The SAS-2 psf file is in the standard psf FITS format for
X-ray astronomy (see the document CAL/GEN/92-018). Columns 1, 2, and 5
have the same definition as in that document. The exception is columns
3 and 4. Though the column names (RAD_LO and RAD_HI) are the same,
their meanings differ significantly from that of X-ray imaging
detectors where the X-ray is given as a position on a 2-dimensional
grid. For the detection of high energy 
-rays where a
sparkchamber is used, such a grid does not exist and therefore the
concept of radial distance is undefined. Instead the zenith and
azimuth angles of the event with respect to the telescope's z-axis is
recorded and therefore the probability of detecting an event depends
upon zenith angle.
The psf of sparkchamber depends on zenith angle and strongly on
energy, since high energy -rays have their direction more
accurately know. The direction of the -ray cannot generally be
determined to better than 0.1 degree. The psf is given for seven
energy ranges and two zenith angles.
Brenden Perry (HEASARC)
HDUDOC/VERS = CAL/GEN/92-018/1.0.0
(see CAL/GEN/92-018).
No information supllied to the CALDB other than that above.
