NIRCam Time-Series Observation Recommended Strategies

This article discusses certain best practices for developing time-series observations (TSOs) with the Near Infrared Camera (NIRCam) onboard the James Webb Space Telescope (JWST). Most of these practices will be useful for both Time Series Imaging and Grism Time Series Observations; exceptions are noted.

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JWST NIRCam offers two observing modes for precision time series observations (TSOs): Time-Series Imaging and Grism Time Series. The unique features of all TSOs are long observing times, lack of dithering, and flexibility in avoiding saturation. Observations using these modes can be specified using the Time Series Imaging and Grism Time Series Templates. In the former, Time Series Imaging Template, both the SW and LW channels are configured for photometry. Each channel can be configured independently by choosing a NIRCam Filter.

In the Grism Time Series Template, the SW channel is configured for photometry and the LW channel is configured for spectroscopy. The SW and LW detectors must be configured using the same readout pattern and exposure time parameters (e.g., NGROUPS, NINTS). 

These "best practices" for NIRCam TSO observations specifically will address the target acquisition requirements and limitations; the nominal use of the dichroic to plan for TSO observations; and choices of filters to isolate which molecules a given program is targeting.  Moreover, data volume is a primary concern for NIRCam observations; while schedulability is more general for all TSO observations. A “best practice” is a procedure accepted or prescribed as being correct or most effective in addressing key issues in observation planning. In the case of TSOs, such issues include (1) balancing long-wavelength (LW) and short-wavelength (SW) exposure times and (2) avoiding saturation. 

Observations using Time-Series Imaging or Grism Time Series are specified in templates of the Astronomers Proposal Tool (APT). Exposure times are calculated by the Exposure Time Calculator (ETC). In the Time-Series Imaging APT template, both the SW and LW channels are configured for photometry. In the Grism Time Series APT template, SW channel is configured for imaging and the LW channel is configured for spectroscopy. For all TSO observations, the SW and LW detectors must be operated by the same readout pattern, subarray selection (e.g., SUBSTRIP128, SUBSTRIP256), and exposure time parameters (e.g., NGROUPS, NINTS).



Balancing LW and SW exposure times

Filters with similar sensitivities and saturation limits should be used for both LW and SW observations. In Time-Series Imaging, photometry could be obtained simultaneously in two wide filters (e.g., F150W and F356W) or two narrow filters (e.g., F212N and F323N). In Grism Time Series, the grism could be used in conjunction with one of 4 wide filters in the LW channel (2.4–5.0 µm): F277W, F322W2, F356W, and F444W. Simultaneous SW (1.7–2.3 µm) imaging is obtained with a weak lens to defocus the image combined with a narrow or medium filter. This combined use of the WLP4 or WLP8 pupils offers saturation limits similar to the long wavelength grism for a given integration time, which must be identical for both channels.



Ephemeris and proper motion

 All Time-Series Imaging and Grism Time Series observations require Target Acquisition (TA) to ensure that JWST places the science target on the exact same physical pixel for every observation to maximize scientific reproducibility.  During Target Acquisition, the observatory places the science target in the center of the central pixel in the SUBTA aperture—(i.e., the center of the 16th by 16th pixel for the SUB32 (32 × 32) subarray), known as the "sweet spot". JWST is expected to have a maximum of 7mas jitter, which may introduced temporal noise or pointing errors. JWST should place the science target at the same sub-pixel position to maximize the reproducibility, such that this 7mas jitter probes the same distribution of pointings. To ensure that the target is indeed at the same location for every observation, the observer must specify the most recent ephemeris (i.e., phase constraints for the exoplanetary orbit), especially the proper motion to reasonable precision—(i.e., Gaia DR2 observations).  The target acquisition aperture for NIRCam (SUBTA) has a 2" × 2" field of view. If the target is uncertain to 3" (2MASS precision) and the proper motion is uncertain to 1.0" (at 1-sigma), then it is highly likely that Absolute Astrometric Precision (0.5") will place the target outside the Target Acquisition subarray.  It is highly recommended that observers find their target sky location and proper motion from the Gaia (DR2) catalog or other catalogs with equivalent precision.

 

Avoiding saturation

Target acquisition requires a minimum, integrated, signal-to-noise (SNR) of 30. Observers are encouraged to use the Exposure Time Calculator (ETC) to estimate the Target Acquisition SNR using the measured (Vega) K mag for their target. Bright targets (with K < 7) may saturate the TA subarray – the ETC will notify users of saturation. In this case, observers should use "Offset Target Acquisition", where the telescope acquires on a nearby background source (within 30") that will not saturate and performs a blind slew from the offset target to the science target.  Knowing the precise relative distance between the acquisition and science target is imperative to placing the science on the exact "sweet spot" of the subarray. Therefore, Offset TA sources are highly recommended to have ephemeris measurements from the Gaia (DR2) catalog.

Weak lenses and subarrays may be used to improve saturation limits.  Subarrays should be >160 pixels on a side, to encompass most of the defocused image and attain a proper background subtraction. Subarrays smaller than the width of the PSF will provide a limited number of background pixels for high precision background estimation.



Grism time series observations

Spectroscopic filter choices

The LW grism can be coupled with several filters; the 2 most functional filters for time-series spectroscopy are the F322W2 (2.4–4.0 μm) and F444W (3.9–5.0 μm) filters. The F277W and F356W are also available for time-series observations, but their wavelength coverage is fully enveloped by the F322W2. The utility of the F277W and F356W filters is to block the order 2 spectrum from nearby targets from overlapping the science frame. 

Spectroscopic overlap concerns: To avoid spectroscopic overlap from the background sources, the Exoplanet Characterization Toolkit has an Overlap Tool (see link below) which takes into account both the 1st and 2nd order overlaps. Only the F322W2 has a wide enough spectral coverage such that the second order of background sources can induce contamination on the primary science (1st order) spectrum. If the user cannot find a reasonable position angle (i.e., observation window and rotation) where overlap is unavoidable, then the F277W and F356W filters can be combined (over 2 visits) to encompass the majority of the F322W2 coverage.

Number of visits requirements: Because the NIRCam grisms have a dispersion of 1nm / pixel, the F322W2 filter spans almost the entirety of the LW detector. Therefore, the use of filters is required by both physical and instrumental constraints. If a program wants to probe the entire wavelength coverage of NIRCam's LW grism (2–5 μm), then it must observe the same target over at least two visits—one for each of the nominal filters: F332W2 and F444W. If spectroscopic overlap cannot be mitigated by proper planning of the position angle, then the program may require observing the same target over three visits – one for each filter: F277W, F356W, and F444W.

Saturation (brightness) limits: Saturation for the LW grism begins to set in near a K ~ 5.4 (Johnson; Vega). This improves slightly at longer wavelengths, when using the F444W filter. Moreover, saturation is only for the brightest pixels (near 3.7 μm). Observations can still be conducted with partial saturation of the spectra. Users are highly recommended to use the JWST ETC to determine the extend of saturation for each pixel. STScI ExoCTK package also includes a planetary spectroscopic SNR calculator (PandExo; see link below) that inlcudes saturation constrains. PandExo is a wrapper for Pandeia (the JWST ETC) that simulates the planetary atmospheric signals with all known JWST noise and throughput considerations.



Time-series imaging observations

Simultaneous SW and LW imaging is possible. There are many filters available for use with both NIRCam channels. The filter choice will affect the saturation ("Brightness") limits. HST and Spitzer observers chose a number of groups (NGROUPS) that would achieve half-well depth for each frame; this balanced temporal and spectral resolution; as well as avoid temporal variations near the non-linear regime. Users should examine the saturation limits per filter to assist in choosing both the filters and the values for NGROUPS & NINTEGRATIONS.

Simultaneous SW imaging and LW grism observations are expected to be beneficial for (1) measuring the relative positional offset of the source during the visit (i.e. the "jitter"), (2) providing a short wavelength constraint to the spectrophotometric measurements, and (3) constraining stellar variability for common mode variations with the LW grism time-series.

So that the relative saturation limits between the SW photometry and LW grism are similar, SW photometry observations are suggested to use either the Weak Lens +4 (WLP4) or Weak Lens +8 (WLP8) dispersion pupils. In particular, the brightness limits for WLP8 are best matched to those of the LW grism depending on the intrinsic source color.

Moreover, the pupil geometry of the WLP8 inludes a "central core" that allows more direct measurement of changes in the relative position of the science target on the frame -- using a 2D Gaussian fitting algorithm, which was chosen for the CalTSO3 pipeline. The WLP4 does not provide a Gaussian-like "core" and, therefore, measuring the relative PSF position over time must nominally be performed using non-gaussian fitting of a highly asymmetric PSF – similar to centering on a canonical defocused ground based telescope.



Data volume

For LW grism spectroscopy, NIRCam simultaneously stores three detectors for TSO observations (LW + 2 SW detectors).  This can generate substantial amounts of data. It is suggested to only read out sub-arrays to minimize the data volume. Reading out the FULL array with short integrations has a large possibility to overload the onboard solid state recorder (SSR)JWST has a finite SSR data volume that is downloaded every 12 hours. TSO observations that generate data volumes close to this level in less than 12 hours should be accompanied with an appropriate justification. If approved, STScI will work with the proposers to optimize the implementation.

The 4–amp mode (reading out with 4 amplifiers at once) in both TSO templates can also overwhelm the SSR data storage. Given a fixed time for the observation (i.e., 1 transit window ~ 3 transit durations), 4–amp mode will read ~4 times faster than 1–amp mode—and, therefore, generate ~4 times as many frames. This will use ~4 times as much space on the SSR as a 1–amp mode observation. In APT, this is controlled by the "number of outputs" drop down menu in the observations folder. It is highly suggested for proposals to set "number of outputs" to 1, unless the science target will saturate within 2 groups when "number of outputs" is set to 1.  In that case, the template must set "number of outputs" to 4 to avoid saturation in 2 groups. The ExoCTK package provides a proposal planing tool to optimize the number of groups (NGROUPS) to avoid saturation and maximize temporal resolution (see link below).



Schedulability

NIRCam detectors (H2RG) are a similar construction to Hubble's WFC3-IR (H1RG) and ground tests have shown similar time correlated, systematic signals—referred to as the "ramp effect". This ramp effect has a decay time scale on the order of ~30 minutes. It is therefore recomended to begin observations of the science target 1 hour before the nominal start time required for the transit/eclipse phase coverage. This time period is referred to as the "detector settling time". For example, if the science target has a 2 hour transit duration, it is suggested to start the observation window 3 hours before the beginning of the transit (1 transit duration + 1 hour), and continue observing to 2 hours (1 transit duration) after the end of the transit. This requires JWST to observe a nominal transit window, with 2 hour transit duration, for a total of 7 hours.

If the observation lasts longer the 10,000 s, then it is likely to occur concurrently with the high gain antenna (HGA) reacquisition. The HGA motion will likely cause a noticable increase in the temporal variations (i.e., noise) related to the increased jitter in the target source during this time. The HGA reacquires every 10,000s; therefore it is impossible to predict at what exoplanetary phase the HGA motion will occur; it may also occur multiple times during a single visit – every 10,000 s.

When providing the phase constraints to APT during the proposal submission process, confirm that the beginning and ending phase range correspond to the exoplanetary phase that JWST should START observations—not for the beginning and ending of TRANSIT phase window. This nominally has the form "0.951 to 0.954"; as oppose to "0.951 to 1.05", which would include the entire transit. If the latter phase constraints are provided, then the proposal is allowing JWST to START observing the science target at any time during the transit, which would significantly limit the scientific viability of the observation.

After the observations have been scheduled, it may be possible to know the range of planetary phases that an HGA motion may occur. It could be useful to check that the HGA will not introduce excess jitter during INGRESS (beginning of transit) or EGRESS (end of transit). This may still be unavoidable.




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