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SAS2MAPS: Help

SAS2MAPS - SAS-2 Map Product Catalog


Overview

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 SAS2RAW database.

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.


References

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.

Description

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 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 details.


Quality

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.

Parameters

RA
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.

Dec
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.

LII
Galactic Longitude

BII
Galactic Latitude

Observation_ID
Original Observation Number

E_Min
Minimum Energy

E_Max
Maximum Energy

Time
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)

End_Time
Stop Time of Observation

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

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

Exposure
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.

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

Class
The BROWSE object classification flag.

File_Image
FITS File Name


Contact Person

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 ledas-help@star.le.ac.uk
 
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