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FOS APERTURE THROUGHPUT VARIATIONS WITH OTA FOCUS

D. Lindler and R. Bohlin
Space Telescope Science Institute

Instrument Science Report CAL/FOS-085
August 1992


The attached figures show the variation in the throughput for several FOS
entrance apertures as a function of the focus error with respect to the HST
"optimal focus" position. The results were derived from point spread functions
computed using the Telescope Image Modeling (TIM) version 27 software package
and should be used to make small corrections to the FOS aperture transmissions
for changes in focus. These small corrections are relative to unity at the
focus positions where the transmission was last measured. The last FOS aperture
transmission data were obtained in June 1991 at focus error=+1 on the blue side
and in December of 1991 at focus error between -8 and -19 microns on the red
side. The reason for the uncertainty in late 1991 is the question of whether
the OTA focus changed gradually or made a sudden jump toward negative focus
error sometime during the hiatus of more than 4 months in precise measurements.

An increase in focus position will increase the throughput by approximately
0.3 percent/micron for the 4.3 arcsec square and by 0.5 percent/micron for the
1.0 arcsec circular aperture.

The optimal focus for the 0.25 arcsec wide slit is in the 0 to +10 micron
range. The throughput for the slit increases as the focal length increases up
to +10 microns at wavelengths above 2000 Angstroms. At 1400 Angstroms, the
throughput for the slit remains within 1% of the maximum in the 0 to +10
range of focus. At a focus position of +5, the throughput for the slit is
within 4% of the maximum throughput for all wavelengths. If the focus is set
below 0, results show a loss of throughput for the slit at all wavelengths. At
a focus position of -10 this loss varies from 2 percent at 1400 Angstroms to 13
percent at 8000 Angstroms. Below -10 the slit throughput is lost at
approximately 0.5 percent/micron. The 0.5 arcsec circular aperture results (not
plotted) are simular to the slit in the focus range analyzed.

The peak throughput for the 0.3 arcsec aperture ranges from -10 at 1400
Angstroms to +10 at 8000 Angstroms. For all wavelengths, the throughput is
within 10% of the maximum throughput for all focus positions between -10 and
+10. Outside this range, throughput decreases as much as 1% per micron.

SUMMARY

No one focus position optimizes all wavelengths and all apertures. For the 4.3
arcsec aperture, a large positive focus is optimal. The optimal focus for the
slit varies from 0 at 1400 Angstroms to +20 at 8000 Angstroms. For the 0.3
arcsec aperture the optimal focus varies from -10 at 1400 Angstroms to +10 at
8000 Angstroms. Our analysis indicates that the optimal focus for the FOS is
between 0 and +5 where both the slit and 0.3 arcsec aperture are within 6% of
the maximum throughput at all wavelengths.

The HST focus was estimated to be at -19 microns on August 20, 1992 (C.
Burrows, private communication). On August 23, 1992, the OTA focus position was
moved from -19 to -5 microns from optimal focus. This focus should be
monitored carefully and refocused to near zero as soon as the focus gets to -10
microns.



RECOMENDATIONS

The safest guess for the red side reference position in December 1991 is
probably at the -12 micron position that is based on a linear change in focus
with time between the bracketing measurements. The possibility of -19 microns
for this focus means that the FOS flux measurements can be systematically off
by as much as 7% for the smallest aperture. The OTA focus should be monitored
more frequently to reduce the chances of getting more than a 5 micron
uncertainty again.

We recommend that the telescope focus be moved to the 0 to +5 micron range
whenever the focus is determined to be outside of the -10 to +10 range.