NIRSpec FS and IFU Mosaic APT Guide

Mosaic patterns are allowed for JWST NIRSpec integral field unit and fixed slits observing modes. This capability allows the user to obtain data for a region larger than the original aperture size. This article explains how to create mosaic patterns in APT templates.

NIRSpec has 5 fixed slits (FSs) cut into the metal plate that holds the 4 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. 

Figure 1. NIRSpec fixed slits and IFU size comparison

A comparison of aperture sizes as projected onto the sky. NIRSpec's 5 fixed slits and the NIRSpec IFU are presented with their corresponding sizes in arcseconds. All apertures are on the same scale. The MSA shutter width is similar to the narrow fixed slits, but can be configured to any length (not shown).
In order to obtain data for a region larger than the original aperture sizes, NIRSpec FS and IFU modes support the creation of mosaic patterns. 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 moasic 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 APT Mosaic Planning.

Figure 2. Mosaic properties tab

When the Mosaic Properties tab is selected, the user can create a mosaic of tiles based on the number of Rows and Columns of each aperture. Overlaps and shifts are also allowed. The View in Aladin button creates a footprint of the mosaic on an Aladin window, outside of APT.

Words in bold are GUI menus/
panels or data software packages; 
bold italics are buttons in GUI
tools or package parameters.

Mosaics are defined by selecting a pattern with a given number of Rows and Columns. Mosaics are composed of several individual 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

Mosaic PropertyDescription
RowsNumber of rows of the mosaic
ColumnsNumber of columns of the mosaic
Row Overlap %This slider gives the percentage of aperture overlap between 2 consecutive rows
Column Overlap %This slider gives the percentage of aperture overlap between 2 consecutive columns
Row shift

This slider selects the displacement between consecutive rows, in angular units of degrees. A Row shift of 30 (degrees) applied to an IFU mosaic offsets the rows as shown in the figures in the 4th row of Table 2. Note that the number 30 actually refers to the angle between the same tile corner on 2 adjacent rows.

Column shift

This slider selects the displacement between consecutive columns, in angular units of degrees. Column shifts can be applied either separately or in conjunction with Row shifts.

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

IFU mosaic footprintRowsColumns





Overlap (%)Shift (degrees)







Table notes:

The footprint plots are ~31" on each side

†  The default parameters are a one row, one column "mosaic" with 10% overlap in each dimension. This is the equivalent to a single pointing and no mosaic. Altering these parameters will add additional pointings to the mosaic to create a mapped shape.

Figure 3 shows 2 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 under the Mosaic Properties tab.

Once the number of rows and columns have been selected, the user can display the mosaic in the Aladin pane 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 2 mosaics, one with 4 columns and the other with 15 columns. For comparison, the lower right panel in Figure 3 shows the same field of view superimposed with the NIRSpec IFU footprint. 

Note that one IFU observation can achieve the same coverage in a single exposure/pointing compared to a fixed slit mosaic with 15 position offsets in the cross-dispersion direction. The IFU observing mode should always be used to map out such a spectral field whenever possible. Keep in mind that some targets may be too bright for the IFU and can saturate the detectors. In order to avoid saturation, the FS mosaics may be needed to map out spectra on bright targets with subarray readouts.

Figure 3. FS Mosaic tiles

Four Aladin views of a ~20' × 20' region in Orion centered on a proto-planetary disk highlighted in green in the upper left panel. The upper right image represents a fixed slit mosaic using one row and 4 columns. The lower left image presents the same field covered with a mosaic generated using one row and 15 columns. For comparison, the lower right image shows what the NIRSpec IFU single tile mosaic footprint looks like using the same target.
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 2 options: included or removed. Users can include or remove tiles depending on their need. Tile splitting is discussed in detail in the articles APT Mosaic Planning and APT Mosaic Tile Splitting Activity. For NIRSpec IFU and FS mosaics, mosaic patterns that are smaller in spatial extent than the visit splitting distance (<40") can all be within one visit, provided the visit duration is less than 24 hours. 
Figure 4. Mosaic tiles

APT creates a table with information on each generated tile. This table allows users to remove tiles and split them to new observations as needed. This strategy is useful if some of the tiles require the use of a different guide star, or if the mosaic cannot be planned in a single visit. The latter may occur when the mosaic is large, or the total exposure time is longer than 24 hours.

Observations using mosaics, where the visit duration exceeds 24 hours, can be easily split into 2 or more visits. (APT does not do this automatically). To split a mosaic into 2 visits, simply make a copy of the observation, remove half of the mosaic tiles from the original, and remove the other half from the copy. The article APT Mosaic Tile Splitting Activity has more information on splitting tiles from mosaics.

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