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Guiding with T-ReCS (and Michelle) can currently only be performed on one side of the chop. The reasons for and implications of this are discussed in more detail in the Mid-IR Resources section.
The default with T-ReCS should be to assign the guide star to peripheral wavefront sensor #2 (PWFS2). We have made some tests of T-ReCS usage with PWFS1 but most of our observations in semesters 2003B and onward have been made with PWFS2.
PWFS2 stars should have a radius greater than 4.8 arcmin. Below 4.7 arc-minutes the probes will start to shadow the detector. The lower limit is specific to T-ReCS and it is different -- typically larger -- for other instruments. The outer limit for PWFS2 guide stars can be displayed on the OT position editor and is discussed here.
Please keep in mind the magnitude limits for PWFS2. It is possible to use PWFS1 if the guide star is brighter than magnitude roughly 12.5, and preferably somewhat brighter than 11.5. In our initial trials of using PWFS1 we observed a significant difference in the PSF when using PWFS1 rather than PWFS2, since PWFS1 generates corrections for trefoil as well as tip/tilt, whereas with PWFS2 only the tip/tilt corrections are done. Typically the PSF observed with PWFS2 has a significant trefoil component.
For PWFS2 guiding, we strongly recommend choosing guide stars bright enough (>13.5 magnitudes in IQ70) to ensure guiding in 100 - 200 Hz. Guiding at 50 Hz may cause elongation of the science target in final nod-stacked images and can degrade the quality of the science data, especially for faint science targets.