NIRSpec Bright Object Time-Series Spectroscopy
JWST NIRSpec bright object time-series (BOTS) mode is optimized for exoplanet transit observations requiring stable observing conditions and high photometric precision time-series spectroscopy. This mode uses the 1.6” × 1.6” fixed slit aperture and all grating-filter combinations available for NIRSpec.
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See also: JWST Time-Series Observations, NIRSpec Bright Object Time-Series APT Template
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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The 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.
- The target acquisition procedure called WATA can acquire and center a bright star directly into 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.
Figure 1. Location of the S1600A1 aperture used in BOTS mode
Click on the figure for a larger view.
The location of the S1600A1 aperture used in BOTS mode observations is identified within the small red box in the MSA focal plane view. On the left is a zoomed view that shows the 1.6" × 1.6" aperture location relative to the S400A1 fixed slit.
Spectral configurations
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 † (μm) |
|---|---|---|
| G140M/F070LP | ~1,000 | 0.70–1.27 |
| G140M/F100LP | 0.97–1.89 | |
| G235M/F170LP | 1.66–3.17 | |
| G395M/F290LP | 2.87–5.27 | |
| G140H/F070LP | ~2,700 | 0.81–1.27 |
| G140H/F100LP | 0.97–1.89 | |
| G235H/F170LP | 1.66–3.17 | |
| G395H/F290LP | 2.87–5.27 | |
| PRISM/CLEAR * | ~100 | 0.60-5.30 |
† 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 BOTS multistripe subarrays can only be used with the PRISM/CLEAR disperser-filter combination, but the multistripe subarrays lose some wavelength coverage in the red due to the interleaved reference pixels (see NIRSpec BOTS Wavelength Ranges and Gaps).
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
See also: NIRSpec BOTS Wavelength Ranges and Gaps, NIRSpec Dithering Recommended Strategies
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.
Subarrays
See also: NIRSpec Detector Subarray Mode
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, the smallest regular subarrays (SUB512 and SUB512S) will record the full wavelength range of the spectrum. The prism BOTS multistripe subarrays (SUB256M2_PRISM, SUB128M4_PRISM, SUB64M8_PRISM, and SUB32M16_PRISM) lose some wavelength coverage in the red due to the interleaved reference pixels at the start of each stripe exposure (see NIRSpec BOTS Wavelength Ranges and Gaps for more details).
- 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. Available subarrays and their properties
| Subarray name | Height (pixels) | Width | Recomended | Frame time | Maximum exposure duration, in seconds (hours)† | Comments |
|---|---|---|---|---|---|---|
| SUB2048 | 32 | 2048 | any | 0.90200 | 177,337.7 (49.3) | Full spectrum range |
| SUB1024A‡,* | 32 | 1024 | any, except PRISM | 0.45144 | 88,755.4 (24.65) | Short wavelength half of the spectrum |
| SUB1024B‡ | 32 | 1024 | any | 0.45144 | 88,755.4 (24.65) | Long wavelength half of the spectrum |
| SUB512 | 32 | 512 | PRISM | 0.22616 | 44,464.2 (12.35) | Both detectors are read out but no illumination of detector NRS2 |
| SUB512S | 16 | 512 | PRISM | 0.14392 | 28,295.4 (7.86) | Both detectors are read out but no illumination of detector NRS2; no un-illuminated pixel in trace |
| SUB256M2_PRISM | 32 | 504 | PRISM | 0.11352 | 22,318.6 (6.20) | NRS1 is fully illuminated but only a 256 pixels wide portion of the subarray is read out for a given integration, no illumination of NRS2 detector |
| SUB128M4_PRISM | 32 | 496 | PRISM | 0.05720 | 11,245.8 (3.12) | NRS1 is fully illuminated but only a 128 pixels wide portion of the subarray is read out for a given integration, no illumination of NRS2 detector |
| SUB64M8_PRISM | 32 | 480 | PRISM | 0.02904 | 5,709.41 (1.59) | NRS1 is fully illuminated but only a 64 pixels wide portion of the subarray is read out for a given integration, no illumination of NRS2 detector |
| SUB32M16_PRISM | 32 | 448 | PRISM | 0.01496 | 2,941.21 (0.82) | NRS1 is fully illuminated but only a 32 pixels wide portion of the subarray is read out for a given integration, no illumination of NRS2 detector |
† All BOTS mode observations use the S1600A1 fixed slit. The stated maximum exposure durations are for 3 groups in NRSRAPID readout mode with the maximum number of integrations/exposure (65,535) unless stated otherwise. These are only allowed after adding the time-series observation (TSO) special requirement.
‡SUB1024A captures the shorter wavelength portion of the spectrum on the NRS1 detector, and the longer wavelength portion of the spectrum on NRS2. Alternatively, SUB1024B captures the longer wavelength portion on NRS1 and the shorter wavelength portion on NRS2. The medium resolution (M) grating and PRISM spectra for BOTS map entirely to NRS1. However, for the high resolution (H) gratings, the use of SUB1024B will enable capturing a (semi-)contiguous mid-wavelength portion of the spectrum across the detector gap.
* SUB1024A should not be used with the PRISM since the spectra do not project to the region of the detector covered but this subarray.
Exposure specification
See also: NIRSpec Detectors, NIRSpec Detector Recommended Strategies
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:
- NRSRAPID
- NRS
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. For prism multistripe subarrays, only the NRSRAPID readout pattern can be used.
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 using the regular subarrays 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 |
| PRISM/CLEAR † | 11.0 | 10.9 | 11.1 |
| G140M/F070LP | 9.0 | 8.9 | 8.9 |
| G140M/F100LP | 9.0 | 8.9 | 8.9 |
| G235M/F170LP | 7.8 | 8.1 | 9.0 |
| G395M/F290LP | 7.0 | 7.5 | 8.8 |
| G140H/F070LP | 7.9 | 7.8 | 7.8 |
| G140H/F100LP | 7.9 | 7.8 | 7.8 |
| G235H/F170LP | 6.8 | 7.1 | 7.9 |
| G395H/F290LP | 6.0 | 6.5 | 7.8 |
† The SUB2048 subarray is assumed in all cases except for PRISM/CLEAR values that were determined using SUB512.
Bright limits are determined using ETC v6.0, which includes on-orbit performance, for a black-body point source using parameters of 2 groups and 1 integration with NRSRAPID readout mode, medium background, and ETC default values for other input.
These values are for the subarray gains indicated above, and a conservative full well depth of 65,000.
The prism multistripe subarrays split the standard SUB512 subarray into "stripes" (i.e., portions of the SUB512 footprint) to provide accessibility to targets brighter than the prism bright limit (J ~ 11.0, as indicated in Table 3). Table 4 lists the new 2-group bright limits that each multistripe subarray provides.
Table 4. Multistripe subarray brightness limits
| J Mag (Vega) | |||
|---|---|---|---|
| Subarray name* | TBB = 10,000K | TBB = 5,000K | TBB = 2,500K |
| SUB256M2_PRISM | 10.26 | 10.35 | 10.22 |
| SUB128M4_PRISM | 9.52 | 9.60 | 9.47 |
| SUB64M8_PRISM | 8.78 | 8.87 | 8.74 |
| SUB32M16_PRISM | 8.06 | 8.15 | 8.02 |
* Disperser-filter combination is PRISM/CLEAR for all subarrays.
Bright limits are determined using ETC v6.0 for a blackbody point source using parameters of 2 groups and one integration with NRSRAPID readout mode, medium background, and ETC default values for other input.
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-axis 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.
Figure 2. A view of multi-integration BOTS mode data
Click on the figure for a larger view.
A representation of a 3-D multi-integration BOTS mode dataset. The spectral trace is from G235H+F290LP point source data acquired under GO program 01729 entitled "A NIRSpec Phase Curve for the ultra hot Jupiter WASP-121b" (PI: T. Mikal-Evans) using NIRSpec slit S1600A1. BOTS mode observations acquires spectral × spatial data for each integration over a series of many integrations in the time series. Program 01729 observed WASP-121b with a total of 3,504 integrations, but 65,535 is the upper limit of the number of integrations.
Figure 3 shows a schematic of a prism multistripe BOTS mode observation. In the time series, each "stripe" (portions of the NRS1 detector with widths varying based on the chosen multistripe subarray) is read out consecutively as sub-integrations, stepping through the SUB512 region in wavelength space until the full prism region is covered. At the start of each sub-integration, 4 reference pixels are interleaved.
Figure 3. A view of BOTS prism multistripe data
Click on the figure for a larger view.
A representation of a BOTS prism multistripe observation. Within the SUB512 footprint, portions of the NRS1 detector ("stripes") are read out as sub-integrations (blue) that step through the prism region in wavelength space (top). The upper figure depicts 4 stripes, as used by the S248M4_PRISM subarray. In the time-series readout, 4 reference pixels (orange) are interleaved at the start of each sub-integration (bottom).
References
Birkmann, S. 2016 ESAC JWST "On Your Mark" Workshop (ppt) (pdf)
Single Object Spectroscopy and Time Series Observations with NIRSpec