Astrophotography Part 5 of 6: Working in Post

The Time-consuming Part

By K David in Articles and Tips on Nov 2, 2016
Astrophotography Part 5 of 6: Working in Post

This article looks at some basic software and editing approaches related to astrophotography, including discussions about and samples of specific photos I've shared over the course of this series. Toward the end you will also find resource links for a number of excellent sources on capture and processing techniques.  Note that this article is not an in-depth exploration of niche astrophotography editing techniques. Many other resources, both here on Pentax Forums and elsewhere, cover that well. Also, the niche editing techniques are not generally intended for DSLR users.

Don't miss parts 1-4 of our astrophotography series before reading on!

Post Processing Benefits

As important as the camera set-up and capture techniques, post processing your star photos will help you deliver the results that you set out for.  Good techniques for astrophotography image editing include a number of specialty and niche techniques and programs. We'll touch on those briefly but this article's focus will be a couple specific compositing use cases. They entail techniques readily accessible to most photographers with a capable photo editing software.

Post Processing Options

Many post processing options exist. Photoshop, Lightroom, GIMP, and Nik are common general-purpose photo editors. RegiStax and StarStax are two popular astrophotography image stacking programs designed specifically for that purpose. For this article, we'll be discussing Photoshop (using version CS6 extended) and comparing both RegiStax and StarStax. We'll also run through some basic post processing principles.

Editing Raw Files

If you don't already shoot raw for your astrophotography work, start. Raw editing affords far greater image recovery room, more star recovery capability, and more foreground illumination control. Also, if you plan to stack your images, you can batch-balance all your raw files to achieve like colors and lighting across each frame prior to stacking.

Editing JPEG Files

JPEG editing, if this isn't already your practice, is best suited for fine tuning. Tasks like cloning out distracting foreground elements, refining colors, adding or removing image noise, and other revisions not accessible in a typical raw editor are well suited for JPEG editing. If you plan to stack images, it's best to do your JPEG editing on the final, stacked image.

Combining Images In-camera

Star Trails Composite Image | K David

If you have a newer advanced Pentax DSLR model, you have the option to easily combine images in-camera using one of two drive modes: interval composite or star stream. The former outputs a single image file, while the latter creates (up to) 4K videos of star trails and works very well for sharing motion-capture time lapse images.  One or both of these modes are available on the Pentax 645Z, K-3, K-3 II, K-1, K-S2, and K-70.

Look in the drive mode menu to see if your camera has the interval composite / star stream modes

These two drive modes make life easy, but they also have their limitations: you get much more control and flexibility by shooting a sequence of multiple images and combining them in post.

If you'd like to create a single star trail image from an interval sequence shot by the camera, the best option is to use your blending mode in Photoshop and select "Bright."

Bright works exactly as you think it might: by layering each photo and keeping only the brightest pixel in the stack. This is the mode to use when shooting a star trail image. You will also pick up clouds, however.

If you have the SD card and hard drive space, it might be a good idea to keep your process as well as the final image. That way, you have more than just your final frame. This has uses if your image has a car drive through the foreground, for example. Going back to the first frame would give you a better foreground to cut and paste into the final image. For more information on Pentax composite shooting modes, check out this article.

Photoshop Extended Statistics Blending

Pentax Forums published an extensive guide on Photoshop blending techniques. For more information on long-exposure compositing, check out this article. A number of useful Photoshop blending modes exist for astrophotography and the results from various modes can be great or, at minimum, possibly useful as editing layers.

Stacking Images

Some Software Options

For photographers who don't need all the to-do Photoshop provides or don't want to pay a monthly fee of the privilege of using Photoshop CC, two free star trail programs provide some reliable astrophotography functionality: Registax and Starstax. Both of these programs lack the ability to support raw files and Registax, at least, caps component imaged a 3,000 by 2,000 pixels. We don't have an in-depth write-up on these, but they are freeware and can be downloaded from the links in this paragraph.


Registax is the more capable of the two and is what many amateur astrophotographers use for lucky-frame image composition. This feature allows a video or multiple, sometimes thousands, of frames to be compared for the sharpest individual pixel groups to deliver far sharper images than typical shots or composites. 


Starstax is a star trails creation software. It also has some options such as making comet-like trails from the stacked images and allows users to remove the gaps that can occur between star photos when the camera ends one frame and starts the next. If you only want to do star trail photos, Starstax is a good, free option.

Layers and Selective Deletion

Layers are a powerful tool for astrophotography edits. It's important to note that editing should not change the nature of the scene but enhance it or fix errors in the capture process. It can also help to create a specific image that recreates a scene that the photographer aimed to capture and, in doing so, create a piece of art. For this section, I'll share three images and then show how layer use resulted in the final image.

Shooting the Moon

 This photo arose from two different images:

I used this shot for the moon— a scaled JPEG otherwise straight out of camera is shown above. The file used in the final image was edited in raw to sharpen, increase contrast, and remove some of the lens-related moon glow.

I used either this shot or the other in the series for the stars. For reference, that star is Vega. The moon was not passing by Vega when the photo was taken that night, which allowed me to capture more stars without light pollution from the moon. I overlayed the moon photo onto the Vega photo in Photoshop.

In this screen capture from the editing process, I turned the transparency on the moon layer to 50% illustrate the overlay. In the final image, the moon layer was left at 100%. There are a number of ways to do the next steps.

Click on the photo for an enlargement. Here you can see my Photoshop interface with the layer blend mode to lighter color. This lets the stars from the lower layer shine through the upper layer. If you elect to use this method, any stars behind the moon that shine through need to be erased. That's easiest by going into the bottom layer and cloning out the subject stars with the healing tool brush set to replace. If you just erase the stars with the eraser tool and the sky colors don't match exacty you will end up with black dots where the stars were.

Again, click on the link for an enlarged view. The image above shows the results of the two approaches. The top circle shows where I cloned out a star. The lower circle shows where I erased a star.

However, that's now how I did my compositing. There's a better way that will deliver better results in situations like these. I noticed in my shots, especially after editing, that the sky in the Vega shot was lighter than the sky in the moon shot. That gave me the idea to give the moon a shadow, to use the empty space behind the moon to give it a complete form. For this approach, I left the moon layer's transparency at Normal.

Here you can see that the moon layer is set to normal transparency. I used grid lines to create an intersection between the furthest point on the moon both on the side and bottom. I chose the bottom only because the top of the moon doesn't show the actual top (notice how the shadow covers the northern pole).

Using the grid lines as a reference, I created a circular mask (CTRL+mask drag) until the moon was surrounded by the mask lines. This is the easiest approach to creating a precisely placed circular mask— find the corners of a square that would fit the circular object snugly and then drag a perfect-circle mask from corner outward.

After creating the mask, I selected the inverse and deleted the background around the moon. This approach created a shadow area for the moon because the moon's background sky was darker than that in the star shot. I did not feather the deleted space because I wanted a crisp moon shape. This empty space created the moon's shape in the final image. After I had just the moon to work with, I placed it where I wanted and cropped to a pleasing 1:1 aspect ratio.


Wading began as an effort to capture star trails. In total, the foreground and sky image resulted from 108 images, though far fewer would have been required. This shot was taken on the shore of South Lake Tahoe in California looking northeast toward Reno, Nevada. I hadn't planned on a person being in the shot, but some drunk kids went for an after-midnight dip and one frame with one of the kids turned out very well.

The shots had to be 30-second exposures because the lights from the casinos in South Lake and the lights of Reno overpowered the sensor on longer exposures. When I pulled the shots into post, I was disappointed with the numbers of stars I had captured. Even in raw, I couldn't pull a significant number of stars out. The foreground had a number of issues, too, with boats and a foreground jet ski moving around in the wind and leaving the smooth lake full of blurry objects.

I blended the 108 frames using Photoshop statistics (median) and obtained a shot totally devoid of stars. I found the shot where one of the waders had stayed in place for the whole frame and also the frame previous where he walked into place and left a wake behind him. I combined those two layers and placed them into the water to hide the blurry red jet ski.

I used some additional layers to remove blurry boat masts and correct color. Because the star trails were lackluster, I brought in the blank sky from the median-blended image and hid the stars from the original shot. That night I had taken a second star trails shot with my K-3 and the same lens only this time pointed directly at the sky to maximize stars. The camera was pointed over the area of this shot but without the same level of light pollution. I used the star trails from that shot in this image and lined their paths up, generally, with the few star trails the original shots captured.

Additional Resources, Tips, and Tricks

For those of you who would like more resources about astrophotography editing, here are some links to websites and organizations with more specialized knowledge. And, of course, your area astronomical society will have a lot of knowledgeable and experienced people who can direct you to editing techniques that will benefit your specific images.

General Resources

Lightroom and Photoshop

Setting Yourself up for Easy Editing

Just for Fun

See also the other parts of our astrophotography series here!

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