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.
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.
Words in bold italics are buttons
or parameters in GUI tools. Bold
style represents GUI menus/
panels & data software packages.
Table 1. Mosaic properties configurable by the user
|Rows||Number of rows of the mosaic|
|Columns||Number of columns of the mosaic|
|Row Overlap %||This slider gives the percentage of aperture overlap between two consecutive rows|
|Column Overlap %||This slider gives the percentage of aperture overlap between two consecutive columns|
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 fourth row of Table 2. Note that the number 30 actually refers to the angle between the same tile corner on two adjacent rows.
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 footprint||Rows||Columns|
|Overlap (%)||Shift (degrees)|
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.