Preparing Time-Series Observations with JWST
Several tools are available for the preparation of time-series observations with JWST.
Preparing time-series observations (TSOs) follows many of the same principles as preparing general observations. A set of tools and applications is available to help with understanding the various mode capabilities, and observational constraints.
Exposure time calculations
The JWST ETC contains the latest information on the sensitivity and saturation limits for the JWST instruments and their various modes. It also allows the user to perform exposure time calculations for target acquisition (TA), where appropriate. Users are strongly recommended to use the ETC when preparing their proposals. The ETC is available as a web application, users can log in using their MyST account so that source definitions and calculation workbooks can be stored. The ETC is also a valuable tool for collaboration, as workbooks can be shared with other users. For detailed instructions on how to use the ETC, we refer to the dedicated section in these Documentation pages.
For time-series observations, the ETC (Pandeia) is available for estimating the expected signal to noise (SNR) for a single integration (ngroups > 2), and extrapolate that for the SNR across the time-series using a Poisson limit: SNR per exposure = SNR per integration x sqrt(number of integrations). The ETC includes an error term for residual flat-field errors which affects long exposures, and significantly underestimates the SNR for exoplanet transit spectroscopy where we take relative measurements. For exposures longer than ~10,000 s, ETC calculations have a "noise floor" above which an increase in exposure time no longer results in an increase in SNR that scales with the square root of the exposure time. Since we are making relative measurements on the same pixels for exoplanet transit spectroscopy, our precision is not affected by the "noise floor" imposed by the residual flat field errors. Using the JWST ETC to compute a full exposure with 18,067 integrations is not yet possible for time-series observations (TSOs). Therefore, as detailed in the Step-by-Step ETC Guide, we must use the Poisson limit to compute the SNR for the full exposure, from the SNR of a single integration.
For calculations specifically tailored for time-series observations of transiting exoplanets, PandExo is a publicly available simulation tool built around the Pandeia ETC engine. We recommend its use for designing such observations.
To address the specific needs of the exoplanet community, PandExo is a community simulation tool that incorporates the Pandeia ETC engine and can be used to perform exoplanet-specific calculations (Batalha et al. 2017). It can be used as both an online tool and a python package for generating instrument simulations of JWST’s NIRSpec, NIRCam, NIRISS and MIRI and HST WFC3. It uses throughput calculations from STScI’s Exposure Time Calculator, Pandeia: Pandeia + Exoplanets = PandExo.
PandExo takes as input the stellar and planetary modeled spectra, as well as other useful and necessary parameters. It then estimates the predicted SNR for the observation across the planetary spectrum, which is equivalent to the uncertainty expected on the transit or eclipse depth at every wavelength along the spectrum. PandExo uses Bokeh, which provides interactive and downloadable plots that provide all of the information on screen to be used by the user on their local machine. Specifically, PandExo provides the predicted planetary spectrum, the predicted stellar spectrum, the predicted error spectrum, and several other factors. For APT, PandExo also provides a table at the bottom of the results page that lists the number of groups and number of integrations required to sustain the SNR in the plots above.
Main article: JWST General Target Visibility Tool Help
Two standalone target visibility tools are available to help proposers evaluate the visibility of targets, and their allowed position angles on the sky, for potential observations by JWST instruments. These python-based tools are included in the STScI Astroconda python disctribution. Using these tools prior to developing a proposal in APT could save time and provide insight into observability. The first is a general tool, the second is tailored to coronagraphic observations. For TSOs, the former is the appropriate tool to use.
For given target coordinates, the general target visibility tool (GTVT) reports (1) the ecliptic latitude and (2) the target visibility windows and allowed position angles versus time. Both tools have been vetted for accuracy against the Astronomers Proposal Tool (APT) calculations, but are primarily intended to provide an overview capability that complements APT. Further command line options give the possibility to save the output, visualize the target's visibility over the requested time period. The results can be shows for all instrument focal planes, or for one particular instrument.
For further details on the use of the General Target Visibility Tool, please visit the dedicated section in these pages.
Astronomer's Proposal Tool
Main article: JWST Astronomers Proposal Tool Overview
The Astronomer's Proposal Tool, APT, is a GUI-based software package used to write, validate, and submit proposals for the James Webb Space Telescope (and the Hubble Space Telescope).
Within APT, observational strategies are specified for the different JWST instruments and their modes in the form of templates. The templates effectively capture the instrument capabilities; as a rule, if a particular observing strategy is not available in APT it is not supported by the observatory. For further information on how to use APT, please refer to the dedicated section in these documentation pages, or jump directly to the sections on the following TSO modes:
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 TOS 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.
Batalha, N. E. et al., 2017
PandExo: A Community Tool for Transiting Exoplanet Science with JWST & HST