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The Palomar Sky Survey
First, let's compare the SDSS to an earlier map of the sky. The First Palomar Sky Survey (POSS I) was carried out in the 1950's using the 48-inch Oschin Schmidt telescope at Mount Palomar in southern California. This survey was carried out using photographic plates. It has since been converted into a digital format and is easily accessible on the Internet.
During the 1970's, the U.K. Schmidt telescope, nearly identical to the Oschin telescope, carried out the Southern Sky Survey. In the early 1980s, Palomar's Oschin telescope was upgraded and carried out a second sky survey called POSS II. POSS II is currently being converted to digital format. Portions of it are available on the web.
Let's start finding out how this survey is different from the SDSS. We will compare POSS I and the SDSS by looking at images of the same portion of the sky taken by each survey.
Let's look at the field containing Pal 3, a globular cluster discovered by POSS (also featured in SkyServer's H-R Diagram project). In the POSS interface, type Pal 3 in the field labeled "Object Name," then click Get Coordinates. You will see the coordinates appear in the box below. Click on File Format and change it to a .gif file (unless you have a special viewer for .fits files and want to use it). Now click on Retrieve Image.
Now to get the SDSS image. Click on the Ra/Dec link in left-hand column the Navigation tool. A small window will appear. Enter the ra and dec of for Pal 3: ra = 151.3801, dec = 0.072. Click OK. This will bring up the SDSS picture in the Zoom window. You should now have two images of Pal 3: one from POSS I and one from the SDSS.
Scroll through the SDSS Navigation tool until you find a field you think is interesting. Note the field's ra and dec. Find the same field in the POSS I survey by entering the ra and dec into the form.