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Image Processing
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Retrieving SDSS Images

Astronomical images are usually saved in a format called the Flexible Image Transport System, or FITS. Unlike the GIF or JPEG images you have probably viewed before, FITS images can't be viewed in web browsers. You need a special viewer to see them - that's why you downloaded Iris! As you retrieve images from SDSS, you will be retrieving FITS files.

Each camera column of the SDSS camera consists of five CCDs covered by five filters

The SDSS camera takes images through five filters called u (ultraviolet), g (green), r (red), i and z (both infrared). Each of these images is a grayscale image. To make a color image, you must combine three of these grayscale images. Iris will help you make tri-color images.

To practice using Iris, we'll start with a beautiful SDSS image of a spiral galaxy, NGC 1087. To get the image, open the Object Explorer. You will see a screen like this:


Click the "Ra,dec" link in the left column; a small window will pop up. Enter the coordinates of NGC 1087: ra = 41.606, dec = -0.499. A new window will pop up containing the Object Explorer entry for NGC 1087.

To get the FITS images, click "FITS" under "PhotoObj" in the left column. You will see a screen like this:

"Corrected frames" are the final step in processing SDSS images, so those are the images you want. To get a corrected frame image, RIGHT-click on one of the filter names (u*, g*, r*, i*, or z*) and select "Save Target As." Save the file(s) in the directory where you have your Iris images.

The files will save as .gz files, compressed files that a program like WinZip can open. (If you don't have WinZip, you can download it as evaluation shareware from Even compressed, the files are still more than 2 MB, so they may take a while to download.

The files will have long and tedious filenames, so you may wish to rename them. You might rename the files NGC1087red, NGC1087green, etc. Choose names that are easy to remember. Change the file extensions from .fits to .fts so the images will open in IRIS.

Now let's open these images up and see what they look like.