PROCESSING
Reduce star trailing in your astro images
An easy processing tip to remedy stretched and unsharp stars
BEFORE
AFTER
Astrophotography is an extremely rewarding aspect of astronomy, but as any astro imager will know, conditions often aren’t in our favour, which is why we have to take advantage of those rare clear nights whenever they come around. It can be disappointing, then, after capturing those precious images, to discover that they suffer from star trailing.
Slightly trailed stars can be caused by a misbalanced mount or one not quite correctly aligned with the polar axis, making your stars look either stretched or appear as short lines. It would be easy to delete your images and accept that your night’s work was in vain, but wait: in some circumstances, you may be able to salvage something from those images by using a simple but effective tip.
In our example we took 21 images of Brocchi’s Cluster, also known as the Coat Hanger Cluster, a tiny asterism shaped like a coat hanger in the constellation of Vulpecula. On cursory examination, the stacked image looked good, but when we looked more closely we noticed the stars were slightly trailed.
Star shape enterprise
With our technique it is possible to improve the star shapes, making the overall image look sharp, and enabling you to process it a bit more to bring out detail. Note the following caveats: this technique works best on images that only contain stars taken through a telescope, not widefield constellation images taken with a camera lens suffering from sky rotation trailing. With images of galaxies or nebulae, the effect of this procedure will slightly blur them while improving the stars, and noisy data will create some interesting unwanted artefacts, so it is not an answer for all situations. Indeed, ideal subjects are star clusters without nebulosity such as our Coat Hanger Cluster image. Note also that images with wiggly trails due to windy conditions won’t work as you will end up with some very interesting artwork!
First, if needed, stack your captured images to make a master image, then load this into your favourite imageprocessing software such as Photoshop, or in our case PaintShop Pro 2021 (see screenshot 1). Right-click on the image shown in the ‘Layers’ tab on the right, then select ‘Duplicate’ to create a copy of the image exactly overlaid on the original. It is this layer we will be working with. Now use the magnify tool to zoom into the image. It is better to focus on mid-range brightness stars, as the brightest ones may be a little overblown, whereas mid-range stars have a better shape and it is easier to see their trail.
Next, in the ‘Layers’ tab select ‘Lighten’ for the blend mode (screenshot 2). At this stage you won’t see any difference; that’s normal. Now select the ‘Move’ tool and the pointer turns into a cross. Holding the left mouse button, carefully drag the copied (top) layer in small increments, keeping an eye on where the trail is moved to. It is here that the lighten blend mode now shows both the upper and base layers’ trails. The aim is to nudge the upper layer until the lower end of its trail overlaps the top end of the trail of the base layer. It will look like all you have done is double the length of the trail, but fear not!
You can click the ‘eye’ icon on the topmost layer, which is the visibility toggle, so you can swap back and forth between seeing both layers or just the original layer, to gauge just how much you have moved the top layer by. Once you are happy with the right amount of overlapping on the trails, you can select ‘Darken’ blend mode for the top layer and voilà: you now have nice round stars instead of trails (screenshot 3).
Right-click on the top-most layer tab and select ‘Merge all’ to flatten the image. Zoom back out to see the full image with sharp stars. You may find you can further enhance the final image with histogram adjustments and a little extra saturation. In our final image (see above), this even brought out the small open cluster NGC 6802 at the bottom left of the Coat Hanger Cluster.
3 QUICK TIPS
1. To prevent overwriting the original image, always work on a copy.
2. Make small adjustments when moving the top layer, rather than one big adjustment.
3. Use the visibility toggle for the top layer to check how much you have moved it.
Paul Money is an astronomy writer and broadcaster, and the reviews editor of BBC Sky at Night Magazine.