STIS Instrument Science Report 98-20 CLEAR APERTURE FRACTIONAL TRANSMISSION FOR POINT SOURCES
نویسندگان
چکیده
The fractional transmission as a function of wavelength for stellar objects is measured for clear apertures by comparing count rates through small apertures to open aperture count rates in STIS first order modes. Because OTA "breathing" causes the throughput of the smallest apertures to vary on short time scales, the best estimate of the aperture throughputs is a theoretical model that is based on PSFs generated by optical modeling software. The photometric repeatability of the slit transmissions improves with increasing aperture size from 12% rms for the 52x0.05 arcsec aperture to 2.7% rms for the 52x0.5 slit to 1.3% rms for the 52x2 slit. Even for the 52x2 slit, the full range of this scatter is seen within one orbit for observations of the same star. The photometric repeatability of shorter echelle slits is probably comparable to the uncertainty in a long slit of similar width.
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