What does the PSF look like?
(3/2/94)
The functional form below is a good description of the mirror and HRI PSF, and is an energy-averaged response.
PSF = A1 * exp( -0.5 * (R/S1) ** 2 ) + A2 * exp( -0.5 * (R/S2) ** 2 )
+ A3 * exp( -R/S3 ),
with R = radial distance (arc sec), and
A1 = 0.9638 S1 = 2.1858
A2 = 0.1798 S2 = 4.0419
A3 = 0.00090 S3 = 31.69
The effect is to scatter about ten percent of the photons beyond the central
core of about 10 arcsec. This region extends out to about 5 arcmin
and is relatively independent of energy. The PSF was derived by
combining data from HZ 43, AR Lac, and LMC X-1. The extra scattering
(a ground calibration showed less than thirteen percent of the photons were outside
10") occurs, due to the introduction of a shield above the
micro-channel plate to remove a serious background problem. The
shield is held at the same voltage as the MCP, so a "field-free"
region is created in the gap. However, ground tests have shown that a
slight positive voltage on the shield would have been better, as a
slightly off-axis photon can scatter off the inter-channel region,
falling into a neighboring micro-channel. That electron then is
amplified, resulting in a larger apparent PSF. Ninety percent of the photons are
within 10 arcsec, while about five percent are scattered beyond 60".The off-axis parameterization is accomodated by modification of S2 as follows:
S2=3.3 + 0.019*theta - 0.016(theta**2) + 0.0044(theta**3)
where theta is the off-axis angle in arcmin.
NB: this expression has not been forced to reproduce the on-axis value. The fact that the off-axis expression gives S2=3.3 for theta=0, compared to the on-axis equation which gives 4.0, reflects the variation in aspect quality from observation to observation.
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