Samuel Oschin Telescope Surveys at Palomar Observatory

How are objects discovered?
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Dwarf Planet Eris

Since 1948 the Palomar Schmidt, now named the Samuel Oschin Telescope has been surveying and monitoring the skies. Initially this was with large, 14-inch square photographic plates that each imaged a 6-degree wide area of the sky. The first Palomar Observatory Sky Survey or POSS I survey, which lasted from 1949 to 1957, was sponsored by the National Geographic Society. It covered the whole Northern sky visible from Palomar Observatory with plates sensitive to both blue and red light. Paper copies of these plates became a basic reference for observatories worldwide. Astronomers would examine these plates to see what was visible at the location of newly discovered infrared or radio sources for example.

     The second POSS II survey, was undertaken between 1985 and 2000 with new, higher sensitivity photographic emulsions from Eastman Kodak. It surveyed the whole northern sky, in 897 fields, in each of three colors, and complemented a southern survey obtained with the United Kingdom and European Southern Observatory Schmidt telescopes. Together these formed the first truly all sky survey, and provided the basis of the digitized DPOSS survey, the Hubble Space Telescope Guide Star Catalog (GSC), and the United States Naval Observatory (USNO-B) catalog of over 1 billion stars and over 50 million galaxies. These catalogs and the digitized plates they are based on are often searched to check the historical record of new discoveries, and to determine precise orbits for moving objects.

     In 2000, the Samuel Oschin Telescope was converted to using a large format CCD camera to replace photographic plates, and the control system was automated so it could work automatically and unattended each and every clear night. The CCD camera was upgraded to the larger QUEST camera in 2003. With these upgrades, the telescope embarked on two new, rapid response, survey capabilities.

     The new QUEST camera on the Samuel Oschin Telescope at Palomar has 161 million pixels, making it one of the largest and most capable digital cameras in the world. Modern digital technology with pipeline processing of the large volume of data produced, enables us to enter the exciting realm of detecting very faint moving and transient objects.

     The telescope and camera operate in two distinct Point and Track and Drift Scan modes.
In the Point and Track mode, especially useful for finding moving objects, the camera is pointed to a piece of the sky, and takes a relatively short exposure. It then goes off and images a pre-arranged sequence of such target fields. After a period of time it comes back and repeats the sequence. Then it does it again after another interval. This way astronomers get many triplets of images, each of the same piece of the sky. Any objects that are visible in all three images, but move consistently with respect to the background star field, are solar system objects such as asteroids, comets or Kuiper Belt objects. Because of the large amount of data, pipeline processing is used to both detect such objects, and to calculate their preliminary orbits from the initial triplet data.

The timing of each exposure (seconds to minutes) and delays between them (typically 30-60 minutes) are referred to as the cadence, with longer exposures and longer gaps between exposures typically necessary to find the fainter, outer solar system objects that appear to move more slowly. Sedna and 2003UB313 were found with this technique, as are a large number of Near Earth Asteroids by the JPL NEAT program.

     In the Drift Scan mode, especially useful for scanning large areas of sky and looking for objects that vary in brightness, the telescope is parked at a fixed declination and hour angle. The camera shutter is opened as the rotation of the earth causes the selected strip of sky to slowly drift past the telescope. There are 112 individual CCDs, each 600 x 2400 pixels, arranged in a mosaic of 4 rows or "fingers" of 28 CCDs each (2400 x 16800 pixels). Starlight crosses each finger in turn. Each finger can detect light through either the same or different color filters, as required. In Drift Scan mode, the CCD electronics are "clocked" along their columns (drift direction), so that the accumulating charge packets from stars move in synchronization with the drifting image, and are eventually read out at the end of each column like a strip chart, or 4 strip charts, each 28 CCDs tall. Exposures in this case can be several hours long. This digital data, now for 15,000 square degrees in each of 8 colors, forms the basis for searching for Supernovae and other time variable sources, and the database for checking the historical record for such objects once found.

     Drift Scan mode works best at declinations close to the equator, between -25 and +25 degrees. This is because the stars trail in curves, not straight lines, and the fingers need to be adjusted for tilt for each declination within this range.

All data is transmitted from Palomar Observatory via the High Performance Wireless Research and Education Network