APT Visit Splitting
APT uses the target position and information about guide star availability to decide if a proposed observation needs to be performed in a single visit or multiple visits.
Users specify observations in APT by filling in the requested information in a selected observation template. However, the unit for actual JWST scheduling is a visit; observations may consist of a single visit or multiple visits, depending on a number of conditions, only some of which are under a user's control. It is the user's job to specify observations, and it is APT's job to determine the number of visits.
A visit is that portion of a requested observation that can be performed on a single guide star selection. If dither motions, combined with other offsets, exceed the limit possible for a single guide star, APT will break the observation into another visit and add a new guide star acquisition activity. A likely example might be an imaging mosaic observation: all of the requested mosaic tiles are part of the same observation request, but if the motion from one tile to another is large enough, APT will break the observation into multiple visits and add a guide star acquisition onto each visit as needed.
Splitting of an observation into multiple visits incurs additional overhead for the new guide star acquisition. See JWST Slew Times and Overheads for more information.
Visit splitting distance
The allowed distance assumed for offsets within a given visit is a function of the galactic latitude of your target position, because the number of potential guide stars drops off toward higher galactic latitudes. Alternatively, one can say that larger areas are serviceable by a single guide star at lower Galactic latitudes where more stars are available. A statistical analysis using the JWST guide star catalog was used to define the visit splitting distance vs. Galactic latitude rules plotted in Figure 1. These are the assumed visit splitting distances used by APT. The visit splitting distance ranges between 30"–80" depending on the Galactic latitude of the target. Any observation requiring total effective motions or offsets greater than this distance will require visit splitting.
For convenient reference, APT reports the visit splitting distance it is using and the number of visits for each observation in the APT observation template GUI directly beneath the target selection box.
APT visit splitting logic is only applied to fixed targets. All moving target observations are assigned a visit splitting distance of 30" by default, with the exception of the WATA target acq for NIRSpec, where the value is set to 38". Refer to Moving Target Acquisition and Tracking for full details.