Girls Exploring the Universe!

July 8 - 12 2019, University of Virginia

 How Astronomers make Images from Telescopes

Telescopes use CCDs to capture light. This information is then recorded so we can reconstruct images. Modern telescopes observe objects through many different filters, capturing different images with the same object with these filters. These images are combined together to make multicolor images. Three of the most popular filters are red, green, and blue (RGB). Today, you will get to make one of these images yourself of the M101 galaxy, just like professional astronomers do!

  • We will use a program called DS9 to combine images from different filters.

  • If you haven't installed it yet, download and install it from here:

  • Our images are in a special file format called FITS. DS9 can read in these files and make images!

1. Loading the image

1. Locate your images.

2.     Make sure your image names are easily identifiable as red, green, and blue — so they are named something like <image>_r_<number>.fits, <image>_g_<number>.fits, and <image>_b_<number>.fits.

3.     Launch DS9.

4.     Go to the “Frame” menu, choose “New Frame RGB”.

5.     Make sure the “Red” band is selected in the “Current” column of the RGB window like this:


Now open a file with “File” → “Open..”  in the main DS9 window. From the folder you've saved your downloaded files in, open the red, or r-band file.

6.     Now change the current band to “Green” in the RGB window. Open the green-band file as you did for the red.

7.     Now change the current band to “Blue” in the RGB window. Open the blue-band file as you did for the red and green.

2. Adjusting the range, brightness, and contrast

8.     First, we need to line up and match the three color images with each other. Go to the “Frame” menu, then select “Match → Frame → WCS”.

9.     In the “Zoom” menu, set your view to “Zoom 1/4” or “Zoom 1/2” so the whole image is in view.


10.   Now change the “View option in the RGB window so that only one color is selected. Make sure this color is selected as “Current” too. The RGB window should now look like this:






11.    We don't see anything yet! We need to set the correct pixel count range. Go to the “Scale” menu and choose “Scale Parameters..”. Drag the red and green limits so that they bound the brightest bounds of the pixel distribution, like this:



12.     We still don't really see the image! We need to change another scale parameter. In the “Scale” menu, select “Log”. This will make the object stand out better from the background.

13.      Now you should see something that starts to look like a star cluster or nebula or galaxy! We can make this look even better by changing the contrast and brightness. Go to the “Color” menu and select Color Parameters. In the new box, play around with the Contrast slider first, then the Bias parameter. How does the image change? A good Contrast would be around 2.5 and a good Bias around 0.5.

14.      Now we have an image of the galaxy in the green band! Repeat steps 10 – 13 for the “Red” and “Blue” images. The best “Contrast” might be a bit different for these two colors.


15.      The final step is now to add these three color images together! In the RGB window, check all three options (Red, Green, and Blue) in the “View” column. Now you have a three color image! It should look something like this:


16.     Save your image! Go to “File → Save Image → JPEG..” and save it as <your_object>.jpeg

This is certainly not the only object you can make colored images of! There are tons of interesting galaxies on the Sloan Digital Sky Survey website here: