Creating Astronomical Images

digital data. Sensors don't detect color. Filters determine the wavelengths saved as a file. All astronomy images are constructed from the data and show some artistic license.

This doesn't make them "false color" but rather representations! Everything is 1s and 0s ultimately.


Astronomy uses a digital file format called FITS (Flexible Image Transport System). This is a 32-bit file which means each pixel or element on the CCD (camera) can hold a value from 0 - 232 or 4,294,967,296! This is the intensity of light detected by that pixel through that filter.


A smart phone camera can only handle 8 bits per pixel! That's only 256 levels per color.






CCD cameras or detectors are really like an array of photon buckets. A FITS file allows for a CCD array made of HUGE buckets! You can see the different sensors with the associated filters.



Astronomical data doesn't use "true color" but representative color. This means different colors can represent different things from actual visible colors, to invisible colors like Infrared, to specific elements!

Colors are assigned digitally using a color table. Colors are assigned for aesthetics and science. The creator gets to choose what channels get what colors. Non-visible light need to be assigned a color to be seen at all!

Now it's your turn. These instructions assume you are using GIMP (free image manipulation software program). It should be preinstalled on the school laptops.


First download the file for NGC 5139 that I have prepared for you.

NGC 5139 - Link

Clicking on the download should launch the GIMP editor

      Open the file in GIMP. You should see something like this!

Lets look at each part


Here is the what your image looks like based on your settings

This is the photo editing toolbar. You will not be using this for this lab.


Here is the layer control panel. You can adjust how strong the different layers are, and even change their color! This image is combined from three images, each with a filter to only allow red, green, and blue light in.


Lets do this now! Select the green layer and adjust the opacity (strength) to 0.



No purple stars in real life? Not a problem with computer science!

Changing the color of each layer is also possible. Open the Color menu and find Colorize near the bottom. You can adjust or change the color for that layer! This will be handy later. You can keep your change or turn it back.


The colors picked for filters is a choice however and the creator sometimes changing things up either to show some particular feature of the object in question or just for aesthetics.



There is a special way to copy the image you created, so you can paste it into your assignment document. Open Edit and then pick Copy Visible. Move to the assignment and paste the image into the box for that image! You may need to use the keyboard shortcut Ctrl-P.


Huzzah! That is it for this image. You may also export your image it to png or jpg so you can show it off.


Extra: Go to the Color menu and find Brightness-Contrast. Explore those options for extra pretty.



Next Image: Jupiter! Download and open the Junocam file. You should see an image of the largest planet in our solar system taken a couple of years ago by the Juno spacecraft currently in orbit around this gas giant.




This image was also taken with three visible light filters, red, green, and blue. You will need to colorize this image yourself!



Extra: Explore brightness and contrast as well as the sharpness setting. There is also a levels control where you have fine grained control over how colors show up in the final image.


Copy visible once you have things just so and paste into your lab sheet. File Export your image as a png or jpg if you want to share it on the Snapagram Books�.

You should create color images for NGC 5139, Jupiter, the Helix Nebula, and Zeta Ophiuchi.

The data sets (FITS images) for the Helix Nebula and Zeta Ophiuchi are not visible light, but are IR data from the Spitzer Space Telescope. The FITS images for NGC 5139 were taken using a remote telescope system by the author.

Helix Nebula in IR

Zeta Ophicius in IR








Video: How to make a 3-color image using GIMP


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