Aurora Post Processing 2: Raw Therapee

Aurora Photography Tutorials

As I eagerly await the release of GIMP 2.10 that will support both 16 and 32 bit color depth, I figured it’s a fine time to continue what I started with aurora post processing 1: white balance and noise and give a more in depth tutorial on processing digital aurora images (using all open-source software). In this installment I’ll be using RawTherapee, a program very similar to Lightroom. If you want some simpler answers to processing aurora photographs (with easier to use software and interface), check out the above link. This tutorial here is more in-depth, should be easy enough if you are new and not totally comfortable with the software, and will make future work easier to automate – it’s definitely more to learn, but if you take a lot of photos time will be saved in the long run.

Tools for these tutorials:

1. 1. UFRaw

(Easy interface)

1. RawTherapee (very similar to lightroom)
2. GIMP
3. G’MIC plugin for GIMP
4. exiftool (only for copying exif info back to a tif if edited in GIMP)

Linux rant – feel free to skip: All of these tools are avilable for Linux, Windows, and Mac. I have had issues with GIMP being unstable in Windows and have had (almost) no trouble running any of these in Linux. If you really want to go the open source route, it might be worthwhile trying a Linux operating system. I’m currently using Ubuntu, but there are plenty of others (Fedora, Red Hat, Mint). With Ubuntu, it’s easy to install as a dual-boot, so you choose which operating system to use on start-up. There’s a huge community base, so it’s pretty easy to find solutions to problems. Actually, a lot more useful than Windows or Mac, because with such a large community and customizability that comes along with Linux, you can almost always quickly find a fix for your problems instead of simply sending an error report and waiting for the bug fix to never come before they move on and try to sell you an upgrade to the operating system or program. I feel much better occasionally donating money to the open source developers knowing that it’s going to the developers and not to a massive corporation where the primary focus is on marketing and not on the product. I’ll never have to pay to upgrade to the newest version and it’s so easy to add user-written plug-ins in GIMP that the software is fully customizable. It’s an addictive world, it may take some time to get to know and love, but you might like it.

The bare-bones tool used in this article is RawTherapee. You can easily finish an image from RAW as a 8-bit or 16-bit tif or a jpg. I often like to do a bit of additional processing, usually level adjustment or further de-noising using GIMP (or G’MIC) software. Since GIMP doesn’t load exif data from tif images (not the case with jpg) I use exiftool to put it back.

The idea is to get the correct color, correct exposure, and eliminate noise without producing too many unwanted artifacts from processing.

1. Open the RAW image in Raw Therapee

I chose this photo as an example because it has a strong foreground element lit by a halogen houselight and aurora behind a thin cloud layer illuminated by a variety of artificial light. Three very different lighting conditions make this a difficult edit, and lead to a lot of discussion.

If the lighting conditions don’t change much while shooting you can try editing just one or two photos and save the profile (right button highlighted in black). Then just load that profile with other photos (or edit as a batch). We’re going to adjust the white balance before anything else, so after opening the photo in Raw Therapee go to the “Color” tab (highlighted in red). I like working in and exporting in sRGB color space, since that’s what most print labs like, so I change this from the default RT_sRGB in the “output profile” in the “Color Management” tab (highlighted in green). I’ve saved this as my base profile as “My Default” (profiles are highlighted in blue) and then set that as my default setting.

White Balance Adjust

The white balance alone can change the entire look of the photo, from the noise profile to the contrast and exposure. For this reason it’s important to adjust white balance first, or else you’ll end up needing to go back to everything you already adjusted earlier once the color changes.

My Color Philosophy

Color in aurora is difficult to pin down correctly. It constantly changes location and brightness, reds appear and fade quickly, and the illumination of the foreground changes with it. If there’s artificial light entering the scene from streetlights, cars, town light, headlamps then it’s even more doubtful that the camera’s auto WB will be correct. Additionally, aurora isn’t opaque, but the sky can be seen through it, influencing the actual color you perceive with your eyes. Twilight, airglow, diffuse backscattering of artificial light can all effect the color of the sky and with it the color of the aurora you see. During the brightest displays, this will have little effect. A diffuse aurora where there is a lot of sky showing through it will have a much larger effect. This is probably the #1 reason to shoot aurora in RAW – it’s easy to correct in post.

For me, it was easy to get caught up in self-battles over color and this hasn’t been an easy war to win. I used to get incredibly frustrated over color, never feeling like it was “right”. Finally, I have come to the conclusion that color accuracy is not the most important thing and that “close” really is good enough. Remember, everything that detects light, whether film, digital sensor, or the human eye has different sensitivities to different colors. If you shoot aurora on film, the film type will dictate what the colors are on the print greatly, with digital we can nearly master control of the color after a night of shooting. It wouldn’t do much to try to pin down the 557.7 nm spectral line on the aurora in the photo, because it might not look like that when you’re watching it. You may have noticed that reds show up much more prevalently on the screen than with your eyes, sometimes I don’t see any red at all. Also, diffuse aurora can be so dim that you can see it, but there’s not enough light for the color receptors in your eyes to pick it up – it looks white, but there it is in brilliant green on the lcd screen. All those other background factors come into play and your perception of color will also change under all these different circumstances. In cases where there is artificial light and you really would need a variable white balance to get the color right, it’s ok to just go with close and employ a few guidelines:

1. Keep mental notes or even written notes on what you observe during the night. This actually helps me a lot when I start to edit a day or two later. Maybe the display was so bright that the snow turned green, the trees were bathed in an orangish glow from a lamp, or very faint pinkish-reds appeared breifly.

2. Avoid the extremes. The 557.7 emission line can be a guideline and the aurora in the photo should (usually) come close to that color. Often I see these steel blue aurora photos against a black sky – which is far from realistic. This isn’t always the case, though. When shooting in bright twilight the aurora takes on an almost cobalt-like color.

Typically, if the camera white balance is not to my liking, I’ll try a few pre-sets first. With aurora photos I sometimes have luck with the “Daylight”, or many of the “Fluorescent” (often cool white, white, or warm white) pre-sets.

In this instance something in between the “Fluorescent > Cool White” and “Fluorescent > White” seemed like it would work – optimally balance both foreground color, aurora, and cloud-cover accurately. Both have the same tint level, so I just switched to “Fluorescent > Cool White” and brought the temperature down until I was happy (4100).

After WB adjust. The ambient lighting on the clouds and trees is much better represented than the pinkish color that came off the camera WB setting and the sky is a bit closer to black.

That’s typically it for me as far as color is concerned. Occassionally I’ll do some adjusting in the RBG curves as well, but that’s a much larger topic that will require its own post.

Chromatic Aberration

The next fix I want to tackle is chromatic aberration. This will have an effect on the detail work in noise reduction and sharpening later, so it’s good to take care of it now. Chromatic aberration can be prevalent, especially in many aspherical or wide angle lenses and can most often be seen around stars or where there are dark edges with bright backlighting. Here I zoom into the edge of the trees.

You can see some magentas around the edges of the trees. This is actually a really small amount, and I don’t really feel the need to fix it in this case (the results might not be great anyway), but I’ll do it here to show how.

The Auto Correction did fix much of the magenta on the edge of the tree with very little or no loss in detail.

Get a handle on noise

Sometimes it helps to bring the exposure up a bit before performing noise reduction operations since it’s easier to see what effects it has in the details. Just make a note what the exposure is set at so you can bring it to the right place after editing. I’ll usually bring it up to a point just before I start clipping highlights in the aurora. You’re going to need to zoom in close (at least 200%), the preview for noise reduction doesn’t work for the full image.

After bringing up the exposure about a full stop and zooming in close, the “salt-and-pepper” and color noise can be seen easily. I usually like to start by just looking at an area that has as much of the image as possible (sky, aurora, foreground).

Raw Therapee: Detail Tab –> Impulse Noise Reduction
Enabled, this will remove some of the salt-and-pepper type of noise. Also, if you have dead or hot pixels, this often takes care of them without needing to use a darkframe. But be cautious, for night images leaving the threshold at the default 50 you may end up removing stars. I’ll typically bring it down to about 15-20. In many cases this won’t even seem to do anything (you might notice a few dark or bright pixels disappear). This also means that the side effects are small so it really doesn’t hurt to use it.

Raw Therapee: Detail Tab –> Sharpening
Sometimes it can be helpful to disable sharpening (usually enabled by default). It’s also possible to simply decrease the sharpening radius or amount, play with it a bit and see what it does. You may want to leave it on if you have a lot in your foreground, but if your picture is mostly an amorphous green blog in the sky this will help in reducing the noise overall. For my case here I just leave it on at the default settings.

Raw Therapee: Detail Tab –> Noise Reduction
Enabled, this will remove quite a bit of the luminance and chrominance noise. The luminance slider will have the most noticeable effect on the image. Find the spot you like, but too far and your photo will start looking like a watercolor painting. I’m usually happy somewhere between 30 and 50 when foreground objects aren’t a main focus. If the foreground is a significant part of the image I’ll keep it between 15 and 30.

The chrominance slider is much more foregiving most of the time. This will eliminate a lot of the strange color noise, often reds and blues, that appear in the image, especially those that occur in underexposed (black) areas. I usually turn this up to at least 30.

After noise reduction is applied. The trees are now a bit washed out in both detail and in dynamic range. The chrominance noise is nearly gone, the “speckled look here is mainly because we’re looking at a region where the aurora is very dim, so there’s a lot of sky blending in from behind. There’s not much that can be done to selectively restore the details in Raw Therapee alone since it doesn’t support layering. I will do some more work on the exposure in the next section.

I could very easily smooth this out more, effectively remove all of the noise, but end up with my foreground looking like a painting. I feel that a bit of noise is perfectly acceptable and even makes me reminisce my old film days since its somewhat similar to grain in aesthetic. I may eventually get to a tutorial on layering in GIMP where you can apply noise reduction only to the sky, leaving the foreground alone. Even this has its drawbacks as it leaves starkly different textures in the photo. I’m more than happy with the results I get here.

Exposure

Often the last edit I tackle in an aurora photo is the exposure and contrast (unless big adjustments are called for early on). This is another area, like with white balance, that you can potentially take a huge amount of artistic license with. I typically go for as realistic as a look as possible. For this reason, I’m ok with leaving the photo a bit “underexposed”. Here I brought the black level up a little bit to help restore the contrast lost in the trees from noise reduction. Taking the black level up really high will start to clip shadows (add another future post to the list – histograms) and more, turning the trees into silhouettes.

The final edit I chose to go with through this workflow. I could bring the exposure up a bit (you can see in the histogram I’m not filling the whole dynamic range), but I usually reserve this for truly bright displays. If anything I would say this is already a bit brighter than it looked to my eyes, and I do try to keep the scene as “true” as possible.

I should note that I tend to limit myself to adjusting overall exposure, lightness, and black levels rather than just pawing at the contrast. The contrast will effect the whole image equally, while lightness and black level will have more of an effect on the darker parts of the image while changing the bright parts less. The same effects can be accomplished using the curves editor with much more control (even with the individual color channels), but once again, this will have to wait for another day.

Subscribe for Weekly Updates