for my image, I used PLUS 3 for Horizontal and -3 for Vertical for the upper left corner ...also check WRAP AROUND at the bottom of the dialog box.

I am using PSCC, latest version - not too sure if there were changes to the box in prior versions.

Once you have both reshaped the footballs into oval stars and still while you have your MARQUEE selection made, click on LEVELS and move the middle slider to the left, while watching the image to match the luminosity of the selection to the rest of the image.

Now once you have that done, go to the opposing corner since those stars are pointing the same way, those same settings will fix them.

Repeat the process until all four corners are to your taste. It is a hassle but it works to perfection.

I meant to add, step 5
Use the COLOR RANGE SELECTION TOOL - at top SELECT---->COLOR RANGE and use your eyedropper to select a dark part of the sky from the bottom layer, the original background layer. Close out the box and copy that selection and paste it to the top layer. This will help reduce artifacts that show up in the dark background areas.

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