INSTRUMENT SCIENCE REPORT CAL-FOS-002
	SPACE TELESCOPE	
	SCIENCE INSTITUTE

TITLE:	FOS Entrance Aperture Sizes

AUTHOR:	J. M. Wheatley, R. C. Bohlin and H. C. Ford 
DATE: 10/7/83
	
ABSTRACT:

The sizes of the 22 F0S entrance apertures are measured directly with
a microscope, and their light trarismittance is measured with the
aperture map technique. The light transmittance is determined by the
area of an aperture, so the aperture map results can be compared with
the areas measured with the rnicroscope. Two sets of aperture maps are
discussed, the first taken during the ambient calibration in March,
1983 and the second during thermal vacuum testing in August 1983. The
March aperture map results give relative areas that are between 8 and
100% larger than the microscope measurements. Particularly discrepant
is the 0.1 arcsec blue side aperture. The two halves of this paired
aperture are supposed to be the same size, but in the March maps the
lower half transmits 60% more light than the upper half, coriflicting
with microscope measurements which show that they are the same size to
within 5%. The August aperture maps also show the two halves to be the
same size, but each is 15% smaller than the microscope
measurements. For the other apertures, the August maps give areas that
are 4  8% smaller than the microscope measurements, which are closest
to the specifications. Uneven illumination, varying lamp brightness,
light scattering from the sides of the small aperture holes, and
cathode non—uniformity are considered as possible sources of error in
the aperture maps, but none of these can adequately explain all the
discrepancies. Further data is required while the FOS is at Goddard
Space Flight Center in order to isolate the cause of the discrepancies.