The definitive 3-component fit to the on-axis response of the
PSPC, as defined in the OGIP Calibration Memo CAL/ROS/92-001,
due to Turner and George.
The model is a combination of three, physically well understood
terms,
- A gaussian for the intrinsic PSPC resolution due to the
inherent statistics of the primary electron generation.
Theoretically the gaussian Sigma is proportional to
1/SQRT(Energy)
- An exponential function due to the finite penetration
depth of the X-rays in the counter gas combined with the
8.5 degree cone angle. The PSPC is focussed for 1 keV; the
'chromatic aberration' is largest for large energies
- A Lorentzian function for the mirror scattering which breaks
into a different power law slope at larger energies.
Theoretically the scattering fraction should increase like
the square of the energy, if the grazing angle remains
constant. Due to the diffraction laws, the shape parameters
should be proportional to 1/Energy.
In principle these three components should be folded with each
other, however, their angular domains are reasonably well
separated that a simple addition is accurate enough. The
detailed PSF parameters and their energy dependence have been
determined using the PANTER telescope calibration data of both
PSPC-A and PSPC-C at the monochromatic energies 0.28, 0.93,
1.49 and 1.70 keV. At lower pulse heights than channel 15
(0.15 keV) additional 'ghost images' appear in the PSPC for
which no analytical fit is possible. These events should be
avoided as far as possible in PSF modelling.