# What does the PSF look like?

### (2/15/94)

The PSPC has an intrinsic Gaussian resolution that appears independent of off-axis angle. It scales roughly as the inverse of sqrt(energy ), with a value of about 25 arcsec at 1 keV.

- a broadening occurs, as the counter is focused for 1 keV photons; particularly for photons of greater energy, there will be additional penetration into the counter. The functional form is that of an exponential. Below 1 keV, the gaussian component is dominant.
- the intrinsic scattering of the mirror (function = Lorentz)
- the blurring due to off-axis photons
- residual blur due to the wobble not being completely removed by the attitude solution (approximately +/- 5 arcsec). This term is a function of the roll angle.

The FWHM of the telescope is approximately given by (in arcmin):

FWHM( tel ) = 0.29 * a ** 1.74
with **a** = off-axis angle in arcmin.

The FWHM of the detector is approximately given by:

FWHM( det ) = sqrt( 409.65/E + 69.28 E ** 2.88 + 66.29 ) arcsec
with **E = energy in keV**. The values and the functional form may be
revised in the coming months. The on-axis, encircled fractions are as
follows: 50 percent, 0.22 arcmin; 90 percent, 0.45 arcmin; 95 percent, 1.55 arcmin.

The on-axis and off-axis psf are described in
**cal_ros_92_001.tex ** and **cal_ros_93_007.tex **
available from the legacy ftp area under
/caldb/docs/rosat/

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