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Near Infrared Camera and Multi-Object Spectrometer Instrument Handbook for Cycle 17 > Appendix D: Techniques for Dithering, Background Measurement andMapping > D.1 Introduction

D.1 Introduction
Multiple exposures with small offsets in the pointing of the telescope between exposures are recommended for NICMOS observations. We distinguish three particular circumstances which may require small offsets:
Dithering to permit the removal of dead or non-calibrated (i.e., non-correctable) pixels on the detectors, and to improve spatial sampling and mitigate the effects of detector’s non-uniformities (i.e. sensitivity variations),
Chopping to measure the background associated with an astronomical source, and
Mapping to map a source larger than a single detector field of view.
The techniques described in this appendix may be used to accomplish any one or any combination of these goals.
Experience with NICMOS has shown that the background is spatially uniform (variations no larger than a few percent across the NIC3 field of view) and does not vary much with time (variations of less than 5% on orbit timescales). The description of the thermal background in Chapter 4, Chapter 9 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 observations at wavelengths greater than 1.7μm are performed. The frequency of such measurements should be about once per orbit, and more frequent measurements should be planned when the background must be measured to high accuracy.
Background images are 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 is a fundamental operational requirement for NICMOS. Starting in Cycle 9, HST programs use a standard pattern syntax, which replaced the old pattern optional parameters, and the even older scan parameters form. The new syntax allows multiple observations (including those with different filters) to be made at each point in the pattern, if desired. Observers should check the “Phase II Proposal Instructions” and APT documentation for instructions on how to set up a pattern, and the “pattern parameter form” that describes the motion. For simplicity, a set of pre-defined observing patterns has been built; the exposures taken under them are combined into one or more associations. A pattern, then, is a set of images of the same astronomical target obtained at pointings offset from each other, e.g. for the purpose of removing bad or grot-affected pixels from the combined image, for creating background images, or mapping an extended target. The associations of exposures are created for the purpose of simultaneously processing all the images (through a given filter) from a single pattern. Thus, dithered images can be easily reassembled into a single image with the effects of minimizing bad pixels, or images taken in the long wavelength regime can be corrected for the thermal contribution, or observations of extended targets can be combined into a single large map.
Starting with Cycle 14, previously unassociated NICMOS observations within a visit, using the same detector and filter, are collected into an association, provided a POS-TARG is specified on the exposure line. The assignment of association numbers is determined from the APT Phase II file exposure line specifications for each visit. Currently, the new association assignments are NOT applicable to previously executed observations, nor does it apply to parallel observations.
Three distinct types of pattern motion are defined:
Dither: Individual motions are limited to no more than 40 arcsec. 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 1440 arcsec are permitted. These are intended for the measurement of the background at one or more locations significantly removed from the target pointing. Non-contiguous background images and target images will be assembled into their own final image by the calibration pipeline.
Mapping: Large motions comparable to the size of the aperture (e.g. 11, 20, or 50 arcsec) are specified. These are intended to cover large regions of the sky in a regular grid pattern.
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 10).

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