GIMP, Fiji and astrophotography on Linux

Ever wanted to know how to process those wonderful Hubble like pictures on Linux? It is not difficult, I will show you how. We will do RGB processing and LRGB is very similar.

On the Web we will encounter plenty tutorials where author uses some proprietary tool to do contrast stretching and after that Photo Shop to do final processing. Usually none of those will be available on Linux. Replacement for Photo Shop is no brainer, it is naturally popular GIMP. Tool for contrast stretching is trickier. For nonlinear stretching I am using Fiji, what is almost the same as ImageJ. Guess there is no distribution which doesn’t offer GIMP so just follow usual install path for your distro. It is good idea to install gimp-plugin-registry as well. For Fiji you will have to download it from http://fiji.sc/wiki/index.php/Downloads and untar it. It comes with or without Java runtime, so we pick what we need depending do we already have Java or not.

Now we installed required programs and we need data. Typically data consists of three or more gray images in FITS format. FITS is abbreviation for Flexible Image Transport System. Good source of FITS data is Las Cumbres Observatory Global Telescope Network and here is their website http://lcogt.net/

They have two two meter reflectors and few smaller telescopes and observation data is freely available under Creative Commons license. If you get data from 2m telescope you will end-up with three 2008x2008 pixels images about eight megabytes each. So go there and pick some galaxy from Observations section, I will go for NGC6946, looks nice. After downloading Blue, Green and Red FITS we can start.

And we immediately see why we need contrast stretching, there is barely few stars. Now from menu we do Process -> Enhance Contrast, tick equalize histogram and hit OK button. Result looks like this:

There is much more to see but also huge amount of noise. We repeat the same story for remaining files and save them as Tiff. If we want we can go to Image -> Color -> Merge Channels and create composite to see how approximately it will look like.

It is nice but too much noise, time for GIMP.

We open all three tifs in GIMP and we do Filters -> Enhance ->Wavelet Denoise with settings like on picture. If you don't have gimp-plugin-registry installed there will be no Wavelet Denoise then just do Despeckle few times.

We do the same on remaining two pictures doing Ctrl+F repeating last filter. Next step is Image -> Mode -> RGB followed by Colors -> Colorify and we apply what is actual color on it.

Now we copy green and paste as layer over red, rename Pasted Layer to something and change layer mode to Screen, we do the same with blue one.

If alignment is OK we can merge layers.

Now we can play with curves, levels or do decomposition to enhance colors and so on, get imaginative here. Here is how it looks like without any additional processing.

If frames are properly aligned we could place them as layers into single image and do Colors -> Components -> Compose what is simpler than doing Mode and Colorify.

If we have LRGB, then we process RGB as above. L we stretch, denoise and at the end use it as value layer and RGB as color layer.