The roadmap to plan a JWST NIRSpec MOS observation using the APT Template is introduced. New MOS related terminology is defined.
Caution: MOS Observations are Created Differently!
NIRSpec's Multi-Object Spectroscopy (MOS) observing mode provides the means to simultaneously obtain the spectra of multiple objects within its 3.4 x 3.6 arcmin overall field of view. The MOS mode uses the NIRSpec Micro-Shutter Assembly (MSA).
The MOS mode APT template should not normally be filled out by the observer. The MSA Planning Tool (MPT) within APT was designed to create MOS observations, and will automatically fill out the template for each observation that is created. To start creating your MOS observation with the MSA Planning tool follow the MPT instructions and Proposal Planning Checklist, linked below.
The NIRSpec MOS Proposal and Observing Update Process
Proposals for the NIRSpec MOS Mode: The NIRSpec MOS planning is unlike other APT templates because users do not simply select a narrow range of parameters in the template. Instead, the MSA Planning Tool (MPT) is used to generate test plans and create observations that request the observing time necessary to execute the science.
An outline of the MOS proposal process and instructions to use the MPT are linked in several planning pages:
- NIRSpec MOS proposal checklist
- The MSA Planning Tool: A Roadmap
- Accessing the MPT
- NIRSpec MPT Terminology
Updating NIRSpec MOS Observations for flight execution (coming soon): NIRSpec MOS observations will not be flight executable at proposal submission time. After a proposal is accepted by the time allocation committee (TAC), the MOS observations are placed onto the Long Range Plan (LRP) and an execution orient and corresponding observing window are assigned. A second interaction process is necessary to replan NIRSpec MOS visits for flight execution readiness, to ensure that science targets are properly aligned in their MSA observing shutters. Target acquisition parameters are defined in this flight update process, not in the proposal.
The MSA Planning Tool: A Roadmap
In NIRSpec's Multi-Object Spectroscopy observing mode it is possible to obtain spectra of many sources at the same time. Early in the development of NIRSpec observation planning, it became apparent that specifying MOS slits on science targets using the fixed grid of shutters in the Micro-Shutter Assembly would benefit greatly from an automated software planning tool. In this article we describe how to use the MSA Planning Tool (MPT) that is built into the Astronomer's Proposal Tool (APT) for NIRSpec MOS planning. Additional information on the MOS hardware is described in the article NIRSpec Micro-Shutter Assembly (MSA).
In brief, the steps to create a NIRSpec MOS observation using MPT are as follows:
- Create and upload a complete catalog of sources.
- Define appropriate candidate sets with primary sources and filler sources.
- Use the MPT planner to create a variety of observing plans.
- Asses the quality of the plans.
- Create an observation.
Observations created using the MPT will send pointing positions, MSA shutter configurations, instrument setups and exposure information to the MOS template. The information from MPT automatically populates the template fields to estimate the science execution time and derive the appropriate information for observation overheads.
Accessing the NIRSpec MSA Planning Tool
For information on APT observations, proposers should read the article JWST APT Observations. Starting from a new APT application in the Form Editor, choose "New JWST Proposal" in the pull-down menu at the top left of the APT window called "New Document", as shown in Figure 1. The next item, "Targets", will be automatically filled in when we create MSA observations. Therefore, we proceed directly to Observations. Select 'Observations' in the APT Tree Editor shown on the left side of the APT GUI window. The only option is to click on the 'New Observation Folder'. When the folder appears it will open in the 'Observation Summary' view.
In the Observation Folder view, there are two upper-level tabs: the Observation Summary and MSA Planning Tool as shown in Figure 2.
For MOS planning, select the MSA Planning Tool tab. In this pane three lower-level tabs are displayed: Catalogs, Planner, and Plans. These tabs refer to three panes in the MSA Planning Tool. Initially, you will see the Catalogs pane for ingesting a source catalog. The other tabs provide access to the Planner pane and to the Plans pane. The panes are organized from left to right in the order of the workflow required to create an observation. The lower-level tabs accessible from this page are briefly described in Table 1.
Table 1. Description and access links to lower-level tabs Catalogs, Planner, and Plans.
In this section the observer imports a catalog containing sources of interest.
It should contain a complete list of sources in the field, including: primary science sources, secondary science sources, and potential stars used for TA reference.
Users are also able to create candidate sets in this area.
In this section the user defines detailed properties of the observation and configuration of the instrument. The shape of the slit used in the MSA is introduced here, the dithering strategy is selected in this section, and the exposure type is also defined. Parameters for plan generation and execution are determined here as well.
Once a plan is generated, its details appear listed under the Plans tab. Statistics of the plan are presented as well as a detailed list of the calculated pointings.
It is under this tab where the observer can create an observation based on the generated plan(s).
Click on the links under Lower-level tab in Table 1 for further details.
Useful NIRSpec MPT terminology
NIRSpec MOS observing has introduced some new terminology. Some existing terminology in the APT has specific meaning for observations created in the MSA Planning Tool. Table 2 gives the definition of these terms and provides links for more information.
Table 2. Glossary of MSA Planning Tool related terms
|A complete list of sources in the field to potentially be observed. This list should include objects of interest for MSA observations, objects of potential use as reference stars for MSA target acquisition, and any other objects that could contaminate spectra. The parent catalog will usually contain more sources than are observed in the NIRSpec MOS observations.|
A user-created list of sources of interest for MOS spectroscopic observations, derived from the Parent Catalog.
Candidate sets can be subsets of the Parent Catalog that meet scientific sample selection criteria. The full Parent Catalog can also be a candidate set.
|Plan||A product from running the MPT for a single set of parameters. Plans contain a list of pointings, MSA configurations, and a table of the MSA targets observed in the MOS configurations.|
|Visit||NIRSpec MOS APT Visits are made by the MPT after clicking "create observations". Visits are observing components that begin with guide star acquisition, followed by target acquisition, and ultimately science exposures and perhaps small angle maneuvers of the telescope.|
|Observation||NIRSpec MOS Observations are sets of visits that are made by the MPT after clicking "create observations" on a preferred plan. The NIRSpec MOS observation template is populated by the plans generated in the MPT. NIRSpec MOS observations must all be created using the same parent catalog. All visits in an observation will execute at the same orient.|
|MSA Configuration||A set of planned open and closed micro shutters in the MSA for one exposure, or one set of nodded exposures (which uses the same pattern of open/closed shutters).|
|Target Set||Subset of sources from the Parent Catalog that are observed in all exposures over a complete set of the nods and/or dithers specified in the MPT. Target sets can include multiple MSA configurations.|
A series of adjacent commanded-open micro-shutters in the cross-dispersion direction forming a slit to disperse the light of a given target.
Indicates the five choices for the limiting shutter centering margin in the MPT Planner: from "tightly constrained" to "unconstrained".
|Repositioning the telescope slightly between exposures to place the targets into different shutters within their respective slitlets in the same MSA configuration.|
|Dither||Telescope offsets used to acquire multiple exposures within a single visit. By definition, dithers require a new MSA configuration, while nods do not.|
Planning NIRSpec MOS Confirmation Images
NIRSpec MOS Confirmation Images can be acquired through the spectroscopy science filter and the open science shutter slitlet configuration (pattern of open shutters to form spectral slits).
There are three available options for acquiring NIRSpec MOS confirmation images:
- No (default): No MOS confirmation images through the open/closed slitlet configurations are acquired at any point in the execution of science exposures.
- After Target ACQ: MOS confirmation images are acquired following the conclusion of the NIRSpec target acquisition activity in a visit, and after the MOS open shutter pattern and science filter are configured. These images can provide verification of target positioning within the first science slitlet configuration used in a visit. TA executes only once during a visit, so this confirmation image will also be taken just once.
- After Target ACQ and New MSA Config: MOS confirmation images are acquired immediately after configuration of MSA shutters into a new open/closed slitlet pattern, every time a new MSA slitlet configuration is used. NOTE: for this option, the grating wheel will move to the imaging mirror to take confirmation images in between science spectra. When the grating wheel is reconfigured to the dispersive element, it might not return to the exact same science position used for prior exposures. As a result, small spectral shifts can occur between multiple MOS spectra acquired with a confirmation image in between exposures.
These confirmation images can be planned by selecting the appropriate option in the Multi-Object Spectroscopy pull-down menu in the APT template, as shown in Figure 3. This option should be selected after observations are created in the template by using the MPT.
Karakla, D. et al. 2014, Proc. SPIE 9149
The NIRSpec MSA Planning Tool for multi-object spectroscopy with JWST