Astrophotography Image Processing

Here we are with the final installment of our astrophotography series. We’ve looked at doing astrophotography in the daytime, shooting simple objects at night, and some more advanced equipment and stacking techniques. If you missed any of these, you can catch them here.

In this series finale, we’re going to take a look at some of the processing techniques to get the most out of your astro images. Most of the work is done in Photoshop. But there are also some extra tools that I cover. These will further enhance the results of the images that you process in Photoshop. They are not necessary, but will make your processing go much smoother.

Let’s jump right in with a before and after example. First is an example of an image shot with a simple set-up, using my old first generation, astro modified Canon 300D Digital Rebel. It was shot through an EF 70-200 f/2.8 lens fixed at 200mm mounted on a small equatorial mount. This image is of the Heart & Soul Nebula, along with the double cluster (IC1805, IC1871, NGC 869 and NGC 884). This is basically how an image might look, right out of the camera.

This image is what we’re striving to achieve, after some stacking and processing.

There are probably a thousand ways to process these images. Sometimes it’s better to do a certain step before another step. It requires a lot of experimentation to get a good recipe. I’ve outlined what seems to work for me, in most of my images. There are always outliers that require something different, or some other tools. But generally, these basic techniques will help you get from a rough stack to a decent final image.

Again, here’s something similar to what you’ll have coming out of your camera. If you’re shooting from dark skies, your starting image may be much better. It’s always easier to process images shot from dark skies as much of the work in post-processing is removing the artifacts added by light pollution.

After stacking, it will have a little more detail and less noise depending on your particular subject.

Next, adjust the levels to reduce the black point. Just be aware that if you move the black pointer past the start of the histogram, you’ll be clipping valuable data. It’s also usually better to leave a little space between the black pointer and the start of the left hand side of the histogram.

This is the result after a levels adjustment.

Some astro images will develop a green hue to them. Many times this has to do with light pollution, other times it will arise from processing. But there is a free tool called HLVG that will attempt to remove the green hue. You can experiment with the level of strength required for your particular image. It’s free and well worth installing and using.

Here’s the appearance after running HVLG on the strong setting.

Several of the next processes use a set of Photoshop actions from Noel Carboni. These actions were developed specifically for astrophotography and are available at very small cost.  They are one of my go to tools for post processing my images.  The tool used on this operation is Enhance DSO and Reduce Stars. This action generally helps pull out some of the faint detail and reduces some of the star intensity. Be aware that this action takes quite some time, so be patient and let it run. Here’s what you get when it’s done.

Similar to the green color filtered above, sometimes astro images will have a stronger gradient across the image.  This is especially true if you’re shooting from an area that’s light polluted.  The gradient is difficult to see in the small blog images.  It is also less severe than what I typically have.  These gradients are typically very difficult to remove manually, but luckily there is another tool that helps to remove them. It’s called GradientXterminator (GXT). This tool loads as a filter in Photoshop. This filter is also available at a small cost. But if you’re planning to get serious with astrophotography, it is almost a must-have addition to your processing toolbox.

Here’s the result after GXT has been run on the coarse detail, medium aggressiveness setting. It was followed-up with a levels adjustment, exactly as performed before.

Then finally, depending on the image, I usually follow up the processing with another action from the Noel Carboni set called Increase Star Color. The change is subtle but many times the entire star field will all appear monochromatic.  Usually the stars are much brighter than the object being photographed.  So the stars can get saturated.  This action will help add the proper color to the stars, which adds more dimension to the image.

There are loads of other tweaks and adjustments that can be done to the images. Here are a couple things to keep in mind when making the final adjustments. First, calibrate your monitor. This is important, but more so with astro images. Second, the sky background shouldn’t be jet black. A pure black background tends to make the images look synthetic and over processed. I generally try to keep my black point (as reported by the Photoshop info/cursor over the darkest areas) at about 20,20,20 (RGB), but certainly no blacker than 10,10,10. Third, I found that less is usually more when doing this kind of processing. Use a light hand and the image will look realistic. Heavy handed processing has an appearance that looks edgy and has many artifacts. Finally, have fun. This is a hobby and should be enjoyed as such.

I hope you’ve enjoyed this astrophotography series as much as I’ve enjoyed sharing it. Doing this kind of imaging has been a dream of mine since seeing my first astronomy magazine more than 40 years ago. Sharing what I’ve learned will hopefully help you shorten your learning curve that was nearly vertical for me. Good luck and dark skies.