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 ~ 1500. 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, 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 4 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).
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
GrismR plus a wide filter:
* 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.)
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. The 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 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 "stripes," each 512 pixels wide that are read simultaneously.
Table 2. Detector read out times for available subarrays and numbers of outputs
|Readout time (s)|
|Readout time (s)|
Each spectrum is dispersed by 10 nm/pix. The undeviated wavelength is 3.95 µm.
As shown in Figure 3, for wavelengths below 4 µm, sources are positioned near the left of the detector (x = 256), and the dispersed spectra are to the right. Above 4 µm, a different reference position is used (x = 1280).
Vertically, the sources are positioned at y = 508 on the long wavelength detectors. At short wavelengths, the defocused images land at y = 1080, a similar position in arcseconds (given the 2× higher spatial resolution in the short wavelength channel) with a shift due to the dichroic + grism's deflection of the light path. Two short wavelength detectors are read out using subarrays with sizes identical to that selected for the long wavelength grism.
Based on preliminary estimates, A-type main-sequence stars as bright as K ~ 5 (Vega mag) may be observed with the NIRCam grism without saturating the detectors 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 below.
For larger subarrays and/or a single detector output, integration times increase, and a K ~ 5 Vega mag star would saturate the detector. Table 3 shows approximate saturation limits for the various subarrays, again assuming 2 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
Approx. saturation limit
|Approx. saturation limit|
(K Vega mag)