The 48-inch is a wide-field Schmidt telescope (also called a Schmidt camera) that was built for surveying the sky. Images of the northern sky used in WWT were obtained from the Second Palomar Observatory Sky Survey (aka POSS-II). The pictures for the survey were first recorded on large (14" inches on a side!) glass photographic plates. They were then scanned at the Space Telescope Science Institute in Baltimore, Maryland. From there they were processed and calibrated at Caltech under the leadership of Caltech Professor of Astronomy S. George Djorgovski. This survey has detected over 50 million galaxies and about a billion stars, as well as many other interesting objects.
The WWT also has data from the ongoing Palomar-Quest digital sky survey, also performed with the 48-inch Samuel Oschin Telescope. The glass photographic plates are now long gone, the Palomar-Quest survey makes use of a 161-megapixel camera. Don't forget to read the full press release on Palomar & Caltech's involvement in WWT.Shown below is one of many infrared images of M81 that you can see on WWT. The image comes from the Spitzer Space Telescope.