JWST mosaic observations, a series of pointings covering an area larger than an individual instrument field of view, can be specified using controls in relevant APT observation templates.
JWST offers the capability to mosaic regions that are larger than a single field of view (FOV). This capability is useful not only for imaging instruments, but also for integral field units (IFUs) in NIRSpec and MIRI MRS. A mosaic is made up of a series of pointings, called tiles, each of which may be dithered. the Astronomers Proposal Tool (APT) implementation of mosaics.
The instruments and their modes of operation that permit the use of the mosaic functionality are shown in Table 1.
Table 1. APT science templates allowing mosaics
Wide field slitless spectroscopy (grism)
|MIRI||Medium resolution spectroscopy (IFU)|
|MIRI||Low resolution spectroscopy|
|NIRSpec||Integral field unit (IFU)|
|NIRISS||Wide field slitless spectroscopy (grism)|
The MIRI MRS is a special case since the field of view sizes for the 4 MRS wavelength ranges are different from each other.
Time series modes, coronagraphy, and templates for observations of individual targets are not useful for mosaics; to avoid confusion, mosaics have been disallowed in those templates.
Mosaics are defined by a single coordinate, an assumed tile footprint (size and orientation), and a designated number of rows and columns of the tile footprint. Each instrument and mode that allows mosaics will have a FOV size and reference axis defined in the Science Instrument Aperture File (SIAF), which is the official repository for all defined instrument-related positional and angular definitions. If you define a dither pattern for a given tile, it is the combined region of the FOV and the dithers that define a tile footprint for a given mosaic. It is then this footprint that should be considered when addressing the overlap of tiles in a given mosaic. Because the various instrument FOVs have different shapes and dimensions, you should always view their defined mosaics in the Aladin viewer in APT to assess the overall shape and coverage of their mosaic.
Because the overall mosaic footprint will rotate with time, you may find it helpful to define overall mosaic footprints that are close to square on the sky. If non-square mosaics are needed to cover a particular target, it means you must fix the position angle of the mosaic and define your tiles to cover the defined region of interest. However, it may or may not be possible to find guide stars simultaneously for all tiles in a highly constrained mosaic. Also, fixing the orientation of mosaics essentially requires fixing the time of the observations, and many such observations can overly constrain observation scheduling. You should only fix the position angle of mosaics when necessary for science. Consider obtaining insight into the available range of position angles using one of the Target Visibility Tools.
For imaging mosaics, it is usually the case that the separation of the tiles is large enough that each tile will be a separate visit* (requiring a separate guide star and GS acquisition activity). For simplicity in this discussion, we will assume this is the case. Because each visit (tile) requires its own guide star (and thus a guide star acquisition), part of the schedulability assessment is that all tiles must simultaneously have guide stars available in order to be declared "schedulable".
As a general rule, the larger the mosaic, the more likely it is that one or more tiles will be missing guide stars at the same time as the rest of the tiles, and hence there will be no time when the entire mosaic can schedule simultaneously. In such cases, APT declares the entire mosaic as unschedulable because by definition the entire mosaic is a single observation. There are ways to remove a tile or tiles into separate associated observations to fill the gap(s) in the original mosaic, but it is a time-consuming task. Another option is to consider a pattern of smaller overlapping mosaics to cover larger regions, leaving each of the smaller units free to rotate and schedule at different times when guide stars are available for that portion. Some experimentation is required to understand the effectiveness of this option.
*(This is not always the case, as it depends on such things as the amount of overlap you specify and the "visit splitting distance" calculated by APT, which is a function of the target's Galactic latitude. However, since APT performs the splitting of proposed observations into visits, it is a detail that only becomes an issue when a tile needs to be removed or split from a parent mosaic.)
See also: JWST APT Mosaic Tile Splitting Activity
See also: APT Simple Mosaic Example
See also: video help with APT Mosaics
Mosaics and spectroscopy
Mosaicking is allowed for many smaller spectroscopic fields of view, such as the NIRSpec IFU and MIRI MRS (also an IFU), but the situation is quite different. In these cases, a target group can be defined and a number of separate pointings can occur within a single guide star region, so offsets can remain on the single guide star without re-acquiring. Also, these modes can take advantage of a reorganization of the activity ordering to greatly reduce mechanism motions. These details are mostly transparent and require no special handling in APT.
See also: JWST APT Targets
Mosaics versus Dithering
The Mosaicking capability for JWST is intended to provide users with a convenient way to cover larger regions of the celestial sphere. It should not be applied to make very small steps in position, to mimic normal dithering. While this could in principle result in somewhat lower overheads, in practice is is a false gain.
Filter mechanisms have the potential to be life-limiting factors for JWST instrumentation. This is particularly the case for NIRCam, which serves as the wave-front sensing imager for the telescope. As a consequence, observers must not circumvent standard observation logic that minimizes filter wheel moves. Programs with either MIRI or NIRCam that involve dithered observations or “mini-mosaics” (that can be observed within a single visit) should step through the dither positions with a single filter before moving to a second filter. Creative alternative ordering for overhead efficiency reasons will not be allowed.