Lessons Learned

Using Photo Shop to Remove Star Trails from image.

Submitted by Bud Guinn - Nov 15th, 2005
Learned: Our Dark Skies Fourms

I was asked to share one of my methods of removing star trails from a photograph.....

I'm not sure where I picked these up....they are not original, even though

I have made some refinements for my own preferences.

First we will start with a pic with good star trails.

Here is a reduced size for display...this was one of 50 some shots of M16...you'll notice abit of the red nebula in the background of the image....no processing was done to this other than converting it to color from the original .fit image.

Now to remove the trails:

What I do in PS is first in layers duplicate the image (called a background copy), next

I change the background copies blending mode to darken, next with background copy high lighted,

I then select the offset tool in filters Filters>Other>Offset (zoom in 500% on the image "Ctrl +") and adjust the horizontal/vertical pixels (undefined areas set to wrap around).

For the full size picture I used Horizontal of "0" and Vertical of "13"...this will very depending on the size of the trails and what direction they are heading....ie up and down...on a 45....sideways....etc.....play with the two settings until the trails are diminshed or eliminated. You can use negative numbers.

Once I get the star shape where I want it, I zoom back out.

I then flatten the image and save. here is the image with the trails removed as above.....I put enlarged insets to be able to see the difference.

This may be all that you want to do, but I also like to smooth and soften the images out a bit......usually when you remove trails or fool with them abit they become somewhat pixelated....so

I do the following to smooth and soften the stars.

Start with a flattened image Duplicate the background layer. you should now have two layers

Now use the magic wand and select the stars. tolerance about 40..anti-aliased not contiguous use the shift key to select more of the stars. now Select>Modify>Expand....about 2 or 3 pixels Now Select>Feather....about 3 pixels Filter>Blur>Gaussian Blur....about 1 pixel Select>Modify>Contract....by the same amount that you expanded...2 or 3 pixels

Now you can adjust the hue and saturation if you like...Image>Adjustments>Hue/Saturation...adjust to your satisfaction.

This will soften the stars and make them abit less pixelated.

Here is an example of the same image with the stars now softened abit...insets again to show the differences.

I sometimes find it is necessary to select the stars a brighten them abit first...or darkent the background....you'll need to fool with it abit depending on your image, but this should give you a good start.

Many of the other software programs have a deconvolution routine that can also do a reasonable job......but this is convenient right in Photoshop.

I have made the softening routine into an action that I sometimes run a couple of times.....again this is personal preference.....as an action it only takes a few seconds to do the softening.

I hope this is helpful,


Check out Bud's web site to see his gallery, well worth the visit. Bud's web site! and Photo Gallery

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