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Data Handling


How much data will Pan-STARRS produce?

Each raw image from a single Pan-STARRS camera will contain 2 Gbytes (2 bytes per pixel). In full survey mode, typical exposures last 30 seconds, so the raw data rate is several terabytes per night for the full telescope. The amount of data produced by Pan-STARRS is so large that it will not be practical to archive every image. Software techniques are therefore being developed to extract the important information from the images, while allowing less crucial information to be discarded.

What kind of data reduction will be necessary?

As soon as a Pan-STARRS image is read out, it must be searched for known stars, which are then used to calibrate the image both astrometrically and photometrically. The images from the four cameras are then compared with each other to remove spurious effects such as cosmic rays, gaps between CCD cells, and bad pixels, and a composite image is calculated based on the four subimages and corrected for measured variations in atmospheric seeing. The composite image is then used in three ways:

First, so long as it passes quality control, it is added to a master image of the sky that is being built up week by week. As the total amount of observing time grows, this cumulative image will have a better and better signal to noise ratio. The cumulative sky image is particularly important for studies of distant galaxies and clusters of galaxies, which are faint, but which are not expected to move or vary.

Second, each composite image will be subtracted from a master image made from earlier observations of the same part of the sky taken with the same filter. Stationary, unvarying objects should cancel out, leaving only those objects which have moved or changed their brightness. The positions and magnitudes of these objects will then be measured and added to a database. Some of them will be known moving or varying objects, others will be new discoveries, the most interesting of which will be reported to the Pan-STARRS staff for further study or action.

Third, each composite image will be digitally scanned and the positions and magnitudes of all objects above a threshold brightness will be stored in a database for future reference.

Finally the composite image, as well as the four individual telescope images, will normally be discarded, freeing up storage space for future images.

Given the speed at which Pan-STARRS produces new images, all the above steps must be completed within about one minute of computer time.

Where will the data be reduced and stored?

Data will be reduced and stored at a sea-level computing facilty consisiting of a large array of realatively simple PCs. The data will be sent down from the observatory by fiber optic link.

Will the data be publicly accessible?

The "3π" deep and ultradeep surveys will be accessible to the public after completion, as will catalogs of moving, variable, and transient objects discovered by Pan-STARRS.

Next: PS1 Prototype

 



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