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Hubble Space Telescope
GHRS Instrument Science Report 072

A Review of the Utility of SPYBALs

Lisa E. Sherbert and Stephen J. Hulbert
October 1995


Many times users want to know how long they can observe without a spectrum Y-balance (SPYBAL) in an effort to limit the motion of the carrousel during an observation. This report discusses the rationale for SPYBALS in terms of the drift of the spectrum across the photocathode with time, taking into account the tilt of the spectrum across the diode array. The results of our investigation reconfirm that the default schedule of SPYBALs should be used to keep the spectrum properly centered on the diode array.

1. - Introduction
2. - Investigation
SPYBAL Drift with Time
Spectrum Tilt Across the Diode Array
Consequences of Tilted Spectra Drifting across the Diode Array
3. - Recommendations