Viewing FITS files in DS9
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What is a FITS file and why do you need special software to view one?
Astronomers use a special file format for storing and sending astronomical data such as images and spectra. These are called Flexible Image Transport System or FITS files for short. This is the format of images you will get from the LCOGT archive.
FITS files are more than just image files because at the start of every FITS file is a header section, which contains important information about the image, such as the number of pixels, the date the image was taken, the telescope it was taken with, the exposure time, and much more.
What is DS9 and what does it do?
SAO/DS9 is a program that can be used to view and manipulate FITS files. It can also save FITS files as other more common image formats such as JPEG and TIFF. However, none of these formats contain the same quality of information as the FITS file. In particular, only 256 brightness levels are available for each pixel in a JPEG, whereas FITS allows up to 65535.
DS9 is available for Windows, Mac and Linux. Download it from the SAO/DS9 website.
Instructions for Windows users and Mac users are available to help you through through the steps of downloading and starting DS9.
How to view your FITS files
Once you have DS9 open, go to File>Open and then choose one of the FITS files you downloaded. You will then see a screen with no obvious image and mostly black. DS9 is displaying the full range of pixel values in the image, from 0 to 65,000. In order to make the image more visible, use the Scale option from the menu bar.
The scale option in DS9 sets the limits for the image, based on the pixel values. The example below shows a scale of 99%. The program plots a histogram of all the pixel values and displays as the image, all those pixels which are within 99% percent of the mean pixel value. In this image, 1% of the pixels are thrown away, and these are the black pixels. This effectively lightens the image so that more detail can be seen.
Experiment with the different scale functions in DS9 to see which one shows your image best. Once you have decided on the best scaling for your image, you can use the Zoom function to zoom in our out of the image, and view the whole image in the main window.
To open another image, go to Frame>New Frame to open a new frame in the same DS9 window. Then go to File>Open and select the 2nd image. Click on the Zoom button, then select Zoom to Fit Frame. Select Scale and 99% again. Repeat for any other FITS files you would like to view.
Below is a screencast you can watch that shows the same steps as described above. HD version coming soon!
What next?
Now that you know how to use DS9 to view and image from the archive, you might want to try:
- Searching the archive for images of asteroids, open them in DS9, then blink the images and spot the asteroid.
- Basic color imaging using DS9 and Gimp
- Use the DS9 manual to explore more functions of DS9
by Sarah Roberts and Edward Gomez, edited by Jessica Barton