Space Telescope Science Institute  Chapter 11:  Techniques for Dithering, Background Measurement and Mapping  11.2 Strategies For Background Subtraction

11.1 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:

The techniques described in this chapter 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 measurements are recommended for all observations at wavelengths longward of 1.7 µm.

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.

Three distinct types of pattern motion are defined:

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).


Chapter 11:  Techniques for Dithering, Background Measurement and Mapping  11.2 Strategies For Background Subtraction
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