FOS Entrance Aperture Transinittance for Point Sources


George Hartig
SPACE TELESCOPE SCIENCE INSTITUTE

Instrument Science Report CAL/FOS 039
November 1986


1 have performed a calculation, for each of the FOS apertures, of the
throughput efficiency, i.e. the fraction of ligbt incident on the OTA
focal plane from a potnt source that is passed by the aperture (with
the image centered on the aperture) - Defined as such, these
efliciencies do not include the losses due to primary and secondary
mirror reflectivity and dust scattering, nor do they account for the
losses due to diffraction of light to points outside the collimator
radius by the smaller apertures. Included are: OTA mirror wavefront
error (at high, middle arid bw spatial frequencies) per PE metrology,
pointing jitter (.007 arcsec, RMS), astigmatism at the 3.6 arcmin
off-axis distance of the FOS apertures, nominal FOS aperture geometry
and dimensions, and the effective limitation of the large aperture
heights by the 1.43 arcsec diode height. Although lab measurements of
the aperture sizes show some variations from the nominal areas,
especially in the case of the A4 lower aperture on the red side, these
rnea.surements are affected by diffraction losses at the collimator
and yield no information concerning the actual shapes of the apertures
(see CAL/FOS-019).


The calculations were performed using a program originally devised by
D. Schroeder (Beloit College) and adapted by Chris Burrows (STScI),
who also provided the astigmatism correction information. The output
of this program, which computes the encircled energy within a given
radius, was used to nurrterically integrate the OTA PSE over the clear
aperture areas to obtain the efficiency estimates.