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Re: [APML]Gradient Mask Question



Gary, maybe this will work - not sure (and I'm no expert). Comes to mind from an S&T article by Sean Walker on removing vignetting.
 
Copy your pic to a new layer in Photoshop. In the new layer use the stamp tool to remove all the bright stars and because this is a Milky Way wide-angle, you need to copy the darkest parts of the milky way onto the brightest parts (you are trying to remove all the bright real data but preserve the general pattern of the sky glow so my thought is that the darkest parts of the milky way are just sky glow and so you want to use that stuff to erase the milky way. Always use parts closest to the thing you are going to erase).
 
Then you should start experimenting with blurring from the filter menu. You can blur it enough to have a mask that is basically a replica of the skyglow pattern. Then in this layer you need to experiment with assigning the blurred image to a mask setting and then play with the combine choices. I've done this on a shot very similar to yours and it worked nicely. Kept most of the signal and removed most of the skyglow.
 
To keep the trees, when you are ready to blur the pic, use the select tools to select all but the trees and then blur the rest (or use the lasso tool if you can't get PS to select all but the trees for you). Try selecting the shadows then select invert from the main select menu.
 
I think.
 
Stuart Heggie
http://www3.sympatico.ca/stuart.j.heggie/Stuart.J.Heggie/
Flesherton, Ontario, Canada
----- Original Message -----
Sent: Wednesday, January 14, 2004 5:07 AM
Subject: [APML]Gradient Mask Question

I'm trying to learn how to do a gradient mask to correct for skyglow near the horizon.  I'm practicing on an image I took of Sagittarius with trees in the foreground and a lot of skyglow.  The picture is way overexposed but I'm trying to learn this technique.  Is it possible to use a gradient mask (linear or radial) to correct it without destroying the trees?  I haven't quite gotten the radial procedure down yet but when I tried the linear it really messed the picture up.  Here's a link to the raw scanned image.  Is it possible to fix something like this or do I need a better picture with less skyglow?  Any advice or tips would be appreciated.
 
Gary
 


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