NIRSpec MPT - Parameter Space
The JWST MSA Planning Tool has been tested using a range of parameters and a variety of catalog densities in order to give a realistic example of performance trends.
For NIRSpec multi-object spectroscopy, observation plans are generated by the MSA Planning Tool (MPT) in APT. Several MPT parameters can significantly affect the number of sources in these plans, including: Catalog density, source centering constraint, number of target sets (based on Number of Configs*), search grid step size, and fixed dither size. Search grid step size also affects the runtime of plans in a predictable way.
Some internal parameters have changed since these tests were completed. The trends shown will remain similar, but the actual values may have changed.
*Bold italics style indicates words that are also parameters or buttons in software tools (like the APT and ETC). Similarly, a bold style represents menu items and panels.
The Micro-Shutter Assembly (MSA) that is used for MOS observations has ~250,000 shutters that can be opened individually, or in columns to create ‘slitlets’.
Spectra with the Medium (M) and high (H) resolution gratings both block other targets in the MSA over a similar area per shutter in each slitlet, so there is not a significant difference (<2-3%) in the number of observed sources in plans made using those two sets of dispersers. Prism spectra are much shorter in the dispersion direction, and can therefore be packed up to 3 to 4 times more tightly in the MSA than the other gratings.
All results shown are for plans using 3-shutter slitlets with nods. Nods are used to subtract background, repositioning the telescope slightly between exposures to place the targets into different shutters within their respective slitlets. Adding nods decreases multiplexing because it requires additional space above and below each planned slitlet to be clear of sources in order to keep the target source's spectra from being contaminated when the open shutters of the slitlet are moved up and down.
Source centering constraint
The source centering constraint sets the bounds of the area inside a mircoshutter where sources can be observed. There are five choices: Unconstrained, Entire Open Shutter, Midpoint, Constrained, and Tightly Constrained. Centered or nearly-centered sources will have higher throughput, but there will be fewer of them.
The effect of the source centering constraint on the number of sources varies, but the effect is greatest at lower Catalog density (below ~200 sources/arcmin2 for M or H gratings and below ~600 sources/arcmin2 for prism). See Figures 1 and 2. At higher Catalog density, there is an excess of potential sources, so the number that are planned is instead limited by the available area of the MSA. As the number of target sets (defined below) increases, and the number of available Catalog sources decreases, the choice of source centering constraint has a greater effect.
Number of target sets
A target set is a group of sources that are observed in one full set of exposures (over nods and dithers). The number of target sets that are observed depends on the Number of configurations that are selected, and the number of primary dithers that are specified in the MPT Plan. MPT will display the number needed to complete each target set. When a multiple, N, of that value is selected, N target sets will be planned.
The number of sources that a target set has depends on a number of input planning parameters. The target set source density is always highest (regardless of input parameters) when the plan is not limited by too few science source candidates in the input Catalog density. For example, the first few target sets created using 3-shutter slitlets achieve maximum multiplexing above ~200 sources/arcmin2 for M or H gratings, and above ~600 sources/arcmin2 for the prism (see Figures 1 and 2). Below these limits, the number of available, unobserved sources in a Catalog can limit the number that are planned within a target set.
As the number of target sets in a plan increases, science sources are observed then removed from the Catalog of available sources. The number of remaining unobserved Catalog sources decreases. As a result, the number of observed sources per target set will also correspondingly decrease.
Search grid step size
The search grid step size sets the spacing of the grid that MPT uses to find the optimal set of pointings for a plan. Searching with a finer grid (smaller step size) increases the total number of pointings that are tested, and often results in plans with more sources being found.
As shown in Figures 3 and 4, decreasing the search grid step size from 20” to 5” can increase the number of sources in 1 target set by 5 to 10% for the highest densities and up to 30% for the lower ones. Increasing it from 20” to 0.2” can increase the number by 10 to 15% for the highest densities and up to 60 to 70% for the lower ones. These increases are not linear though, the results also depend on how the sources in the catalog happen to fall, relative to each other and to the center of the search grid.
While choosing a smaller search step size can result in better multiplexing, it will also make generating plans take longer. However, the time needed to generate a plan can be estimated. With everything else held constant, including computing resources, the run time of a plan will be roughly inversely proportional to the search grid step size in log-log space, as shown in Figure 5.
As noted previously, the level of multiplexing achievable with 3-shutter slitlets in the MSA approaches a limit at Catalog densities of ~200 sources/arcmin2 for M or H gratings and ~600 sources/arcmin2 for the prism. This is seen in the distribution of the results for the different densities in Figures 3 and 4. It is also apparent (more so than in Figures 1 and 2) that the maximum number of sources that will fit on the MSA is ~70 for M or H gratings and ~200 for prism in the 3-shutter slitlet observing case.
A dither is a translation of an MSA configuration by a given number of shutters in the dispersion, X, and spatial, Y, directions, which allows spectra to be observed on multiple parts of the detector. On average, larger dithers result in fewer selected sources than smaller dithers, and even small dithers result in fewer sources than no dithers. However, the results will depend on the Catalog. There can be dips or bumps in the number of sources as step size decreases, and, in general, higher density catalogs will lose a lower percentage of sources than lower density ones, but that also is not uniform (see Figures 6, 7, 8, and 9).
Two reasons for the decrease in number of sources that affect both dithers in X and Y are: distortion, which can cause sources to move out of the area allowed by the source centering constraint within their planned shutters, and the fact that only sources in areas that are covered by the MSA in all dithers can be selected. Both of these get worse for larger dithers, and, therefore, fixed dithers of more than 10” (~38 shutters in X and ~18 in Y) are not recommended.
For spatial dithers, there is an additional effect that causes a sharp drop in the number of sources between no dither and a 3-shutter dither: the MSA has some rows that are unusable because of failed open or failed closed shutters. Since each slitlet is 3 shutters tall, dithering in the spatial direction by 1, 2 or 3 shutters requires 4, 5, or 6 consecutive rows to be usable, respectively, while any dither of more than 3 shutters will require 3 then 3 consecutive rows. As shown in Figure 9, the effect is most apparent for the Prism, where sources can be packed more tightly.
About the Catalogs
The catalogs used in these tests were derived from a catalog of galaxies in the Hubble UDF. The original catalog had approximately 830 sources/arcmin2 and was distributed evenly over an area slightly smaller than the 3.4” × 3.6” MSA footprint. Subsets of the full catalog were randomly chosen to create similar catalogs with densities of 800, 400, 200, 100, 50, 25, 12.5, and 3.125 sources/arcmin2.