Retrieving SDSS Images
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Each camera column of the SDSS camera consists of
five CCDs covered by five filters |
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.
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 (Ctrl-click on a Mac) on one
of the filter names (u, g, r, i, or z) and select "Save Link As" or "Save Target As." Save
the file(s) in the directory where you have your Iris images.
The files will save as .bz2 files, which are a type of compressed file that cannot be
opened by all programs.
StuffIt
can open .bz2 files; you can download it as evaluation
shareware from www.stuffit.com.
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.
Now let's open these images up and see what they look like.
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