Making Your Stars Round in Photoshop

the NGC 5560 area

Hi All, here is an updated guide to making your stars round when they are egg shaped. I just got back from a good 2 weeks in Texas and am heading out again so I am posting this today. While in Texas I had some great nights capturing data, but I also had some nights where I lost guiding because of dew on the guidescope. It happens, and on 5 minute subs with a DSLR it can lead to results that aren't good.

Still, there is a quick and easy fix if they aren't too bad. Slight eggs, those are okay, grains of rice? That isn't happening. We are using Photoshop for this. First here is an example from one of my stacks:

stars from bad guiding that are oval

So what you do in Photoshop is to make a duplicate layer of the whole image. You will select it all, then copy it, then paste it as a new layer over the original. You will set it as a darken layer.

screencap showing where to set layer type in photoshop

Next you will go to Filter > Other > Offset. Play with your vertical and horizontal, understanding that if you go more than 5 in either direction you will likely have something that cannot be fixed with this method.

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screenshot showing the offset dialog box and corrected stars

As you can see by this it can work to clean those star shapes up, but you may have some residual stuff in the background that you might need to bury or clean up. Still, it takes an image from unusable to usable. Also, don't be afraid to play with the opacity and fill sliders for the layers.

You can even take this image, copy it, paste it into another tab in Photoshop, then remove the stars, enhance the nebula, and use that as a layer. Copy and paste this same image into a new tab again and then do a filter to remove that little bit of background, then take this image and darken things to bury the background, the paste the other two over as layers using them as lighten color for the stars and lighten for the nebula part. It can certainly save an image!

I've added a quick process version of this taken with a astro modded Pentax K5 DSLR, with a Samyang 135 f/2 at f/2.8, and on an iOptron CEM25EC.

DSLR image of the Witch Head Nebula and Rigel area

Of course it wouldn't be right to post without a nice image from Telescope Live. The header image for this post is NGC 5560 which can be acquired here:

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