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SAS2RAW - SAS-2 Photon Events Catalog



The SAS2RAW database is a log of the 28 SAS-2 observation intervals and contains target names, sky coordinates start times and other information for all 13056 photons detected by SAS-2. The original data came from 2 sources. The photon information was obtained from the Event Encyclopedia, and the exposures were derived from the original "Orbit Attitude Live Time" (OALT) tapes stored at NASA/GSFC. These data sets were combined into FITS format images at HEASARC. The images were formed by making the center pixel of a 512 x 512 pixel image correspond to the RA and DEC given in the event file. Each photon's RA and DEC was converted to a relative pixel in the image. This was done by using Aitoff projections. All the raw data from the original SAS-2 binary data files are now stored in 28 FITS files. These images can be accessed and plotted using XIMAGE and other columns of the FITS file extensions can be plotted with the FTOOL FPLOT.


Fichtel, C.E., Hartman, R.C., Kniffen, D.A., Thompson, D.J., Bignami,
G.F., Ogelman, H., Ozel, M.E., & Tumer, T. 1975. ApJ, 198, 163.

Thompson, D.J. 1993. private communication.


The second NASA Small Astronomy Satellite (SAS-2) was dedicated to gamma-ray astronomy in the energy range above 35 MeV. The satellite carried a single telescope using a 32-level wire spark-chamber. The satellite was spin stabilized with the telescope axis along the spin axis. SAS-2 was launched on 1972 November 15 and became operational on 1972 November 19. On 1973 June 8, a failure of the low-voltage power supply ended the collection of data. During the approximately six months of the mission, 28 pointed observations, typically of a week duration, were made resulting in about 55 percent of the sky being observed, including most of the galactic plane. The field-of-view of the telescope is about 35 degrees (full width at half maximum) with an angular resolution of a few degrees. In addition to the general galactic emission, high-energy gamma-rays were also seen from the Crab and Vela pulsars.

The low fluxes involved in the study of gamma-ray sources make it desirable to minimize the background flux from cosmic-rays. Therefore a low Earth equatorial orbit was chosen having a 2 degree inclination; an apogee and perigee of 610 km and 440 km, respectively; and an orbital period of about 95 minutes. During the short lifetime of the mission, there was some noticable decrease in sensitivity due to deterioration of the spark-chamber gas. The calibration of the SAS-2 experiment was done using both the flight unit and an identical flight spare unit. The range of energy studied at the National Bureau of Standards (NBS) Synchrotron, Gaithersburg, Maryland, was approximately 20 to 114 MeV. The energy range between 200 to 1000 MeV was studied at the Deutsches Elektronen-Synchrotron (DESY), Hamburg, West Germany.

Data Products

This database contains one entry for each FITS file of the original 28 SAS-2 events file observations. Each file consists of 3 extensions: a GTI (Good Time Intervals), an EVENTS and an EXPOSURE extension. the GTI extension contains consecutive 'up-times' within the observation. The Events extension contains pointing, timing and energy information about each detected photon. The Exposure extension contains pointing and on-time information about each exposure from the old 'OALT' file (see Database Overview for more information).

This data can be accessed and analyzed in two ways. The events can be plotted for quicklook observing using the BROWSE on-line command XIMAGE, or a file can be extracted from the database and the analysis FTOOL FADMAP can be used on the file to produce source and background maps from the extracted file.

To run FADMAP, type xp # (where # is a number between 1 and 28, for the desired observation). When the file has been downloaded into your captive account area, run the FTOOL FADMAP, entering the desired FITS file name to be analysed at the prompt. If you have a FADMAP.PAR file already in your account area, you can change the hidden parameters to values other than the default by either editing that file, or by typing any parameters you want to change on the initial command line (type 'fhelp fadmap' from the command line for more details). Fadmap will create 4 files: 'source count', 'source exposure', 'modified background count' and 'modified background exposure'. These files can be further analyzed by running the FTOOL FARITH from your captive account. One option is to first divide the 'source count' map by the 'source exposure', divide the 'background count' map by the 'background exposure' map, then subtract the first resultant file by the second. This will give a fairly accurate 'intensity' map. For more information SAS-2 data analysis, consult the on-line "Explanatory Supplement to the SAS-2 database". See the references section of this dbhelp entry for more information


The SAS-2 mission lasted about 7 months. During this time the instrumental sensitivity declined at nearly a linear rate. This has been documented and incorporated into the calibration database. Due to the limited number of events, only rough instrumental calibration was possible. Of minor concern were changes in the background rate, due to the low Earth orbit.


The Right Ascension (1950) in degrees of the optimum observation direction. With a FOV of approximately 35 degrees, photons associated with a particular observation should be within 35 degrees of this RA.

The Declination (1950) in degrees of the optimum observation direction. With a FOV of approximately 35 degrees, photons associated with a particular observation should be within 35 degrees of this DEC.

Galactic Longitude

Galactic Latitude

Original SAS-2 observation identification number. Numbered from 01 to 28.

The start time of the observation. The time is displayed in the format 'yy.ddd' where yy is the last two digits of the year and ddd is the day number within the year (cf: 73.118, is 1973, day 118)

The stop time of the observation. The time is displayed in the format 'yy.ddd' where yy is the last two digits of the year and ddd is the day number within the year (cf: 73.118, is 1973, day 118)

The source name. If one does not exists, this parameter defaults to 'N/A'.

Elapsed time between start of an observation file and the end of an observation file. Inclusive time, in seconds of entire observation.

The sum of all the continuous, uninterrupted exposures within an observation in seconds. Exposure is approximately the "up-time" of the instrument during each of the 28 observations.

The number of photons collected during an observation. Dividing PHOTONS by EXPOSURE gives an approximate value for the flux of an observation.

The BROWSE object classification flag.

The name of the events file in the archive associated with each observation. File is of the form s2_hhmmVddP**, where s2 is the instrument (SAS-2), hhmm is the right ascension in degrees and minutes, V is N for positive declinations and S is for negative declination, dd is the declination in degrees, P is for 'Pointed' observations '**' is the original observation number, from 01 to 28, t is the datafile 'type' (b for 'background', s for 'source'), b is the 'band' (l for 'low', h for 'high', and f for 'full') and xxx is the FITS file 'type' (img for 'image', exp for 'exposure', flux for 'modified flux map' or gif for 'graphical image format').

Contact Person

Questions regarding the SAS2RAW database table can be addressed to the HEASARC User Hotline.
Page Author: Browse Software Development Team
Last Modified: Monday, 08-Nov-2004 18:28:30 EST