Target acquisition (TA) for JWST time-series observations may be necessary for observations taken at different epochs that require repeated target placement at sub-pixel accuracy.
Target Acquisition (TA) may be important for TSOs if multiple observations taken at different epochs are to be combined to increase signal to noise. When studying exoplanet atmospheres, faint signals may be detected by combining observations of multiple transits. In these cases, the ability to place the target at the exact same location with sub-pixel accuracy may help with subsequent calibration and analysis of the data.
Astronomers Proposal Tool
The Astronomers Proposal Tool, is a GUI-based software package proposers use to write, validate, and submit proposals for the James Webb Space Telescope. It allows users to both enter information necessary for their proposal, including target acquisition, and gather information about the absolute time usage and data volume usage. APT also includes many useful features for planning observations and placing the FOV on the sky. It is required to submit your proposals through the canonical proposal system. Moreover, full observational constraints (what HST refers to as `Phase 2`) are required for JWST submissions. Use of the ETC (Pandeia) or PandExo (Pandeia for Exoplanets) is strongly recommended to develop JWST proposals.
Exposure Time Calculator
The Exposure Time Calculator Old is a pixel-based ETC paired with a modern graphical user interface. It supports all JWST observing modes: imaging, spectroscopy (slitted, slitless, and IFU), coronagraphy, aperture masking interferometry, and target acquisition calculations for all relevant instrument modes. For time series observations of transiting exoplanets, the PandExo simulation tool provides dedicated ETC functionality built on top of the Pandeia ETC engine. It takes stellar and planetary modeled spectra as input, and produces the optimal groups and integration settings for the required mode and SNR (as well as a range of predicted model outputs). For calculations of the science exposures, we therefore recommend the use of PandExo. See also the Preparing Time-Series Observations with JWST article.
Many TSOs will however require target acquisition to place the target in a particular aperture with sub-pixel accuracy. TA exposure parameters must also be specified by the user in the APT template. The ETC provides dedicated calculation options for TA with settings consistent with the APT templates and instrument requirements. Failed target acquisition can result in aborted observations or a wrong pointing location. We strongly recommend that TA exposures are calculated using this functionality in the ETC.
We refer to dedicated documentation pages on JWST ETC Target Acquisition, JWST ETC NIRCam Target Acquisition, JWST ETC NIRISS Target Acquisition, JWST ETC NIRSpec Target Acquisition and JWST ETC MIRI Target Acquisition for instructions on these calculations.
Several Use Cases have been written to show how to use APT, ETC, and PandExo. They give precise examples, with values, to teach users how to develop the exposure settings for a given Time-Series Observation (TSO) proposal. All of the Step-by-Step guides for ETC (Pandeia) include examples for how to estimate the Target Acquisition saturation limits and sensitivity limits. The ETC Step-by-Step pages include the necessary information to compute the SNR (and saturation limits) for target acquisition with each instrument.
Target Acquisition Algorithm
Each JWST Instrument Operations Team has chosen parameters for target acquisition that work best for their instrument. In particular, each instrument has its own brightness limits, sensitivities, region of interest on the detector, and other parameters for target acquisition. These parameters can be found on the instrument-specific target acquisition pages:
Here is limited summary table for important information. See Instrument-specific links above for full information:
Offset target acquisition
In the event that the science target saturates the target acquisition subarray, here are two options for acquiring on a saturated target:
It is highly recommended that the proposer read the target acquisition pages, and follow the guidelines in the APT template pages, for the instruments that are relevant to the proposal.
Batalha, N. E. et al., 2017