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. 

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).

Filters

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

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 ). 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

Dispersion in the LW channel

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.

Saturation limits in the LW channel

See also: NIRCam Bright Source Limits

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).

Table 3. Subarray saturation limits

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

Figure 7. Tilt events in the SW time series imaging

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