HRI Option

The HRI model is an analytic fit to the in-flight response, performed by MPE/GSFC/SAO. The functional form of the on-axis HRI PSF is :

$$PSF(R) = A_{1} e^{-0.5({\frac{R}{S1}})^2} + A_{2} e^{-0.5({\frac{R}{S2}})^2} + e^{(\frac{-R}{S3})}$$

where the radial distance R from the psf centre is in arc seconds. The constants A and S have values :

A1 = 0.9638 S1 = 2.1858" A2 = 0.1798 S2 = 4.0419" A3 = 0.001168 S3 = 31.69"

The on-axis model provides a reasonable description of the PSF out to radii of 100 arcsec, but does not model the azimuthal asymmetry present within the PSF core [at a radii less than 12 arcsec] of the PSF.

The on-axis model does not vary with position in the field of view and consequently " ... the PSF starts to degrade beyond 5 arc minutes off axis. Beyond 12 arc minutes off axis, the images become very asymmetrical."

"A parametric representation of the azimuthally averaged off-axis HRI PSF has also been derived which is a simple extension of the on axis expression ... The best fit values of s2 are well parameterised by a cubic polynomial in the off-axis angle, and are given by

where T is the angle off-axis in arc minutes. Substituting this into the HRI PSF gives the general off-axis representation."

The ASTERIX XRT_HRI option uses the off-axis HRI PSF parameterisation.

Documentation on HRI can be found at :

http://hea-www.harvard.edu/rosat/rsdc_www/HRI_CAL_REPORT/node12.html. Feb-97
http://heasarc.gsfc.nasa.gov/docs/rosat/newsletters/hripsf10.html. Mar-96.