NICMOS Instrument Handbook for Cycle 11

Phase II Proposal Instructions for Patterns

---

We discuss the Phase II instructions for patterns in this section in order to illustrate the options available. This is not, however, an exhaustive description and is not the appropriate reference to use when preparing a Phase II proposal. While writing your Phase II you should refer to the 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. The pattern parameters form allows for two nested parameters (pattern 2 is performed at each point in pattern 1) but many of the predefined patterns already use this capability: you cannot always "nest" the "canned" patterns. A pattern can apply to a series of exposure logsheet lines, and the entire sequence is done at each point in the pattern. Thus the special requirement PATTERN n [<exposure list>] (where n is the number of the pattern parameter form to be applied to these observations) should only apply once to any exposure logsheet line, and should only be specified on one exposure in <exposure list>. Be very careful not to "call" a line in two different patterns!

Note that the canned patterns are available only through the proposal editor PED. You must select this as your editor from the RPS2 toolbar, so that this interactive editor will come up when you push the RPS2 "edit" button. PED will automatically fill in the default values and will prevent you from editing pattern values that shouldn't be changed. You should also be able to download filled in pattern templates from the NICMOS web site. You should use the RPS2 description generator's "FOV Tool" to make sure the pattern is doing what you expect it to.

The set of exposures resulting from a pattern is sorted by filter, and an association is created for each filter.

Patterns may use the FOM instead of moving the whole telescope, however this is an "available" (not "supported") mode of observation. The FOM is not exactly repeatable, so artifacts on the mirror may appear to move around. Observations requiring small and rapid offsets (or making observations in parallel to another instrument) may benefit from this mode. Note that the astigmatism changes with FOM offsets. Observers who plan to use the FOM for dithering should ask for assignment of a Contact Scientist to their proposal.

Types of Motions

The OFFSET optional 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 maneuvers (SAMs) where FGS Fine Lock guiding is maintained. Such SAMs are typically limited to < 2 arcmin 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 pointing control). These are necessary for larger motions (> 2 arcmin). The telescope will drift at a rate of 1 to 2 milliarcsec per second of time (elapsed time since dropping to gyro-- not exposure time).
• SAMs with RE-ACQuisitions of guide stars 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 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 control. If a subsequent motion returns the telescope to a point where the original guide stars become available then the guide stars will be RE-ACQuired. 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 control.
• SAM-NO-GYRO will use guide stars for all exposures. If guide stars are not available, the observations will not be scheduled.
• FOM will use the Field Offset Mirror to perform the pattern. See the description above; this is an "available" operating mode, for use only for coordinated parallels.