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42.4 Prism Aperture Calibration

The HSP prism mode split the light from a single target into two separate apertures on adjacent filters. By moving the read beam, the light through the two aperture-filter pairs was alternately observed. The initial post-launch verification tests produced unexpectedly low transmission results. Other HSP tests to calibrate the prism apertures were run, but showed the throughput to be lower than expected. The current estimate is a lower throughput of 3-4 magnitudes.

Due to small misalignments of the beam splitter prisms, PRISM mode was even more highly susceptible to the effects of telescope jitter than normal one-color photometry.

Table 42.2 summarizes the throughput of the prism apertures relative to pre-flight calibrations.

--Throughput of Prism Apertures

Filter/Detector

eff (Å)

Loss Factor

F248/UV1

2462

9.5

F135/UV1

1549

6.0

F262/UV2

2606

7.0

F145/UV2

1556

8.8

F551/VIS

5482

20.4

F240/VIS

2192

3.9

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