NIRCam Grism Time Series

JWST NIRCam's grism time-series observing mode performs rapid spectroscopic (R ~ 1,600) monitoring of bright, time-variable sources at 2.4–5.0 µm.

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See also: NIRCam Grism Time-Series APT Template

The NIRCam grism time-series observing mode is designed to monitor bright, isolated, time-varying sources at 2.4–5.0 µm with spectroscopic resolving power R ~ 1,600. It is one of two modes available forNIRCam time-series observations (TSO), the other being time-series imaging. Both of these modes allow very long exposures (>50 hours with some interruptions and limitations) and disallow dithering and mosaics. A separate non-TSO grism mode is available for wide field grism observations.

In grism time-series mode, the module A "grism R" is used to disperse the target's spectrum along (parallel to) detector rows. The grism is used in conjunction with one of four wide filters in the long wavelength channel (2.4–5.0 µm): F277W, F322W2, F356W, and F444W.

Simultaneous short wavelength (1.7–2.3 µm) imaging is obtained with a weak lens to defocus the image combined with a narrow or medium filter. This combination offers saturation limits similar to the long wavelength grism for a given integration time (which must be identical for both wavelengths).

Integration times may be shortened for rapid cadence monitoring by using detector subarrays and/or multiple detector outputs. The shortest integration times enable observations without saturation of some stars visible to the naked eye (~5th magnitude).

Figure 1. Sample NIRCam Grism R + F444W data

Sample NIRCam Grism R + F444W data, obtained during Integrated Science Instrument Module (ISIM) testing at Goddard. The bright test source included a CO2 absorption feature at ~4.25 µm. Wavelength increases left to right.


Filters

Table 1. Available combinations of filters, weak lenses, and grism

Short wavelengths (1.7–2.3 µm)
Long wavelengths (2.4–5.0 µm)

Weak lens offering +8 or +4 waves defocus
combined with a narrow or medium filter
longward of 1.7µm:

  • WLP8 + F182M
  • WLP8 + F187N
  • WLP8 + F210M
  • WLP8 + F212N
  • WLP4+F212N2

Grism R plus a wide filter:

  • F277W
  • F322W2
  • F356W
  • F444W

† The WLP4 weak lens is joined with a F212N2 narrowband filter with a 2.3% bandpass, to form a single optical element. (Note that this F212N2 filter is wider than the 1% bandpass F212N filter in the filter wheel that can be paired with WLP8.)


Figure 2. Throughput response for NIRCam grism and long wavelength filters (1st order)

First order throughputs of the module A and B grism and the four wide filters (module A is shown, module B is similar) for grism time series, including all JWST and NIRCam optics and detector quantum efficiencies. The grism throughput must be multiplied with that of a selected filter (not shown). The module B grisms are AR coated on only one side and therefore have throughputs ∼33% lower than the module A grisms.
Figure 3. Throughput response for NIRCam grism and long wavelength filters (2nd order)

Second order throughputs of the module A and B grism and the four wide filters (module A is shown, module B is similar) for grism time series, including all JWST and NIRCam optics and detector quantum efficiencies. The grism throughput must be multiplied with that of a selected filter (not shown).


Subarrays and readout times

Bright science targets require short integration times to avoid saturation. To shorten detector readout times, a subset of the detector rows may be read out (and the rest discarded). These subarrays will contain the entire spectrum for isolated compact science targets. Module A available subarrays for this mode contain 64, 128, or 256 pixel rows. Each of the subarrays include all 2048 pixel columns (2040 sensitive to light + 8 reference). The full 2048 × 2048 pixel array may be read out instead if desired.

The detector may be read out either through four outputs simultaneously (to speed readout and minimize saturation) or a single output (to reduce the data volume rate). This is the only NIRCam observing mode to offer this option; other modes always use four outputs to read the full detector (and a single output for subarrays). When four outputs are chosen, the detector output is split into four columns, or "stripes," each 512 pixels wide that are read simultaneously.


Table 2.  Detector read out times for available subarrays and numbers of outputs

Rows
(pixels) 
Columns
(pixels) 
Readout time (s)
one output
Readout time (s)
four outputs
2048204842.2094010.73677
25620485.294201.34669
12820482.657400.67597
6420481.339000.34061



Dispersion

Each spectrum is dispersed by 1 nm/pix. The undeviated wavelength is 3.95 µm.

As shown in Figure 4, for wavelengths below 4 µm (F277W, F322W2, F356W), sources are positioned near the right of the detector (x = 1581), and the spectra disperse to the left (towards shorter wavelengths). Above 4 µm (F444W), a different reference position is used (x = 887) because the source disperses to the right (towards longer wavelengths).

Vertically, the sources are positioned at y = 34 on the long wavelength detectors. At short wavelengths, the defocused images land in the vertical center of the corresponding subarrays, which traverse two short wave detectors (with a 4″–5″ gap).

Figure 4. Grism dispersions within a grism detector subarray

Approximate grism dispersions within a grism detector subarray for sources at two pre-defined reference positions, for wavelengths below and above 4 µm. The vertical size of the subarray is to scale, but filters are spaced vertically solely for clarity.


Saturation limits

See also: NIRCam Bright Source Limits

Based on preliminary estimates, A-type main sequence stars as bright as K ~ 4.5 (Vega mag) may be observed with the NIRCam grism without saturating the module A detectors at any wavelength when using the smallest subarray (2048 × 64 pixels), stripe mode (four outputs), and a short integration time of 0.68 s (two reads). Still brighter stars may be observed at the longest wavelengths, as shown in Figure 5.

Figure 5. Grism saturation limits in the 2048 × 64 pixel subarray
Approximate grism saturation limits for an A0V star in the Mod A 2048 × 64 pixel subarray read out in stripe mode (four outputs), assuming a detector reset and two reads (0.68 s integration). Please use the Exposure Time Calculator (ETC) to obtain saturation limits for your proposed observations.
For larger subarrays and/or a single detector output, the minimum integration times increase, and a K ~ 4.5 Vega mag star would saturate the detector. Table 3 shows approximate saturation limits for the various subarrays, again assuming two detector reads between resets. These limits are given for 2.7 µm, the wavelength most prone to saturation. F277W and F322W2 observations will experience such saturation. Longer wavelength observations may observe somewhat brighter stars without saturating. Please consult the Exposure Time Calculator (ETC).


Table 3. Subarray saturation limits

Rows
(pixels) 
Columns
(pixels) 

Approx. saturation limit
(K Vega mag)
1 output

Approx. saturation limit
(K Vega mag)
4 outputs
2048204813.612.1
25620489.07.5
12820487.56.0
6420486.04.5



References

Greene, T. et al. 2017, JATIS, 035001

λ = 2.4 to 5 μm spectroscopy with the James Webb Space Telescope Near-Infrared Camera




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