Instructions for designing JWST NIRCam time-series imaging observations using the Astronomer's Proposal Tool (APT) are provided in this article.

See also: NIRCam Time-Series Observations, JWST Time-Series Observations Roadmap, NIRCam Time-Series Observation Recommended Strategies

Time-series imaging is one of the 5 NIRCam observing modes and one of 2 NIRCam time-series observing modes. Each mode has a corresponding template in APT for users to design their observing programs.

No telescope moves (dithering nor mosaics) are allowed during NIRCam time-series observations.

Lists of allowed values for each input parameter are documented and maintained in the NIRCam Time Series Imaging Template Parameters article.

Step-by-step APT instructions are provided below.

Generic parameters

NIRCam Time Series tab

Target Acquisition Parameters

Target ACQ

See also: NIRCam Time-Series Imaging Target Acquisition, JWST ETC NIRCam Target Acquisition

Target acquisition may be performed either on the science target (Acq Target = Same Target as Observation) or any other target nearby (within 55"). Target acquisition occurs with a 32 × 32 pixel subarray on the long wavelength channel of Module B located near the point source subarrays. The available filters for target acquisition are F335M and F405N. The F405N filter is used to perform target acquisition on bright science targets that would saturate if observed with the F335M filter.   

Figure 1. Target acquisition location

Acq Exposure Time

See also: Understanding JWST Exposure Times, NIRCam Readout Patterns

Acq Readout Pattern: The NIRCam detectors are read out continuously using readout patterns. Patterns with longer exposure times typically average more frames to reduce data volume (less of a concern for subarrays).

Acq Groups/Int: This value is the number of groups to include during an integration. Each group results in a saved image, perhaps averaged from multiple frames (reads).

Only one integration per exposure is permitted. Thus the exposure time and integration time are equal. 

Time series parameters

Module

See also: NIRCam Modules

Module B is used for this observing mode.

Subarray

See also: NIRCam Detector Subarrays

Users may opt to either read out 2 detectors completely or more quickly read out 2 detector subarrays, one in each wavelength channel. Smaller subarrays allow for brighter saturation limits in each integration. NIRCam's point source subarrays are available in this observing mode. The subarrays are read out through a single detector output; the full detector is read out through 4 outputs simultaneously.

Number of exposures

See also: NIRCam Filters, Understanding JWST Exposure Times, NIRCam Pupil and Filter Wheels

Multiple exposures may be performed in sequence to increase the total exposure time. Each exposure is executed as defined in the remaining sections below.

Short Wavelength

In this APT template, users select both pupil and filter wheel elements. Light passes through both elements in series in each wavelength channel. Only certain combinations of these elements are allowed in APT.

In addition to broad-, medium-, and narrowband filters, the NIRCam weak lens WLP8 is available in this observing mode in the short wavelength channel. The weak lens defocuses incoming light, mitigating uncertainties (jitter and flat fields) and allowing for observations of brighter objects before saturation in a given integration time.

Long Wavelength

The pupil and filter wheel elements allowed in the long wavelength channel are restricted based on those selected in the short wavelength channel. For example, if the weak lens is selected in the short wavelength channel, then the long wavelength observations must include a narrowband filter as well. These restrictions guide users to select elements with similar saturation limits in both wavelength channels, since exposures are obtained simultaneously in both.

Exposure Time

See also: Understanding JWST Exposure Times

Each exposure is defined as a Readout Pattern, number of groups per integration (Groups/Int), and number of integrations per exposure (Integrations/Exp). The resulting total exposure time is reported. This readout configuration applies to both wavelength channels (short and long).

Users should consult the Exposure Time Calculator (ETC) to achieve a sufficient signal-to-noise ratio for their science without saturating during each integration. Approximate saturation limits may be found at NIRCam Time-Series Imaging.

Each group yields saved data. Each integration accumulates charge for its duration, preceded and followed by detector resets. Shorter integrations may prevent saturation. Saturated sources may be recovered (unsaturated) in earlier groups during the integration.

Each exposure is performed without moving the telescope nor any mechanisms, with one exception. Exposures of more than 10,000 s are permitted in this observing mode, but users are warned that the High Gain Antenna may need to move during a longer exposure. That movement may introduce jitter and affect data quality.

Other tabs

Special Requirements

A variety of observatory level Special Requirements may be chosen under the Special Requirements tab. 

When NIRCam is used in the time-series mode, 2 special requirements are automatically included and required: Time Series Observation and No Parallel. The users are encouraged to determine Phase limits by making use of the ExoCTK's Phase Constraint Calculator.

Comments

The Comments field (under the Comments tab) should be used for observing notes.