Two of the JWST NIRSpec observing modes (Integral Field Unit and Fixed Slits) may be used to create mosaic patterns. This capability allows the user to obtain data for a region larger than the original aperture size. This article explains how to create IFU and FS mosaic patterns in APT templates.
NIRSpec has 5 fixed slits (FSs) cut into the metal plate that holds the four quadrants of the micro-shutter assembly (MSA). The individual sizes of these slits and their names are presented in Figure 1. NIRSpec also has an integral field unit (IFU) that can obtain spatially resolved imaging spectroscopy of a contiguous 3" × 3" area on the sky. An aperture the size of the IFU is also presented in Figure 1 for comparison. The NIRSpec Integral Field Unit article includes diagrams that describe in detail the location of the fixed apertures with respect to the MSA quadrants.
This article provides a step-by-step guide on how to create mosaic patterns using either the NIRSpec Fixed Slit Spectroscopy APT Template or the IFU Spectroscopy APT Template.
Creating NIRSpec mosaic patterns with APT
The NIRSpec FS and IFU observing modes allow for the creation of mosaics. This feature is available under the Mosaic Properties tab shown in Figure 2. General information on the mosaic planning for JWST instruments is given in the article JWST APT Mosaic Planning.
Mosaics are defined by selecting a pattern with a given number of Rows1 and Columns1. Mosaics are composed of tiles. For example, a mosaic with 3 Rows and 4 Columns contains 12 tiles. The parameters that can be defined by the user are listed in Table 1. A visual description of these parameters is in Table 2.
Table 1. Mosaic properties configurable by the user
In Table 2 we present several examples of simple mosaic configurations created using the mosaic properties listed in the last columns. An image of the corresponding mosaic as viewed in Aladin is listed in the first column for the IFU case. Each pattern should be suitable for different science programs. Click on each images to see a more detailed version of the figures.
Table 2. Example IFU mosaic configurations. The footprint plots are ~31" on each side.
Figure 3 shows two mosaics created using the Fixed Slit Mosaic pattern to cover a proto planetary disk in the Orion Nebula shown in the upper left panel.
Users can select the number of 'Rows' and 'Columns' in the Mosaic Properties pane.
Once the number of rows and columns have been selected, the user can display the mosaic as an Aladin plane by clicking on the 'View in Aladin' button. It is usually useful to display an image of the target as well. As an example, Figure 3 shows two mosaics, one with four columns and the other with fifteen columns. For comparison, the lower right panel in Figure 3 shows the same field of view superimposed with the NIRSpec IFU footprint.
Once the mosaic properties are defined, APT automatically creates a table of mosaic tiles (Figure 4) with a tile number, a tile state, the visit in which the tile will be observed, a column to split the tile to a new observation, and a column to copy the tile to a new observation. The tile number and the visit value are not editable. The tile state provides two options: included or removed. Users can include or remove tiles depending on their need. Tile splitting is discussed in detail in the articles JWST APT Mosaic Planning and JWST APT Mosaic Tile Splitting Activity. For NIRSpec IFU and FS mosaics, the mosaic patterns that are smaller in spatial extent than the visit splitting distance (<40") can all be within one visit per spectral configuration.