Improved Dark-Sky Restrictions for HST Observations

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Session 9 -- HST Observing and Instruments
Display presentation, Monday, 9, 1995, 9:20am - 6:30pm

[9.07] Improved Dark-Sky Restrictions for HST Observations

L. Petro, P. Bely, C. Burrows, D. Elkin, J. MacKenty, F. Paresce, A. Storrs (STScI), K. Ratnatunga (JHU), R. Lyons (UCSD)

One of the advantages of performing astronomical observations above Earth's atmosphere is the lower level of diffuse background light. The high ecliptic and galactic latitude sky is twice as dark in space as that observed from Earth's surface. The principal sources of diffuse background in HST observations long ward of 2,500 \AA \ are the Zodiacal Light, and earth shine scattered by the Optical Telescope Assembly. For typical observations the range of possible background brightness from those sources is approximately a factor of 5. When low-background HST observations have been required it has been standard procedure to control the scattered earth shine by observing only while the spacecraft was in Earth's shadow. We present 1,300 measurements of the sky background from the Wide Field Camera Medium Deep Survey and the Faint Object Spectrograph Sky Background Survey that demonstrate the dependence of sky brightness on bright earth angle and Zodiacal Light. In practice, with the current bright earth to optical axis limit set at $20\deg$, Zodiacal Light is found to be a more significant source of background, and little difference is found between the sky background during earth shadow and during the sunlit portion of the orbit. New scheduling restrictions have been developed that can be used, when required, to minimize background light from both earth shine and Zodiacal Light and to limit the sky background in exposures to no more than 30\% above the minimum value. These restrictions will allow longer exposures per orbit and will provide greater yearly scheduling flexibility than the previous earth shadow restriction, and will guarantee that the sky background is never more

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