Jeffrey W. Kruk, Samuel T. Durrance, Gerard A. Kriss,
Arthur F. Davidsen, William P. Blair, Brian R. Espey
Department of Physics & Astronomy,
Johns Hopkins University,
Baltimore, MD 21218
&
David S. Finley
Eureka Scientific, Berkeley, CA 94602
We describe changes to the instrument following Astro-1 and
the in-flight photometric calibration,
which is based on a comparison of our observations of the hot DA white
dwarf HZ 43 with a model atmosphere whose parameters were derived from
optical observations. The peak effective area is 
at
1160 Å, where the inverse sensitivity is 
. This is an improvement by a factor of 2.3 over
Astro-1, largely due to installation of new
optics coated with ion-sputtered silicon carbide.
Observations of several other white dwarfs indicate that the
calibration is accurate to 
5%, after correction for modest, but
significant, time-dependent degradation during the mission. The
spectral resolution varied from 2--4 Å over the first order
wavelength range of 820--1840 Å. The wavelength scale is established
to better than 1 Å. As on Astro-1, dark counts and scattered light were
extremely low. Airglow line intensities were much lower due to the
lower level of solar activity. When all factors are considered
together, HUT performance on Astro-2 was a full order of magnitude
better than that achieved on the highly successful Astro-1 mission.
Subject headings: space vehicles --- instrumentation: spectrographs --- instrumentation: detectors --- telescopes --- ultraviolet: general