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 (near infrared) and z (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.

You can download SDSS images by clicking here. You will see the interface shown below.

Search by Run/Rerun/Camcol/FieldID/ObjectID. Use the default columns for the information.

We will start off by looking at a nice spiral galaxy. The galaxy is in run 94, rerun 7, camcol 2, field 446, and the object id is 6. Type these in the large box, separating them with only a space - that is, type "94 7 2 446 6". Check Atlas Images for product type. Select all five bands (u,g,r,i, and z) and return the file as a .zip file. Click Get Products. Save the zip file to the directory you specified earlier. Unzip the files to that directory.

The files will have long and tedious filenames, so you may wish to rename them. The galaxy we are looking at is NGC1087. 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.