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JWST NIRSpec bright object time-series (BOTS) mode uses the 1.6” × 1.6” fixed slit aperture and is optimized for exoplanet transit observations requiring stable observing condition and high photometric precision time-series spectroscopy.

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Introduction

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 spectroscopy of any bright target time-series science made possible with the NIRSpec's observing capabilities. 

BOTS mode can only be used with NIRSpec's S1600A1 aperture. This 1.6" square aperture specifically enables high precision time-series spectroscopy of bright stars with transiting planets.  The S1600A1 aperture can be accessed in both the fixed slits (FS) and this BOTS mode, unlike other NIRSpec apertures which can only be used in a specific mode and the corresponding template.  

BOTS mode provides a specific set of observing options that are optimized for exoplanet transits:

  • A target acquisition procedure can 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 of multiple integrations for the time-series spectroscopy, which distinguishes it from the fixed slit (FS) mode with the S1600A1 aperture.


Properties of the BOTS mode 

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 signal to noise, 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.

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Figure 1. Location of the S1600A1 aperture used in BOTS mode

Location of the S1600A1 aperture used in BOTS modeImage Modified

Figure caption

The location of the S1600A1 aperture used in BOTS mode observations is identified by 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, with its location relative to the S400A1 fixed slit.


Spectral configurations

All NIRSpec modes, including BOTS, use the same set of disperser and filter combinations to provide spectra with R ~ 100, ~ 1000, and ~ 2700.

Table 1. Available disperser and filter combinations


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PageWithExcerptNIRSpec Dispersers and Filters

 

The overall system throughputs for BOTS modes are slightly higher than other modes because of the larger S1600A1 aperture, which is taken into account in the Exposure Time Calculator (ETC).

Note that the S1600A1 aperture is positioned such that no spectrum wavelengths are lost to the physical gap between the 2 NIRSpec detectors when the PRISM or the medium resolution dispersers are used. For high resolution dispersers, there are some wavelength losses, as shown in Table 4 in the NIRSpec Fixed Slits Spectroscopy page (for the S1600A1 aperture used in FS Mode).

 


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 enumerates the available subarrays and their properties. Observers are encouraged to use the largest subarray possible that avoids saturation.

  • The smallest subarrays that will record the entire spectrum when using the PRISM are SUB512 and SUB512S.
  • For other dispersers, only the SUB2048 option will record a full spectrum.

The ETC can provide precise estimates of count rates to guide the subarray selection.

 

Table 2.  Subarrays and exposure parameters for BOTS mode

Subarray name

Size1
(pixels) 

DisperserFrame time2 (s)

Maximum total duration3 (s (hr))

Comments
SUB20482048 × 32any0.902118167.0 (32.82)Full spectrum range
SUB1024A1024 × 32any0.45159113.0 (16.42)Short wavelength half of spectrum
SUB1024B1024 × 32any0.45159113.0 (16.42)Long wavelength half of spectrum
SUB512512 × 32PRISM0.22629585.0 (8.22)Both detectors are read out but no illumination of NRS25.
SUB512S4512 × 16PRISM0.14418827.0 (5.23)Both detectors are read out but no illumination of NRS25.

Table notes:

  1. Subarray sizes are in detector pixels, in width (dispersion direction) × height.
  2. Frame time is the time to read out the subarray, in seconds.
  3. 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. 
  4. The SUB512S subarray option will be included in version APT 25.0 (January 2017).
  5. Both detectors, NRS1 and NRS2, are read for all subarray exposures.

 


Exposure specification

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 two 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.  A maximum number of 65,535 integrations can be used per exposure in BOTS mode.  

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.

 


Sensitivity and bright limiting magnitudes

Table 3 lists the brightest magnitude (K 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 NIRSpec Exposure Time Calculator.

 

Table 3.  Configurations and estimated brightness limits

 K Magnitude
Disperser/filterTeff = 10,000 KTeff = 5,000 KTeff = 2,500 K
PRISM/CLEAR10.210.29.7
G140M/F070LP7.97.87.2
G140M/F100LP8.08.07.5
G235M/F170LP7.07.57.3
G395M/F290LP6.06.56.7
G140H/F070LP6.86.66.1
G140H/F100LP6.96.86.2
G235H/F170LP5.96.36.3
G395H/F290LP5.05.55.6

Table note:

  • The SUB2048 subarray is assumed in all cases except for PRISM values that were determined using SUB512.

 


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 dimensions in the observed data cube is the time series of multiple integrations captured over the duration defined in the exposure specifications.

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Figure 2. A view of multi-integration BOTS mode data

A view of multi-integration BOTS mode dataImage Modified

 

Figure caption

A representation of a 3D multi-integration BOTS mode dataset. The white spectral trace is from G235H+F170LP point source data acquired during NIRSpec instrument tests. BOTS mode observations will acquire spectral × spatial data for each integration over a series of many integrations in the time series.

 


 

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Related links

JWST User Documentation Home
Near Infrared Spectrograph, NIRSpec
NIRSpec Overview
NIRSpec Observing Modes
NIRSpec Micro-Shutter Assembly
JWST Astronomers Proposal Tool, APT
JWST APT website 
JWST Exposure Time Calculator, ETC
JWST ETC website

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References

Birkmann, S. 2016 ESAC JWST "On Your Mark" Workshop (ppt) (pdf)
Single Object Spectroscopy and Time Series Observations with NIRSpec

JWST technical documents

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Last updated

Published December 29, 2016


 

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Published March 02, 2017


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