Re: [APML]Gradient Mask Question

[Jerry Lodriguss <jerryapml@astropix.com>]

Hi Gary,

If you have milky way all the way down to the horizon like this, you have a 
pretty good observing site.  You also have a well exposed image here, I 
don't think it is terribly overexposed at all.

I'm not even sure you have a light pollution problem with the image. It may 
just be air glow or extinction near the horizon. Things in the sky just 
tend to get brighter / milkier near the horizon, even in the daytime, even 
at a perfect site.

You can easily adjust the gradient though, with a linear gradient fill in 
Photoshop.

Try this:

1. Layer > New > Layer.
     Mode: Soft Light.
     Check the box that says "fill with soft-light-neutral color (50% gray).
     Opacity 100%.

2. Set the foreground color to a dark gray, something like 30,30,30.
     Set the background color to middle gray 128,128,128.
     These colors will be the starting and ending points of the linear fill.

3. Click on the linear gradient tool.
     Click on the fill layer to be sure it is active.
     Drag the tool from the bottom of the picture to the top.
     Click the eye icon on and off next to the fill layer to see what 
effect it has on the image.

4. Experiment with different foreground colors. You can make the gradient 
fill affect the image more or less by using a darker or lighter tone of 
gray. A gray of 128,128,128 produces no change to the original image.

Use the eyedropper set to a 5x5 pixel size and read the RGB pixel values in 
an area of blank sky at the top and bottom of the image (say in the Pipe 
nebula) to see when they are roughly equal, then you have removed the gradient.

5. Layer > Flatten Image
Flatten the image when you have corrected to your taste.

You can also do some color correction with this technique by using a 
non-gray color in the foreground color box that is a compliment to the 
color you want to correct.  It's pretty much the same technique I have 
detailed at: http://www.astropix.com/HTML/J_DIGIT/VIGNET.HTM for removing 
vignetting.

<Plug mode on> Naturally, all of these techniques are covered in my book. 
<Plug mode off>

Jerry







At 05:07 AM 1/14/2004 -0500, you wrote:
>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.


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