NIRCam-Specific Time-Series Observations

JWST NIRCam-specific strategy tips for preparing and carrying out time-series observations (TSO)

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Main articles: NIRCam Grism Time SeriesNIRCam Time Series Imaging
See also: NIRCam Time-Series Observation Recommended Strategies 

NIRCam's time-series observation (TSO) mode implements a slitless spectroscopy observation with special configurations to optimize the temporal photometric and/or spectroscopic signal to noise (SNR) from a single object. 

NIRCam splits the light into two channels using a dichroic.

  • the SW channel can detect light from 0.6 to 2.3 µm and provides dedicated photometric observations for TSO mode. 
  • the LW channel can detect light from 2.4 to 5.0 µm and provides either photometric or spectroscopic observations for TSO mode.



Photometric time-series observations (assumed for SW observations)

Main article: NIRCam Weak LensesNIRISS Detector Subarrays

For imaging observations, both of the NIRCam SW & LW channels can be coupled with the full suite of filters. For the SW channel, the F150W2 is not considered to be spectroscopically stable because of wavefront errors. Time-series photometry can use any subarray apertures. The shortest photometric temporal resolutions can be achieved with the 32 × 32 subarray; but it provides the least area for background estimation. For reference, a PSF observed using the WLP4 and WLP8 pupils has a FWHM ~ 128 pixels. WLP8 provides a central core that can be used to track the position of the PSF over time; however, WLP4 does not. The target is placed in the center of the selected subarray.



Spectroscopic time-series observations

Main article: NIRCam Grisms
See also: NIRCam Detector Subarrays

NIRCam spectroscopic observations use one of two NIR grisms that both span 2.4–5.0 µm for high precision spectro-photometry. LW grism spectroscopy is automatically accompanied by SW photometry. To sustain the same FOV as the LW detectors, 2 SW detectors are read out with the LW detector.

In STRIPE mode, four detector outputs are read out simultaneously, quadrupling the data usage. STRIPE mode provides shorter integration times and is recommended only for bright sources to minimize saturation, such as the extrasolar planet 55 Cnc e. 

For grism observations, NIRCam LW can be coupled with 1 of 4 wide or double-wide filters: F277W, F322W2, F356W, and F444W. The spectroscopic range of the F277W filter (2.4–3.1 µm) is completely covered by the F322W2 filter (2.4–4.0 µm). Observations using the double wide filters permit 2nd order light from sources in the field. For some telescope orientations, 2nd order spectra of nearby, background sources may overlap with 1st order spectra of science targets. Whether 2nd order background spectra will contaminate target spectra can be determined using the ExoCTK Spectroscopic Overlap Tool. The F444W filter (3.9–5.0 µm) is the only NIRCam TSO approved filter for detecting photons at wavelengths longer than 4.0 µm. 

Spectroscopic subarray choices

For spectroscopic observations, the science target is placed in one of five dedicated subarrays that place the first order on the same physical pixels—34 pixels above bottom of the detector. All NIRCam Grism TSO observations require a target acquisition observation to confirm that the spectrum will always fall on the same rows for maximum reproducibility between visits. The five dedicated TSO subarrays are

  • SUBGRISM32: this subarray has 32 × 32 pixels and has a frame time of 0.01496 s (WINDOW) or 0.00374 s (STRIPE).
  • SUBGRISM64: this subarray has 64 × 2048 pixels and has a frame time of 1.339 s (WINDOW) or 0.341 s (STRIPE).
  • SUBGRISM128: this subarray has 128 × 2048 pixels and has a frame time of 2.657 s (WINDOW) or 0.676 s (STRIPE).
  • SUBGRISM256: this subarray has 256 × 2048 pixels and has a frame time of 5.2942 s (WINDOW) or 1.347 s (STRIPE).
  • FULL: this aperture has 2048 × 2048 pixels and has a frame time of 10.568 s (WINDOW) or 2.688 s (STRIPE).

The saturation limit of the 64 × 2048 subarray is approximately Kmag ~ 2-4, depending on the wavelength. This encompasses almost all known exoplanet host stars, including 55 Cnc e.

At an R ~ 1,600, NIRCam disperses 1 µm over ~1000 pixels. Therefore, the F322W2 spectrum fills up almost the entire FOV.  The dispersion – more than the filter –  forces the requirement to use 2 visits if the science observations require full spectroscopic coverage from 2.4 to 5.0 µm.

For the SW channel to achieve similar saturation limits as the grism observations in the LW channel, it is highly recommended to use the WLP8 pupil lens and spread out the light during simultaneous photometric observations. This spreads the light across ~128 pixels. For the 64 × 64 subarray, more than half of the light will fall off the detector when using WLP8; and for the 128 × 128 subarray, there is still considerable aperture loss. Subarrays 256 × 256 or larger are needed to include sufficient background pixels to achieve maximum photometric stability.

Advantages and disadvantages for NIRCam grism timeseries

The main advantages of the NIRCam grism spectroscopy mode:

  • The absence of slits removes pointing-related flux variations
  • NIRCam can observe the science target both photometrically and spectroscopically, providing critical extra information to monitor stellar activity and provide a short wavelength atmospheric data point, which is useful for comparing with aerosol models.
  • NIRCam can spectroscopically capture wavelengths from 2.4 to 5.0 µm over 2 visits.
  • NIRCam has complimentary wavelength coverage to NIRISS, but enables observations of brighter stars.
  • The NIRCam grisms provide a relatively linear trace that minimizes complexities during spectral extraction.
  • NIRCam is the only JWST instrument that can observe exoplanet host stars with K < 4 mag (Vegamag).

The main disadvantages of the NIRCam grism spectroscopy mode:

  • NIRCam cannot currently spectroscopically observe an exoplanet host star at λ < 2.3 µm.
  • To achieve full LW spectroscopic coverage, NIRCam requires 2 visits.
  • It is possible that the 2nd order from nearby targets can overlap the science target spectrum. Spectral overlap can be explored using the ExoCTK Spectroscopic Overlap Tool website to plan for the appropriate position angle that minimizes contamination.



NIRCam TSO APT template

Main articles: NIRCam Grism Time-Series APT Template, NIRCam Time-Series APT Template

NIRCam TSO observations use either the NIRCam Grism Time Series Template (spectroscopy) or NIRCam Time Series Template (imaging) in APT. The grism template uses the FULL (2048 × 2048), 256 × 2048, 128 × 2048, 256 × 2048, and 32 × 32 subarrays. For imaging, users can choose FULL (2048 × 2048), 400 × 400, 160 × 160, 64 × 64, or 32 × 32 subarrays. If the user chooses any TSO mode, the observation will force the 'no dithering' special requirement, as well as waive the 10,000 second exposure time limit. 



NIRCam TSO operations

Main article: NIRCam Time-Series Imaging Target AcquisitionNIRCam Grism Time-Series Target Acquisition

Target acquisition

Given the importance of target placement for TSOs, target acquisition (TA) is mandatory for NIRCam TSO observations. For this mode, TA is performed in a 32 × 32 subarray with readout cadence of 0.015 s; TA requires one integration with at least 3 reads up the ramp (ngroups = 3). TA observations must be taken with the F355M filter, which provides a saturation limit of K ~ 6.92 (7.23 in update). This will saturate for a number of exoplanet host stars. 

To acquire a bright host star that will saturate the SUBTA frame in ngroups = 3, it is suggested to use the offset targeting mode.  This entails targeting on a nearby source and slew to the science target after acquisition of the offset target.  To minimize the uncertainty in positions, it is highly suggested to select nearby stars approximately 10"–20" away from the science target.  The final uncertainty for the position of the science target is a quadrature sum of the uncertainty in acquirng the offset target, the uncertainty in the delta-RA & delta-Dec, the uncertainty in the slew rate, and the intrinsic uncertainty in the JWST pointing precision (<7 mas).  For an offset target that is 20" from the science target, the combination of these uncertainties can generate a combined uncertainty in the position of the science target by up to 20 mas, or more.

For spectroscopic time series, the science target is then placed on a "sweet spot" in the science subarray. Then, the GRISM pupil is placed in the path of light. On the SW channel, the filter of choice and/or WLP8 pupil are place in the path of light after the target has been placed on the associated sweet spot for the LW grism observations.

For photometric time series observations, the science target is then placed at the geometric center of the science subarray for both the LW and SW channels. 

Users are still strongly recommended to perform TA calculations in the JWST ETC to ensure successful and non-saturated TA exposures. The minimum number of groups required for the TA centroiding algorithm is 3; and the recommended minimum SNR for TA is 30. Note that any readout mode other than BRIGHT1 and BRIGHT2 are permitted for TSO TA. For more TA specifics, please visit the NIRCam Time Series Target Acqusition (photometry) and NIRCam Grism Time Series Target Acquisition (spectroscopy) pages.

Exposure setup

In general, NIRCam has a number of read mode, subarray, and exposure settings to select from for its various observing modes. For TSO, the choices are fairly constrained:

  • Subarray is either SUB64, SUB128, or SUB256

To define the NGROUPS, NINTS, NEXP setting, the following rules of thumb should be considered:

  • Use the STScI package ExoCTK Observation Planning Tool (see link below) to determine the optimal NGROUPS and NINTS for TSO observations. ExoCTK includes the PandExo package to provide spectroscopic SNR and phase considerations.
  • For well-calibrated data, we recommend NGROUPS > 3. For very bright targets, NGROUPS = 2 are permitted, but non-linearity and cosmic rays may not be properly calibrated.
  • Special care should be taken to confirm that the 2nd order spectra of nearby targets does not overlap 1st order spectra of the science target, especially if the nearby target is as bright as the science target.



References

ExoCTK Spectroscopic Overlap Tool

ExoCTK Observation Planning Tool




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