SAS2MAPS - SAS-2 Map Product Catalog
This database is a collection of maps created from the 28 SAS-2 observation
files. The original observation files can be accessed within BROWSE by
changing to the SAS2RAW database. For each of the SAS-2 observation files,
the analysis package FADMAP was run and the resulting maps, plus GIF images
created from these maps, were collected into this database. Each map is a
60 x 60 pixel FITS format image with 1 degree pixels. The user may
reconstruct any of these maps within the captive account by running
FADMAP from the command line after extracting a file from within the
The parameters used for selecting data for these product map files are
embedded keywords in the FITS maps themselves. These parameters are set
in FADMAP, and for the maps in this database are set as 'wide open' as
possible. That is, except for selecting on each of 3 energy ranges, all
other FADMAP parameters were set using broad criteria. To find more
information about how to run FADMAP on the raw event's file, the user
can access help files within the SAS2RAW database or can use the 'fhelp'
facility from the command line to gain information about FADMAP.
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.
This database contains 9 FITS images for each of the original 28 SAS-2
photon events files. This database was created by running the FTOOL
FADMAP on the events files 3 times, in 3 different energy ranges (low,
high and 'full'). Running FADMAP on an events file produces four product
maps: a "source count" map, a "source exposure" map, a "corrected
background count" map and a "corrected background exposure" map. Retained
here for the database are only the source count and source exposure maps.
The source count map was divided by the source exposure map using the
FTOOL FARITH to produce a "modified flux" map. For quicklook observations
this product is the most useful. A GIF image was also created for each
file in this database. Thus, for each pointing there exists a "source
counts". a "source exposure" and a "modified flux" map in each of the
3 energy ranges, plus of GIF of each of these maps.
The energy ranges selected for SAS-2 are as follows: "low" (35-100 MeV),
"high" (100-1000 MeV) and "full" (35-1000 MeV).
For each database entry, you will be able to extract any of the 9 maps
for viewing or further analysis. Reproduction of any of these files,
using the original events files can be done by running FADMAP on them
with user selected parameters or the defaults. This method can also be
used to reproduce the "background count" and "background exposure" maps
for analysis. To do this, change databases to SAS2RAW, extract the desired
events file, and run FADMAP on that file. See the database SAS2RAW for more
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 20 degrees, photons associated with a particular
observation should be within 20 degrees of this RA.
The Declination (1950) in degrees of the optimum observation direction. With a
FOV of approximately 20 degrees, photons associated with a particular
observation should be within 20 degrees of this DEC.
Original Observation Number
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)
Stop Time of Observation
The source name. If one does not exists, this parameter defaults to
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.
FITS File Name
Questions regarding the SAS2MAPS database table can be addressed to the
HEASARC User Hotline.
If you have any problems, please consult the help
page or mail firstname.lastname@example.org