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Phase II Proposal -Instructions for Patterns

We discuss the Phase II instructions for patterns in this section in order to illustrate the options available, however, this is not an exhaustive description and is not the appropriate reference to use when preparing a Phase II proposal. At that stage you should refer to the Institute's Phase II Proposal Instructions, which contain a complete and up-to-date guide.

This section is not crucial for preparing the Phase I proposal, but it may be relevant to know beforehand which parameters will be available, and what values these parameters can have. One, and only one, pattern may be defined on an individual exposure logsheet line. A pattern may only be specified on the prime NICMOS camera. The motions between each exposure are carried out by expanding the pattern into a sequence of individual exposures separated by POSTARG operations. Only a single filter may be used within a pattern (i.e. only one filter may be specified on a Phase II exposure logsheet line).

The set of exposures resulting from a pattern is called an association. A number of optional parameters are implemented for associations, and are listed below.

These patterns may also be used with motions of the Field Offset Mirror. While the FOM is "available" (i.e., not supported) for Cycle 7-NICMOS, observations requiring very rapid small offsets may benefit from its use and observers can discuss this with their Contact Scientist during the Phase II proposal process.

Parameters

PATTERN=NONE (default) or Pattern_Name (see Table 10.1).
NUM-POS = 2 to 40 (number of steps in dither or chop pattern to perform).
DITH-SIZE = 0.0 to 40.0 (size of each step in arcsecs).
CHOP-SIZE = 0.0 to 1800.0 (size of each step in arcsecs).
OFFSET=SAM (default), (use SAMs; gyros for large moves with re-acquire); SAM-NO-REACQ, (use SAMs; do not attempt to re-acquire guide stars); SAM-NO-GYRO, (use SAMs; must use same or new guide stars).
PATTERN-ORIENT=DETECTOR (default), (move in detector X-Y system) 0.0 - 360.0 degrees from PA=0 increasing to East (Note that this does not constrain telescope roll; use existing ORIENT requirement).

Types of Motions

The OFFSET parameter defines which type of telescope motion will be performed during a pattern, in order to dither or chop. Telescope motions fall into three categories:

Small angle motions (SAMs) where FGS Fine Lock guiding is maintained. Such SAMs are typically limited to < 2 arcmins from the point of the initial guide star acquisition. This is the practical limit of the radial extent of the pattern. Often it will be smaller due to guide star availability.
SAMs without FGS guiding (i.e., GYRO HOLD pointing control). These are necessary for larger motions (> 2 arcmins). The telescope will drift at a rate of 1 to 2 milli-arcsecs per second of time (elapsed time-not exposure time).
SAMs with RE-ACQuisitions at each return to the target position. This can be used to chop between a target and an offset background measurement pointing (which would be observed with GYRO HOLD pointing control). The available options for OFFSET are:
SAM, the default, will use guide stars whenever possible. If a motion, or the sequence of motions, moves the telescope sufficiently from the original position that guide stars are no longer available then exposures will be obtained using GYRO HOLD. If a subsequent motion returns the telescope to a point where the original guide stars become available then it will RE-ACQuire the guide stars. This incurs an overhead of ~3 minutes for each RE-ACQuisition.
SAM-NO-REACQ will use guide stars (FGS Fine Lock) until the first instance in the pattern when guide stars become unavailable. The remainder of the pattern will be executed using GYRO HOLD pointing control.
SAM-NO-GYRO will use guide stars for all exposures. If guide stars are not available, the observation cannot be scheduled.

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