NIRSpec Bright Object Time-Series Spectroscopy
JWST NIRSpec bright object time-series (BOTS) mode uses the 1.6” × 1.6” fixed slit aperture. This mode is optimized for exoplanet transit observations requiring stable observing conditions and high photometric precision time-series spectroscopy.
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 the NIRSpec's observing capabilities.
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BOTS mode provides a specific set of observing options that are optimized for exoplanet transits:
- 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 (see Table 2 in the NIRSpec Detector Subarrays article) of multiple integrations for the time-series spectroscopy, which distinguishes it from the fixed slit (FS) mode with the S1600A1 aperture.
- Multiple exposures can be used to span the necessary duration of the requested time series. Each exposure contains the same number of integrations.
- A target acquisition procedure called WATA that can acquire and center a bright star directly in the S1600A1 aperture. It is also possible to use an offset target for BOTS target acquisition.
Properties of the BOTS mode
See also: NIRSpec Fixed Slits
The NIRSpec S1600A1 aperture is 1.6 arcsec square, and is large enough to pass about 95% of the light from a point source. This helps improve the signal-to-noise ratio, and the large aperture also makes the throughput insensitive to the small amount of pointing jitter. The location of the S1600A1 aperture in the micro-shutter assembly (MSA) focal plane and a zoomed view of the BOTS aperture is presented in Figure 1. Because the S1600A1 aperture is so large, the spectral resolution in the resulting data will typically be determined by the PSF size of the source.
See also: NIRSpec Dispersers and Filters
All NIRSpec modes, including BOTS, use the same set of disperser and filter combinations to provide spectra with R ~ 100, ~ 1,000, and ~ 2,700.
Table 1. Available disperser and filter combinations
|Disperser-filter combination||Nominal resolving power||Wavelength range † |
† Wavelength range values presented here are approximate. Note that the nominal spectral ranges for medium and high-resolution dispersers may be shortened due to red-end detector cutoffs. The cutoff wavelengths depend on the target aperture location (slit or shutter). Detailed limits on the wavelength ranges and gaps are found in the ETC.
The overall system throughput for BOTS (and FS) mode with the S1600A1 aperture is higher than the other fixed slits and MSA shutters because of the larger aperture. This is taken into account in the Exposure Time Calculator (ETC).
Detector wavelength gaps
There is a physical gap between the 2 NIRSpec detectors in the focal plane array. This affects NIRSpec BOTS observations with the high resolution (R = 2,700) gratings because the spectra are long enough to span both NIRSpec detectors. The S1600A1 aperture is positioned such that no spectrum wavelengths are lost when the PRISM or the medium-resolution dispersers are used. A full description of the position of the gaps and wavelength ranges for each filter-grating and subarray combination are available in the NIRSpec BOTS Wavelength Ranges and Gaps article.
See also: NIRSpec Detector Subarrays
Targets observed in BOTS mode are expected to be very bright, so bright that full frame readouts of the detectors would saturate. Detector saturation is avoided by reading out a smaller portion of the detectors. Table 2 lists the available subarrays and their properties. Observers are encouraged to use the largest subarray possible that avoids saturation.
- When using PRISM/CLEAR, even the smallest subarrays (SUB512 and SUB512S) will record the full wavelength range of the spectrum.
- For other dispersers, only the SUB2048 option will record the full wavelength range of the spectrum (i.e., there are no red or blue end detector spectral cutoffs, but there are still losses of some wavelengths due to the gap between detectors).
The ETC can provide precise estimates of count rates to guide the subarray selection.
BOTS mode exposure time estimates can be performed with a fixed slit calculation using the S1600A1 slit in the ETC. The BOTS subarrays are a subset of the FS subarrays.
Table 2. Subarrays and exposure parameters for BOTS mode
Maximum total duration‡
|SUB2048||2048 × 32||any||0.902||118225.14 (32.84)||Full spectrum range|
|SUB1024A||1024 × 32||any, except PRISM||0.451||59170.241 (16.44)||Short wavelength half of spectrum|
|SUB1024B||1024 × 32||any||0.451||59170.241 (16.44)||Long wavelength half of spectrum|
|SUB512¶||512 × 32||PRISM||0.226||29642.791 (8.23)||§Both detectors are read out but no illumination of NRS2 detector|
|SUB512S¶||512 × 16||PRISM||0.144||18863.594 (5.24)||§Both detectors are read out but no illumination of NRS2 detector|
* Subarray sizes are in detector pixels, in width (dispersion direction) × height (cross-dispersion direction).
† Frame time is the time to read out the subarray, in seconds.
‡ The maximum total duration is the longest time that can be spent observing a multi-integration exposure when the highest level of time resolution is selected for a given subarray. This is based on use of the NRSRAPID readout mode for an exposure, and is limited by a maximum of 65,535 one-group integrations.
§ Both detectors, NRS1 and NRS2, are read for all subarray exposures.
¶ Subarray SUB512S reads out only pixels that are illuminated by the S1600A1 aperture, whereas SUB512 reads out extra unilluminated reference pixels
The NIRSpec BOTS mode exposure durations are tied to the subarray selected, the associated detector readout pattern timing (see column 4 in Table 2), and the number of integrations used for the time series.
There are 2 readout patterns available for NIRSpec BOTS mode observations:
The NRSRAPID readout pattern has a single frame, and NRS is 4 frames averaged into a single group. BOTS mode users will specify exposure times by defining the subarray, readout pattern, number of groups in each integration (integration time), and the number of integrations within the observation. Each exposure in BOTS mode is limited to a maximum of 65,535 integrations and a maximum of 196,608 frames.
Additional information on NIRSpec BOTS exposure specification and how this translates to exposure time and sensitivity can be found using the JWST Exposure Time Calculator (ETC).
Options for dithering
No dithers are allowed in BOTS mode. This minimizes changes in source flux due to resulting variations in sensitivity and throughput.
Gain, sensitivity, and bright limiting magnitudes
Only the NIRSpec subarray readout gain is appropriate for BOTS exposures since BOTS is restricted to subarray readout. The gain for subarray readout, which in addition to BOTS may also be used in fixed slit observations, will be approximately a factor of 1.43 higher than the full frame readout gain. This corresponds to an actual gain of ~1.4 e–/DN for NRS1 and ~1.5 e–/DN for NRS2.
Table 3 lists the brightest magnitude (J band) that can be observed in BOTS mode without saturation for 3 stellar temperatures. These values are intended solely for guidance and precise values should be calculated with the JWST Exposure Time Calculator (ETC).
Table 3. Configurations and estimated brightness limits
|J Magnitude (Vega)|
|Disperser-filter||TBB = 10,000 K||TBB = 5,000 K||TBB = 2,500 K|
† The SUB2048 subarray is assumed in all cases except for PRISM/CLEAR values that were determined using SUB512.
Bright limits are determined using ETC v3.0, including on-orbit performance, for a black body point source using parameters of 2 groups and 1 integration with NRSRAPID readout mode.
These values are for the subarray gains indicated above, and a conservative full well depth of 65,000.
What do BOTS mode data look like?
Figure 2 presents an example exposure for a BOTS mode observation. BOTS data consist of multiple integrations captured in a time series. In the figure, each individual integration is shown as a full spectrum across the 2 detectors. The x- and y-axis in the figure represent the spectral and spatial ranges, respectively. The 3rd dimension in the observed data cube is the time series of multiple integrations captured over the duration defined in the exposure specifications.