NIRSpec BOTS Operations
Operational features of the JWST NIRSpec bright object time-series (BOTS) mode include target acquisition methods and other special considerations.
Time-series observations (TSO) are carried out to monitor astrophysical sources that are time variable. A time-series observation is typically a long staring observation, optimized to detect and characterize faint temporal modulations in the source flux. Common examples are observations of stellar variability, eclipsing variables, brown dwarf variability, and exoplanet transits.
The JWST NIRSpec bright object time-series (BOTS) mode is for observations of bright sources that require high throughput and stable time-resolved spectroscopy. This mode is optimized for the study of transiting exoplanets around their bright host stars; such observations are expected to be the primary use of the BOTS mode. Additional use cases include any time-series science from spectroscopy of bright targets made possible with these NIRSpec observing capabilities.
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- BOTS mode always uses the S1600A1 aperture.
- It uses WATA target acqusition to acquire and center a bright star directly in the S1600A1 aperture.
- No dithering is done in BOTS mode. The object's spectrum is kept on the same detector pixels at all times (only modulated by the pointing jitter of JWST), thereby optimizing spectro-photometric stability and precision.
- Several detector subarray options can be selected to limit detector saturation when observing very bright stars.
- The BOTS mode provides the capability to take extremely long exposures (up to 48 hours) that utilize multiple integrations for the time-series spectroscopy, which distinguishes it from the fixed slit (FS) mode with the S1600A1 aperture.
WATA operational sequence
The NIRSpec wide aperture target acquisition (WATA) method is used to acquire suitably bright objects. It may be used with all templates for situations when roll correction is not critical (i.e., when the orientation achieved through blind pointing is sufficient). Following the guide star acquisition, the operational sequence of WATA is:
- The telescope is slewed to place the acquisition target in the S1600A1 aperture.
- An Ngroups = 3 image covering 32 × 32 pixels, centered at the location of the S1600A1 wide aperture, is acquired.
This exposure is taken using the NIRSpec imaging mirror in the grating wheel assembly. Note that the flux sensitivity of the image is set by (1) the TA filter selected for the WATA observation and (2) the exposure time that is defined by the selected detector readout pattern and subarray. All NIRSpec TA images are acquired with Ngroups = 3 (Acq Groups/Int parameter in APT).
- Cosmic ray and hot pixel amelioration is performed on the image.
- The star's centroid is calculated and pixel coordinates are transformed to a sky coordinate system.
- The telescope performs a small angle maneuver to center the star in the S1600A1 aperture.
- A mandatory post-target acquisition "reference image" of the centered TA object is obtained as part of the procedure. This exposure must be obtained in the same visit as all of the science exposures that depend on the acquisition.
If it is necessary to split an observation into multiple visits, additional observations and acquisitions will be needed. Any slew needed between the acquisition pointing and all subsequent science pointings must fit within the visit splitting distance and there must be at least one guide star that can support all the needed pointings. This will limit the size of mosaics and other offsets that can be supported following the WATA acquisition.
The operational sequence of activities carried out by WATA is hard-coded in the NIRSpec observation execution onboard scripts. The only user-selectable parameters are the subarray type, filter, and detector readout pattern for the observation:
- Three subarrays, which correspond to 3 different readout times, are available: SUB32, SUB2048, and FULL.
- Two readout patterns are available: NRSRAPID and NRSRAPIDD6. The choice will depend on target brightness and achievable S/N: NRSRAPIDD6 is meant for fainter targets that need longer exposure time for the S/N in TA. The NRSRAPIDD6 readout option observes a longer exposure and so it takes a bit longer to execute. The exposure timing for NRSRAPIDD6 and NRSRAPID with different TA subarray options will be reflected in the ETC results. These readout patterns are described in NIRSpec Detector Readout Modes and Patterns.
- Three NIRSpec filters are available: F110W, F140X, and CLEAR. These are expected to allow coverage of the brightness range ~ 12.8 mag < AB < ~26.2 mag.
BOTS observation considerations
Time-series observations (TSOs), as enabled by the NIRSpec BOTS mode, put specific performance needs on the JWST observatory and its instruments. These design considerations ensure the highest possible level of stability over lengthy exposures.
NIRSpec features a 1.6" × 1.6" wide aperture (called S1600A1) that enables high precision time-series observations of bright targets. Because the S1600A1 aperture is so large, the spectral resolution in the resulting data will typically be determined by the size of the source. The aperture is also wide enough to limit aperture losses and associated small variations in throughput due to pointing jitter and drift of the JWST, yet small enough that background and possible contamination by other nearby sources will usually not be an issue. JWST pointing stability is required to be better than 7 mas over 10,000 s. Assuming the pointing stability requirement is met, the 1 sigma slit and diffraction loss variability is below 25 ppm (Dorner 2012). See also Rigby (2022) for an up-to-date summary of the in-flight JWST pointing accuracy. This effect contributes very little to the total noise over the wavelength range of the different NIRSpec dispersers.
NIRSpec BOTS observations, which require long duration staring observations with high stability requirements, have the following operational features:
- Detector readout options can be selected for long time-series exposures.
Very long exposures on bright targets can be acquired with many short integrations, and the number of groups in an integration should be chosen to prevent saturation. The limit on the number of integrations is 65,535; this is set by a software limit of less than 216 integrations per exposure (see Table 2 in Detector Subarrays). Detector readout patterns NRS and NRSRAPID are both available for BOTS observations.
- High gain antenna moves are allowed during the exposure. During normal JWST operations, the high gain antenna (HGA) needs to re-position to continually point toward the ground communication stations. As a result, a HGA check is done every 10,000 sec, and a re-point is executed if necessary. However, for exoplanet transit/occultation observations, the expected exposure times necessary to monitor a full eclipse light curve can often far exceed the 10,000 sec exposure limit imposed by the HGA re-points. Exposure breaking around HGA moves would be difficult to implement in an optimal way that would not affect the light curve stability. The pointing stability of the observatory is not guaranteed to meet the requirement during the brief period associated with an HGA move. The plan is for NIRSpec BOTS mode to observe straight through the HGA moves and to later exclude the ~60 sec excursions in signal flux during data analysis. After the HGA moves, the guide star lock and repositioning is expected to be repeatable to better than 5 mas, and therefore should not significantly alter the transit light curve signal outside of the move time.
- Exposures longer than 10,000 sec are allowed in NIRSpec BOTS mode. The exposure duration in the NIRSpec BOTS template is defined by the choice of detector subarray and the number of groups and integrations. The primary planning limit for BOTS mode observations is that the total visit duration must be shorter than 48 hours. However, there will be visit initiation and visit closeout activities (see Figure 1). So, the maximum possible single exposure time in NIRSpec BOTS observations will be slightly less than 48 hours.
- BOTS observations cannot be dithered.
BOTS observation timeline
While, in general, all JWST observations are event driven, the JWST Astronomers Proposal Tool (APT) allows users to provide timing constraints for time critical observations. The absolute timing constraints on JWST NIRSpec observations of exoplanet eclipse or phase curve targets is defined by the following timing special requirements:
Table 1. Timing special requirements for time-series phase constraints
PHASE RANGE (start)
|Specification of the earliest time for the start of the first BOTS science action in the visit.|
PHASE RANGE (end)
Specification of the latest time for the start of the first BOTS science action in the visit.
|PERIOD specifies the period of the time series source (in days)|
Specifies the time of the nominal zeropoint reference for the target (as a Heliocentric Julian Date).
For example, PHASE RANGE (Start) = 0.0 to PHASE RANGE (End) = 0.1 would specify that the start of the first science activity must be scheduled within the first 10% of the time-series period. Another example for a transiting exoplanet observation is discussed in the article Step-by-Step APT Guide for NIRSpec BOTS Observations of WASP-79b.
Tight timing constraints that result in a narrow observation execution window (less than 24 hrs) will result in a direct scheduling overhead of 1 hour. All tightly constrained NIRSpec BOTS observations will be affected by this overhead.
The shortest absolute timing constraint for JWST is 5 min; as a result, the window defined by the earliest and latest phase start time must be at least 10 min. Because of the thermal settling time of the detectors that will happen at the beginning of the exposure, it is advisable to allow for extra "padding'' of about 15 min at the beginning of the time-series observation by adjusting the phase range accordingly.
Figure 1 presents the sequence of activities in a single exposure NIRSpec exoplanet transit observation. The red curve shows an example exoplanet light curve with flux as a function of time increasing to the right. The visit starts with the slew to the star, followed by the guide star (GS) acquisition and the instrument target acquisition to place the star within the NIRSpec S1600A1 aperture. Depending on the timing of the visit, there might be a brief wait period prior to the configuration of NIRSpec for the first science activity. The start of the first science activity is seen as the magenta line and arrow in Figure 1. This is the point that must be defined by the timing special requirements: the phase range, period, and zero phase special requirements (see the BOTS template parameters specification). A detailed sample use case, with corresponding APT and ETC strategies is documented in the article NIRSpec BOTS Observations of WASP-79b.
The duration of the science actions to configure the instrument (setting the filter wheel assembly [FWA] and grating wheel assembly [GWA] for the first exposure is expected to be less than about 5 m. The timescale for instrument configurations is expanded in Figure 1 for clarity of the flow. The first (or only) exposure starts after the mechanism moves, and any thermal settling will happen while data is acquired in the exposure. In the case of only one exposure, it will continue uninterrupted for the duration of the visit. If there is more than one exposure requested in the visit, they will be taken back-to-back, but there will be a short gap of about 20 sec for exposure setup. This will introduce small thermal transients at the beginning of new exposures. After the exposures are complete, brief close-out activities (moving the FWA to its closed position) will occur at the end of the visit.
Dorner, Bernhard. (2012). PhD Thesis
Verification and science simulations with the Instrument Performance Simulator for JWST - NIRSpec. (A model-based approach to the spatial and spectra calibration of NIRSpec onboard JWST).
Rigby, Jane et al. (2022)
Characterization of JWST science performance from commissioning