XRT PSPC

Four alternative psfs are supplied for the XRT PSPC - the user selects them by name using the MASK prompt. The options are,

VARPROFILE

An analytic fit to enclosed psf energy radii distributed to US observers. This model varies with both off-axis angle and energy and should be regarded as the current best model for the PSPC. However, the model is relatively slow to calculate, and use of the spatial modelling POLAR option is recommended.

VARGAUSS

A gaussian fit to the combined telescope and detector response. The width of the gaussian is a function of both off-axis angle and energy. This model has the merit of being very fast even when used in a spatially varying manner.

ONAXIS_2

A 2 component fit to the energy integrated on-axis psf (derived by Jon Mittaz).

ONAXIS_3

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,

1. 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)
2. 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
3. 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 proporional 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.