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Orienting Patterns
Observations will often require that patterns are executed with specific position angles (PA) on the sky. For instance an observer may need to map an elongated region with PA=n degrees on the sky, using the SQUARE-WAVE-DITH pattern. Since the use of the ORIENT special requirement usually constrains the scheduling opportunities for a target (and may result in the unavailability of suitable guide stars), NICMOS implements an optional parameter called PATTERN-ORIENT (see next section). With this parameter, the orientation of the pattern on the sky can be set independently of the orientation of the three NICMOS detectors (and vice-versa). If a specific orientation of the detectors is also required, then the observatory level special requirement ORIENT must be set together with PATTERN-ORIENT.
The default value for PATTERN-ORIENT is DETECTOR, and the pattern motions are in the NICMOS (x-y) focal plane orientation, i.e., rotated 225 degrees from the U3 axis. The orientation of the patterns on the sky is then determined by the orientation of the telescope with the ORIENT special requirement. Other values for the PATTERN-ORIENT parameter are given by the standard convention for PA, from 0.0 to 360.0, North through East. For example, the XSTRIP-DITH pattern with PATTERN-ORIENT=0 will track from East to West on the sky (the +Y, vertical in the preceding figure, axis is pointing North), and with PATTERN-ORIENT=45 from Southeast to Northwest. The YSTRIP-DITH pattern with PATTERN-ORIENT=0 will track from South to North.
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Last updated: 07/24/97 15:33:47