Free Post Processing
In this post I will walk you through post processing a set of astrophotography images using only free programs.
Getting the Data
Let’s start with getting our raw images. I’m going to be using a 8-in Celestron Schmidt-Cassegrain telescope with a Cannon EOS Rebel T2i. I am using an advanced GT star tracking mount so that I can collect long exposures. Tonight I decided to fix my sights on Orion’s nebula. I chose this nebula because I was setup in an area with a lot of light pollution and knew I could still get Orion’s stunning color.
Once I finish setting up; I knew the telescope and camera had come to temperature and took several dark frames. Dark frames are images taken with a cover on the camera so that no light can get in. This allows you to know how much noise the camera is producing at a given temperature and exposure time. Pixels that always register above their actual value are called hot pixels, and they are easy to find and correct for with dark frames. A general rule of thumb, is you always want to take more than 15 dark frames at the ISO, exposure, specific camera, and temperature you are shooting at.
After I finished taking my dark frames, I started to take light frames. These are the frames of the object you are looking at, in this case Orion. I ended up taking around 20 light frames of the nebula. Each one had an exposure time of 30 seconds and an ISO of 1600. Below you can see one of the raw frames to get an idea of what the camera could pick up. As you can tell there’s a lot of stray light and grain in the image. In the next step I’ll go over how to use multiple frames to clean up the image.
Astrophotography Notes:
Two quick notes for anyone new to astrophotography. First, always shoot in at least RAW format for your camera. This stores more of the data and you can correct for bad settings later. I shoot in RAW+L and carry an extra SD card if I think I’m gonna run out of data. Second, try to keep your hands off of the camera when taking a picture. I have a remote trigger for my camera, they are about $15 and one for Cannons can be found here. If you forgot your remote trigger at home, you can always set your camera to have a 1-2 second delay on taking the picture after pressing down on the trigger. This will reduce the amount of vibrations from your hand on the camera and improve your picture quality, especially on long exposures.
Stacking
Deep Sky Stacker, DSS, is a free astrophotography stacking software. You place in a set of images of the night sky, along with the dark frames, and it will use the stars visible in each of the images to overlay them. DSS can be found here, and I suggest getting version 3.3.4 .
Now there are many in depth tutorials on how to use DSS by people with more experience using it than me so I’ll just stick to my workflow for this picture. DSS is quite simple and intuitive. I began by importing all of my light frames using the “open picture files” button. Next I imported all the dark frames using the “dark files” button. After selecting all of them I then clicked “register checked pictures”. I set DSS to use only the top 80% of photos. The rest of the process should be automated, and result in a picture similar to the one below. This process can take a while to work so give it some time.
Once the image appears I increase the saturation to between 20 to 30. This draws out some color. Finally I save the file in Tiff 16 bit with the changes embedded in the photo.
(Note: I massively downscale the quality of this image so that his page wouldn’t take forever to load. It started at ~.5GB and is now at 430KB)
Raw Adjustment
If we look at the final image from DSS it looks very de-saturated and lacking a lot of the color from the original single photo. To boost the saturation and make some other small adjustments I turned to Raw Therapee. Raw Therapee is a raw photo editor, similar in purpose to Lightroom. It’s free and can be downloaded here. As you can tell from other photos on this website, I’m not the worlds best photographer. I use Raw Therapee to adjust my photos, so that if I mess up with the cameras settings I can fix small mistakes.
Once I imported the photo from DSS into Raw Therapee, I began by increasing the saturation and contrast. This brought out the colors from the nebula. Additionally this also showed that there was a bit of a red haze over parts of the image that ended suddenly. This is an artifact from the stacking process. This sudden transition from the black of space to the red of the nebula wasn’t realistic or visually appealing so I upped the black in Raw Therapee. More of the pixels were now registered as black and made the transition more realistic. Finally, I increased the sharpness of the image to hopefully show a bit more detail. I actually played around with more settings but these were the only ones I could see a difference from. If you want the full edit file you can find that here.
(I’m currently, 2018/03/08, using this photo as my phones background)
Final Photo Editing
In the above photo, as well as in the thumbnail image (reproduced below), you can see my logo as a watermark in the lower left corner. This can’t be done (or at least I don’t know how to do it) in Raw Therapee. In order to do this I moved over to GIMP, a photo editing program. GIMP, which stands for the GNU image manipulation program, is an opensource alternative to Photoshop. It’s much less powerful, but then again it’s also free. Again there are many tutorials for using GIMP, so I’ll just go over what I did. I added in my logo, inverted it’s color from black to white, and made it 10% opaque. The thumbnail, reproduced below in larger format, is just a cropped version of the main photo.
If you want more Gereshes
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