NIRCam Grism Time Series
JWST NIRCam's grism time-series observing mode performs rapid spectroscopic (R ~ 1,600 at 4µm) monitoring of bright, time-variable sources at 2.4–5.0 µm. At shorter wavelengths (0.6-2.3 µm), this mode utilizes the Dispersed Hartmann Sensor (DHS) to achieve rapid spectroscopic monitoring with a resolution of R ~ 300.
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See also: NIRCam Grism Time-Series APT Template and NIRCam Short Wavelength Grism Time Series Observing Strategies
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 at 4µm. It is one of 2 modes available for NIRCam 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, on the long wavelength (LW) channel (2.4–5.0 µm), the module A "grism R" is used to disperse the target's spectrum along (parallel to) detector rows. The LW grism is paired with one of 4 LW wide filters: F277W, F322W2, F356W, and F444W.
Simultaneously, the NIRCam short wavelength (SW) channel (0.6–2.4 µm) can be used to take data in either imaging or, starting from Cycle 4, spectroscopic mode. SW imaging is obtained with a weak lens to defocus the image. This choice offers saturation limits similar to the long wavelength grism for a given integration time (which must be identical for both wavelengths). SW spectroscopy is obtained via the Dispersed Hartmann Sensor (DHS). For more details on SW Grism Time Series, see the NIRCam Short Wavelength Grism Time Series article.
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 unaided eye (~4.5th magnitude when using LW spectroscopy + SW imaging, ~4th magnitude when using LW and SW spectroscopy, due to the smaller size of subarrays adopted in the latter case).
Figure 1. Sample NIRCam LW Grism R + F444W data
Filters
Table 1. Available combinations of filters, weak lenses, and grism
Short wavelength channel (0.6–2.3 µm) | Long wavelength channel (2.4–5.0 µm) | |
|---|---|---|
Imaging | Spectroscopy |
Grism R plus a wide filter:
|
Weak lens offering +8 or +4 waves defocus
| Dispersed Hartmann Sensor, producing 10 spectra per source in the field of view combined with a filter among:
| |
† 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 LW grism and LW filters (1st order)
First order throughputs of the module A and B LW grism and the 4 LW 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-only throughput (shown above) must be multiplied by the throughput of a selected filter. The module B grisms are AR coated on only one side and therefore have throughputs ∼33% lower than the module A grisms. Only module A is available for grism time-series mode.
Figure 3. Throughput response for NIRCam LW grism and LW filters (2nd order)
Second order throughputs of the module A and B grism and the 4 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-only throughput (shown above) must be multiplied by the throughput of a selected filter. The module B grisms are AR coated on only one side and therefore have throughputs ∼33% lower than the module A grisms. Only module A is available for grism time-series mode.
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, when paired with SW imaging, contain 64, 128, or 256 pixel rows. When paired with SW grism spectroscopy, the Multistripe readout mode is utilized (new in Cycle 4, see NIRCam Multistripe Subarrays for Grism Time Series Spectroscopy ). This mode allows subarrays as small as 40 rows (correspondingly increasing the brightness limit).
Each of the subarrays include all 2,048 pixel columns (2,040 sensitive to light + 8 reference pixels). When LW spectroscopy is paired with SW Imaging, the full 2048 × 2048 pixel array may be read out instead if desired.
The detector may be read out either through 4 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 4 outputs to read the full detector (and a single output for subarrays). When 4 outputs are chosen, the detector output is split into 4 columns, or "vertical stripes," each 512 pixels wide that are read simultaneously. Note that when using the SW Grsim Spectroscopy pairing, only 4 detector outputs are available.
Table 2. Detector read out times for available subarrays and numbers of outputs
| LW + SW combination | Rows (pixels) | Columns (pixels) | Readout time (s) 1 output | Readout time (s) 4 outputs |
|---|---|---|---|---|
LW Spectroscopy + SW Imaging | 2048 | 2048 | 42.23000 | 10.73677 |
| 256 | 2048 | 5.31480 | 1.34669 | |
| 128 | 2048 | 2.67800 | 0.67597 | |
| 64 | 2048 | 1.35960 | 0.34061 | |
LW Spectroscopy + SW Spectroscopy†† | 256 | 2048 | - | 1.34669 |
| 160 | 2048 | - | 0.84365 | |
| 80 | 2048 | - | 0.42445 | |
| 40 | 2048 | - | 0.21485 |
†† The subarrays used for this combination utilize the MULTISTRIPE readout mode, see NIRCam Multistripe Subarrays for Grism Time Series Spectroscopy for details
Dispersion in the LW channel
For information on the dispersion for the SW channel, see NIRCam Short Wavelength Grism Time Series
Each LW 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 ~ 952) because the source disperses to the right (towards longer wavelengths). These horizontal positions are the same when pairing LW grsim spectroscopy with either one of the SW imaging and SW grism spectroscopy options.
Vertically, the sources are positioned at y ~ 35 on the LW detectors when the LW grism is paired with SW imaging, and at y ~ 1024 when paired with SW spectroscopy. For SW imaging, the defocused images land in the vertical center of the corresponding subarrays, which traverse two short wave detectors (with a 4″–5″ gap). For the corresponding placement of the spectra for SW spectroscopy, see NIRCam Short Wavelength Grism Time Series.
Figure 4. Dispersions within a LW grism detector subarray
LW channel footprint of the spectra in grism time-series mode. Red boxes indicate the 256 pixels subarray for this mode (to scale). The footprints for the different filters are spaced vertically for clarity (the actual subarray on the detector is the same for the 4 filters). The reference position is marked with an X symbol and a vertical dotted line. The reference position corresponds to the location of the undeflected wavelength of 3.95 μm, or alternatively would correspond to the direct image position if the grism was not in the beam. Filters F277W, F322W2 and F356W share the same reference position, while the F444W reference position is shifted to smaller x values in the science reference frame. The thick portion of the footprint corresponds to wavelengths for which the total throughput is larger than 50% of the maximum throughput for that setting. The thinner part of the footprint corresponds to wavelengths for which the total throughput is larger than 1% of the maximum throughput for that setting.
Note
The reddest part of the F444W footprint (above 5.03 μm, where the throughput drops below 20% of the maximum) extends beyond the detector. The loss of data in this low-throughput region is balanced by 2 factors. First, the high throughput portion of the spectrum has more overlap with the corresponding portions for the other 3 filters. This allows a better characterization of systematics at the subpixel level in the region in common.
Saturation limits in the LW channel
See also: NIRCam Bright Source Limits
For information on the saturation for the SW channel, see NIRCam Short Wavelength Grism Time Series
Based on in-flight characterization of NIRCam performance, when using LW grism spectrscopy, paired with SW imaging, A-type main sequence stars as bright as K ~ 4.5 (Vega mag) may be observed with the NIRCam grism on the LW channel without saturating the module A detectors at any wavelength when using the smallest subarray (2048 × 64 pixels), stripe mode (4 outputs), and a short integration time of 0.68 s (2 reads). Still brighter stars may be observed at the longest wavelengths, as shown in Figure 5. Thanks to the even smaller subarray (2048 × 40 pixels) available when LW grism spectroscopy is paired with SW grism spectroscopy, this limit is extended to even brighter magnitudes of K ~ 4 (Vega mag).
Figure 5. Grism saturation limits in the 2048 × 64 pixel subarray
Approximate grism saturation limits in K-band Vega magnitudes for 3 stellar types (A0V, G2V, M2V) in the module A 2048 × 64 pixel subarray with stripe mode readout (4 outputs), assuming a detector reset and 2 reads (0.68 s integration). Using the readout times of Table 2, these limits can be scaled by -2.5*Log10(0.34061/0.21485) = -0.5 magnitudes to obtains saturation for the 2048 × 40 pixel subarray used in LW + SW grism spectroscopy. Saturation is defined here as 70% of the pixel well capacity. Results are from the Exposure Time Calculator (ETC) v4.0. Please use the ETC to obtain saturation limits for your proposed observations.
Table 3. Subarray saturation limits
| Rows (pixels) | Columns (pixels) | Approx. saturation limit | Approx. saturation limit (K Vega mag) 4 outputs |
|---|---|---|---|
| 2048 | 2048 | 9.7 | 8.3 |
| 256 | 2048 | 7.5 | 6.0 |
| 128 | 2048 | 6.8 | 5.3 |
| 64 | 2048 | 6.0 | 4.5 |
Tilt events
The NIRCam time-series imaging and grism modes are the only modes currently supported that allow a simultaneous high-cadence wavefront sensing along with the time series. An analysis of commissioning observations, covering a transit of HAT-P-14b (PID 1442) have revealed a jump in the target flux around the mid-transit in both the SW (weak lens) and LW (grism, band-integrated) light curves (Figure 6 and 7). The jumps in flux correspond to changes in the mirror surface, known as “tilt events”, which have been hypothesized to be structural microdynamics in the telescope that may occur when stresses in the backplane structure behind the mirrors are suddenly relaxed. As these stresses are released, the frequency of tilt events is expected to decrease in time.
The short wavelength Weak Lens +8 PSF is part of the wavefront sensing tools aboard JWST and is thus highly sensitive to changes in the mirror surface. Identification of tilt events can be done by choosing an arbitrary reference image from the SW time series of rate images and dividing the rate from all other integrations by it. The tilt events show up as clear wave patterns in these differential images due to changes in the mirror surface (Figure 6). For the HAT-P-14b data, the mirror changes appear on a timescale faster than the 27 second cadence of integrations. Photometry analysis on the pairwise-subtracted group images shows that each of the two tilt events happen on a timescale as fast or faster than the 1.351 second time between BRIGHT2 detector groups, which contain 2 frames each. Tilt events can also be sensed from the LW grism time series by measuring the FWHM as a function of time (Figure 7). Observers making use of the NIRCam time-series and grism time-series modes are strongly encouraged to examine the data products described above for their observations and routinely identify tilt events, if any.
Figure 6. SW Time-series imaging and LW grism transit light curves with tilt events
The light curves exhibited jumps most noticeably in the long wavelength spectroscopy, shown here as a broadband time series. One way to detect these jumps is by measuring the FWHM of the long wavelength spectroscopy.
Figure 7. Tilt events in the SW time series imaging
Click on the figure for a larger view.
Weak Lens ratio images before and after two tilt events identified in the commissioning transit observation of HAT-P-14b.
Detector settling time
Transit observations of exoplanet HAT-P-14b, obtained during commissioning (PID 1442) suggest that the settling time for the short wavelength channel is in the range from 5 to 15 minutes and even shorter for the long wavelength channel. Observers are advised to continue assume the more conservative pre-flight settling time of 30 minutes in the preparation of their transit observations until a further notice.
SW and LW timing synchronization
Due to the on-board subarray configuration sequence being executed serially for the different NIRCam detectors, the timing of the SW and LW channels can be offset from each other from zero seconds up to one frame time, which can be smaller than the group time.
References
Beatty et al. in prep (reference will be updated when ready)
Greene, T. et al. 2017, JATIS, 035001
Rigby, J. et al 2023, PASP, 135, 048001
The Science Performance of JWST as Characterized in Commissioning
Schlawin et al. 2017, PASP, 129, 971
Two NIRCam Channels are Better than One: How JWST Can Do More Science with NIRCam’s Short-wavelength Dispersed Hartmann Sensor
Schlawin et al. 2023, PASP, 135, 1043
JWST NIRCam Defocused Imaging: Photometric Stability Performance and How It Can Sense Mirror Tilt