Most observational modes will be available for DD ERS programs, however some have specific special requirements for implementation, relevant largely to just DD ERS or Cycle 1.
Target Visibility and Alternate Targets
DD ERS teams must establish that the visibilities of their proposed observations will allow them to be obtained in the first 5 months of Cycle 1 (planned to be from April to August 2019), with a substantive subset observable within the first 3 months. DD ERS proposers must also describe the steps they will take to identify alternate observations in the event of a change to the scheduled start of Cycle 1. Teams with accepted proposals will then be required to specify observations of a minimum set of alternate targets to allow the program to be executed at any point during the year. DD ERS alternate observation lists will be published, and GO Cycle 1 proposals including such observations will be allowed.
NIRSpec Multi-Object Spectroscopy
The multiplexing capabilities offered by the NIRSpec Multi-Object Spectroscopy (MOS) modes represent a major opportunity for JWST observers. NIRSpec MOS programs might involve observations of anywhere from a handful of sources to more than 100 targets. Larger-scale programs introduce significant complexities in planning, scheduling and implementing specific observations.
Orientations, optical distortions and target selection: The target selection process for NIRSpec MOS observations must take account of how objects are projected onto the micro-shutter array, and therefore depends on the orientation of the observations and the distortions present along the NIRSpec optical path. The flight optical distortion maps can only be measured after launch, during and after NIRSpec commissioning. The exact orientation for any observation will not be determined until that observation is placed on the Long Range Plan (LRP) for Cycle 1. Consequently, at the time of the DD ERS proposal submission (and Cycle 1 GO as well), proposers will not be able to specify which of their targets will actually be observable.
Catalog sizes: The number of targets within a given observing catalogue that can be observed during a single NIRSpec MOS observation is limited by the availability of suitably-positioned micro-shutters to accommodate the appropriate nod and dither patterns, and, if relevant, by the need to avoid overlapping spectra on the detector. Taking this into account, the NIRSpec team has conducted analysis to determine that up to ~190 objects can be targeted at low spectral resolution (R=100, no overlap) and ~55 at high spectral resolution (R~1000 and 2700, no overlap). This work also shows that approaching these asymptotic multiplex values requires a large input catalog with high target densities of ~720 sources arcmin-2 and 240 arcmin-2, respectively, corresponding to ~7,000 and ~2,400 targets within the NIRSpec field of view. Thus, to maximise the multiplex capabilities of NIRSpec MOS, catalogs provided by observers at the time of proposal submission should include many more targets than can actually be observed (Jakobsen et al. 2017). Conversely, in many cases a substantial number of potential targets will remain unobserved at the conclusion of a program.
These considerations lead to several operational consequences:
- In order to accommodate the full range of possible orientations and the small nominal pointing adjustments necessary to maximize the science return from an observation, observers should specify a potential target list covering an area of radius at least 3 arcminutes for any particular pointing. If possible, the catalogue should be oversized in the number of targets to maximize the NIRSPec MOS multiplexing; this will not be possible for all science cases.
- By policy, proposers do not reserve access to the field of view covered by a NIRSpec MOS observation. Consequently, proposers may not reserve the full list of targets associated with any accepted observing proposal. At the time of submission, proposers may flag high priority targets, but there is no guarantee that any one target will be observed.
- By policy, proposals for MOS observations may be submitted with source catalogues that overlap with those of previously accepted proposals. Proposers must identify potential duplications with prior programs, and must provide an appropriate scientific justification and a demonstration that sufficient targets are available to justify the additional observations. If the Telescope Allocation Committee accepts such a proposal, the previously accepted proposal will have priority in target selection; thus, in JWST Cycle 1, GTO programs have priority in target selection over DD ERS programs, which have priority over Cycle 1 GO programs.
- Multiple proposals using overlapping source catalogues may be proposed and accepted by the Telescope Allocation Committee during the same cycle. In such cases, the TAC will provide a clear specification of the relative priority of those proposals with regard to target selection.
Duplicate observations with JWST are generally not allowed without an approved scientific justification. However, in order to maximize the scientific return, NIRSpec MOS observations may include a limited number of duplicate observations of individual targets without specific scientific justification. The latter sources may not exceed 10% of the total targets within a given NIRSpec MOS observation as implemented for execution. The final target lists will be reviewed for compliance and, if necessary, subjected to adjudication by the STScI Director.
MIRI and NIRCam Mosaic and Dithering Strategy in Regards to Filter Wheel Moves
Filter mechanisms have the potential to be life limiting factors for JWST instrumentation. This is particularly the case for NIRCam, which serves as the wave-front sensing imager for the telescope. As a consequence, observers must take steps to minimize filter wheel moves. Programs with either instrument that involve mosaics or closely spaced observations should step through the observing positions with a single filter before moving to a second filter. Exceptions to this protocol must be justified scientifically. Increasing observing efficiency is not an adequate justification except under exceptional circumstances. Regarding the MIRI IFU, restrictions are already in place to prevent the abuse of its gratings.
Data Volume Limitations
Some observations will generate a high data volume per visit and/or high data rates that may exceed limits in the storage capacity of the solid state recorder (58.8 Gbytes) and/or in the downlink rate (28.6 Gbytes per 12 hour period). In some cases, APT has implemented limitations regarding the readout patterns to avoid exceeding these limits, but other observing options enabled by APT could create problems. APT (25.2) will only create an error if the visit exceeds the capacity of the solid state recorder. If this is the case, the user is required to change the observing strategy to comply with solid state recorder storage limits. Users should keep in mind, however, that data volume issues can only be fully identified downstream and the Visit Scheduling Subsystem and the Visit Planning Subsystem are designed to take these issues into consideration. To facilitate the scheduling of the observations, users are encouraged to keep the data volume under 28.2 Gbytes in a 12 hour period (0.654 MB/s). Please, refer to APT documentation on how to obtain data volume and data rate information. The Contact Scientists and Program Coordinators will iterate with proposers to finalize observations in accordance with the TAC recommendations.
Jacobsen, P. & the NIRSpec Instrument Science Team, "White Paper: NIRSpec Multiplexing and the Need for Proposal Target List Over-booking" (Feb 27, 2017)