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Introduction

Multiple exposures with small offsets in the pointing of the telescope are usually necessary or recommended for NICMOS observations. We distinguish three particular circumstances which may require small offsets:

Dithering to permit the removal of dead or non-calibrated pixels (i.e., non-correctable) on the detectors.
Dithering or chopping to measure the background associated with an -astronomical source.
Mosaicing to image a source larger than the NICMOS field of view. The techniques described in this chapter may be used to accomplish any one or any combination of these goals.
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.

Background images will be obtained by offsetting the telescope from the target to point to an "empty" region of the sky. The ability to routinely offset the telescope pointing will then be a fundamental operational requirement for NICMOS. To make it easier to plan observations beyond 1.6 microns, a set of pre-defined observing patterns was built; these patterns combine exposures taken at different telescope pointings into an association. A pattern is a set of images of the same astronomical target obtained at pointings offset from each other, for the purpose, for instance, of creating background images. The associations of exposures are created for the purpose of processing simultaneously all the images from a single pattern. In addition to background subtraction at long NICMOS wavelengths, the patterns will be useful for creating maps of extended targets at any wavelength.

Two distinct type of telescope motion are defined:

Dither: Individual motions are limited to no more than 40 arcsecs. These are intended to be used to perform small dithers, to measure backgrounds for compact sources, and to accomplish sequences of overlapping exposures for the construction of mosaics. Such sequences will be assembled into a single final image by the calibration pipeline.
Chop: Motions up to 1800 arcsecs are permitted. These are intended for the measurement of the background at one or more locations significantly removed from the target pointing. Each non-contiguous background pointing will be assembled into its own final image in addition to the target pointing by the calibration pipeline. Telescope motions involve overheads for physically moving the telescope and, if necessary, for re-acquiring the guide stars. Therefore, significant time overheads may be incurred by observations which need background subtraction or propose to map extended regions of the sky. A careful estimate of the overheads associated with a specific observation or set of observations is necessary to evaluate the number of orbits required (see Chapter 9).

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