Category Archives: Gaps in Trails

Dealing with gaps in star trails – technical and practical advice.

Out Darn Gaps!

In this installment of our BLOG, I present to you Mozzer502 (as he calls himself on Flickr).  His real name is not often used in polite company, unless of course you know him in which case you’d probably call him “Andy”.  I met Andy recently at a Star Circle Academy Workshop. He’s a hoot which is American slang for “wickedly clever” not to mention that he’s all around a fun guy who loves slinging burning steel into the ether. See his photostream to see what I mean.

I asked Andy to write up his uber cool trick for eliminating gaps in Star Trails.  Here is his work, slightly edited. Andy’s clever trick applies to star trails that are circular (i.e. revolve around the north or south celestial pole).

Dotty Problem

Now that all good discerning photographers stack their long exposure star photos, I’m sure you’ve noticed your dotty problem. I may have a solution for you for that. Take a look at these two photos. The one of the left is the original after stacking, the one on the right is the nice smooth one after processing.

Is processing a photo wrong? I won’t even bother answering that. If you think so, move along. If you want to know how to make your pictures a little less Morse Code and a little more creamy silk, read on.

We’re going to use a little used Photoshop filter called the Radial Blur. If we do this right, we’ll blur the stars just enough to fill the gaps that appeared while your camera was processing. Of course this is only going to work if we have a radial star pattern with a center we can pin.

Adobe neglected to provide a way to accurately point the radial center, so you have two options to make it work. You can start the filter and click randomly until you get it right, or you can follow this convoluted process.

Load your circular star picture. We need to make the polar star the center of the image. To do that make your window big enough to play with, select the crop tool, set the width, height and resolution to blank, and draw a crop frame. You’re trying to make a crop that has the little X in the center, right over the North Star [Editor’s note: Polaris is close enough for guv’ment work – but the actual center of rotation lies away from Polaris toward the next nearest star in the Little Dipper]. It doesn’t matter how big you enlarge your photo.

We only want to blur the stars, so the next step is to switch off all the foreground in your image. I use a combination of the magic wand, the quick selection tool, and the magnetic lasso to select the foreground and to mask it out.