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How the ROSAT PSPC survey was generated

This document describes the steps used in generating the SkyView ROSAT PSPC survey.

Description of the Survey

The SkyView ROSAT PSPC survey is a mosaic of public PSPC observations available at the High Energy Science Archive Research Center. It includes data from all PSPC observations with the following exceptions: data taken with the Boron filter, data taken prior to 1991-1-1, and observations for which there was no good exposure. [Currently there are also 6 observations not included because the number of photons in the observations makes the mosaicking program overflow memory limitations.] A total of 4174 observations are included in the mosaic. 274 verification period observations are excluded as well as 40 observations for which there is no good data.

The mosaic comprises 600x600 pixel tiles. Each pixel is 15" square so that the coverage of an individual tile is approximately 2.5° square. Each tile is a gnomonic projection about the center of the tile, i.e., the intersection of the four central pixels.

The tiles are arranged on the sphere in 'rows' at declinations separated by 2°. Each row has tiles separated in right ascension by no more the 2°. Thus tiles should have an overlap of at least 30 arc minutes (or 120 pixels). The total number of tiles needed for complete sky coverage would be 10359.

For each observation a determination was made of the tiles to which it would contribute and then the counts and exposure were added to cumulative counts and exposure maps for the tile. When all observations had been processed an intensity map was created by dividing the counts maps by the exposure.

Three different mosaics where created by using different cut-off radii for included observations. The three cut-off radii used were 2°, 1° and 0.6°. For these three cut-offs data were found for 5998, 4540 and 3984 tiles respectively. For each tile the contributing observations were recorded and are included in the tile headers. Virtually all tiles have only partial coverage.

Total sky coverage is estimated at 14% for the 2° survey, 4% for the 1° survey and 2% for the 0.6° survey. Coverage in 'interesting' regions of the sky is typically much higher.

Using the ROSAT surveys in SkyView

SkyView uses the tiles as a standard SkyView survey. The 1° and 2° intensity maps are available directly through the forms interfaces. Ten surveys total are available to users either from the results of previous search request, or through the Batch interface: Intensity, counts and exposure maps for the 0.6°, 1° and 2° calculations, and the old PSPC intensity survey (which should be similar to the 2° maps with substantially fewer observations).

SkyView does not know anything about which parts of a tile may have been observed. It may return a blank image the user requests an area near but not overlapping other observations.

How was the mosaicking done?

This section describes the algorithms used in mosaicking the data. Throughout we have used the term tile to refer to the mosaic fragments, while observation refers to the ROSAT observation being added to the mosaic.

Phase 1: Generating the counts and exposure maps

The system looped recursively over the directory structure containing the PSPC observations. Whenever the job was interruped it was restarted using an observation catalog to ensure that data were not reprocessed accidentally.

For each directory:

Phase 2: Generating the Intensity Maps

Once a complete set of counts and exposure maps was generated, a loop over the counts maps was initiated. Each counts and exposure map was read and the ratio -- the intensity map was computed. The value of the intensity map at all points where the exposure was 0 was set to -1. A FITS header was created which included the projection information for the map and information about which observations contributed to each image. A counts, exposure and intensity FITS file was then produced using this header with an appropriate BUNIT's field added. The output files were sorted in 9 directories.

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SkyView has been developed with generous support from the NASA AISR and ADP programs (P.I. Thomas A. McGlynn) under the auspices of the High Energy Astrophysics Science Archive Research Center (HEASARC) at the NASA/ GSFC Astrophysics Science Division.

We gratefully acknowledge the support of NASA and contributors of SkyView surveys.

A service of the Astrophysics Science Division at NASA/ GSFC
and the High Energy Astrophysics Division of the Smithsonian Astrophysical Observatory (SAO)

HEASARC Director: Dr. Alan P. Smale
HEASARC Associate Director: Dr. Roger Brissenden
Responsible NASA Official: Phil Newman
Web Curator: Laura McDonald

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