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The early SMOV results show that the background is considerably fainter than was expected prior to the deployment of NICMOS (see Chapter 3). While only a preliminary analysis of the SMOV data has been completed, it appears that the thermal background is spatially uniform with variations no larger than a few percent across the field of view of NIC3. Temporal variations on orbit timescales with <10% amplitude have been observed and more significant variations cannot yet be excluded. The description of the thermal background in Chapters 3 and 6 and the Exposure Time Calculator provide a basis for estimating the relative contributions of source and background. It is strongly advised that provision for direct measurement of the background be included in proposals whenever the background is significant relative to the source(s) of interest. While the frequency of such measurements is presently difficult to define, at least once per orbit should suffice when the background is not overly critical to an observation and more frequent measurements should be planned when the background must be measured to high accuracy.
All observations at wavelengths beyond 1.6 microns should at least consider the potential need for background measurements.
Two distinct type of telescope motion are defined: