PandExo, The Exoplanet ETC 

PandExo, the Exoplanet Exposure Time Calculator (ETC), used for planning JWST exoplanet observations, provides a framework for performing calculations that help users optimize their observations of transiting exoplanet.

On this page

See also: JWST Exposure Time Calculator Overview

When preparing JWST proposals for transiting exoplanet science, a crucial part of the process is determining the exposure parameters and optimal instruments that are needed to successfully perform detections of the sought-after atmospheric features. Although the JWST Exposure Time Calculator provides a framework to estimate signal-to-noise ratios (SNRs) for the different instrument configurations offered by JWST, the transformation of these to actual observables of interest for transiting exoplanet science, such as transit or eclipse depths, is non-trivial. These transformations depend on, among other things, the actual orbital parameters of the exoplanet under study and the total number of integrations in a given exposure.

PandExo, the Exoplanet ETC, (Batalha 2017) provides a simple interface to perform these calculations in order to aid exoplanet observers with their proposal planning. The tool makes use of the latest version of Pandeia to perform the SNR calculations, which, in turn, are used to estimate the expected precision on observables of interest.

The web-based version of PandExo, which is part of the STScI's Exoplanet Characterization ToolKit (ExoCTK) suite of web-based services. PandExo is also available as a python library, which is recommended for performing a large (>10) number of sequential runs.



Using the web interface

The web-based version of PandExo for JWST can be accessed from ExoCTK's website upper menu. To start a new calculation, click on the green New Calculation button under the James Webb Space Telescope icon, as shown in Figure 1.

Figure 1. PandExo landing page on ExoCTK

When a new calculation is started, the user is asked to define 2 sets of properties for the PandExo run. The first set, outlined below, relates to the specific exoplanetary system under study.

Under Name, provide a name that will act as an identifier for the PandExo run. Next, define the properties of the exoplanet (such as the planetary radius and the transit duration) and its host star (such as the stellar effective temperature, radius, metallicity, log-gravity and apparent magnitude) in their respective fields. Alternatively, you can directly query these properties from exo.MAST by entering the name of the exoplanet under Target Name and clicking the green Resolve Target button; this will then fill the properties of the exoplanet under study if they are available — the user can then decide to use the retrieved properties or change subsets of them as desired.

The user then needs to define a stellar spectrum model in the Stellar Model section. Here, they can either use the closest Phoenix model (Husser et al. 2013) to the previously-entered stellar properties by selecting Get Default From Phoenix, or upload a stellar model spectrum by selecting User Defined Stellar Spectrum.

Similarly, for Planet Model, the user can select a planetary model:

  • Constant Value: The transit or eclipse depth (for the latter, the estimated temperature of the planet's dayside has to be provided) will be assumed to be constant over the entire wavelength range.
  • Upload Spectrum: The user can upload their own transit or eclipse model spectrum.
  • Select From Grid: The user can select one of the Fortney et al. (2010) models as inputs to PandExo (for details, see the Forward Modeling Tool article).

Finally, in the Baseline section, the user must define the length of out-of-transit observing time to be spent on the target and the number of transits/eclipses to be observed under Number of Transits. An example of the webpage and all these inputs are shown in Figure 2.

Figure 2. JWST PandExo Calculation online interface 

The second set of properties are related to the specific instrument used in these simulations. An example is shown in Figure 3.

In the Instrument section, the user defines the instrument, modes, and dispersers they will be using for the PandExo run. In the Number of Groups per Integration field, enter either the number of groups per integration or simply use the default value, optimize, so the tool will optimize this number for maximum observational efficiency. Under Saturation Limit, define either a percentage or total number of electrons that will be used as saturation by PandExo. Note the percent full well is, in reality, the signal limit defined in the Time Series Observations (TSO) Saturation article. Finally, the user can also define a maximum achievable precision level under Noise floor; this can be defined as constant across the entire wavelength range by selecting Constant Minimum Noise or the user can upload their own model under User Defined Noise Model.

Figure 3. Continuation of the JWST PandExo calculation online interface 

Once all the properties have been defined, click the green Submit button to start the PandExo run. This will redirect users to their dashboard (shown in Figure 4), where they will be able to track the progress of their runs. Once the PandExo run has completed the calculations, the Status of the run will change from Running to Finished, and the "eye" icon under Tools will be a clickable button. By clicking it, the user can gain access to the results of the PandExo run.
Figure 4. Example of a PandExo Dashboard where users can track their calculation progress.



Exploring PandExo results

Once the run has finished, and after clicking the "eye" button on the right in the dashboard, the user will be redirected to the pages showing the results of the PandExo runs. The first set of results shown by the tool are a set of 1-D plots, which will look similar to the one in Figure 5 where we show example results of a PandExo run for transmission spectroscopy using the exoplanet HAT-P-1b with the NIRISS SOSS instrument.

Figure 5.  Example of PandExo 1-D results


The top plot shows the transit depth as a function of wavelength, which includes both the model and the simulated transit depths by the tool. The slider on top of this figure can be used to either combine points in wavelength space (i.e., bin in wavelength; # pixels to bin) or in terms of additional transits (Num Trans). The bottom panel shows several interesting diagnostics such as the SNR per integration (SNR per Int) and the transit depth precision as a function of wavelength (Precision).

It is important to note that the SNR per integration shown by PandExo might be slightly higher than some SNR per integration values estimated by the JWST Exposure Time Calculator. The reason for this is that this latter tool for non-TSO modes includes an additional noise term due to flat field errors, which will most likely not be important for most transiting exoplanet observations due to the relative nature of these measurements. (See the JWST ETC Residual Flat Field Errors article for more details.) 

PandExo also presents some 2-D images that are useful for inspecting the overall distribution of flux, SNR and saturation across the detector.  Figure 6 shows an example for NIRISS SOSS.

Figure 6. Example of PandExo 2-D results

The top panel simply shows the flux levels on the detector; the bottom panel, on the other hand, presents the SNR across the detector (when clicking the SNR tab) as well as saturated pixels (by clicking on the Saturation tab), i.e., pixels that fall above the signal limit imposed by the user in the previous section.

The tool then presents the inputs used for the calculation under the "Table of Original Inputs" (not shown), and, finally, two tables summarizing the results: one with timing information ("Timing Info"), which includes valuable information to be entered in the Astronomer's Proposal Tool (APT), and one with a set of warnings ("Warnings"), which report various details to pay attention to on the current PandExo run, shown as an example in Figure 7.

Figure 7. Summary of inputs and warnings

The "Timing Info" table includes all details about the current exposure and, if observations were requested to be optimized in terms of the number of groups, presents the number of groups (in APT: Num Groups per Integration) and integrations (in APT: Num Integrations per Occultation) to be inserted in APT that match the predicted PandExo results. The table under "Warnings," on the other hand, present a set of checks PandExo performs in order to inform the user if any limits in terms of number of groups, non-linearity regimes, saturation regimes and/or closeness to the signal limits are either being crossed or close to being crossed. Depending on the messages, the user might consider rerunning the calculation with a different instrument, mode and/or target. For optimal decision making on these later warnings, users should follow the recommendations provided by the different JWST instrument teams. Information relevant to saturation and number of groups to choose in the context of TSOs can be found on the TSO Saturation article.

Finally, the user can download the results of the PandExo run to their local machine by clicking on the Download Data button at the bottom of the results page. This is a "pickle" file that can be opened using Python's pickle library. The file contains a Python dictionary with all the details about the results of the run. For a full description of each of the keys of that dictionary, see the relevant PandExo ReadTheDocs article.



References

Batalha, N. E., et al. 2017, PASP, 129, 976, 064501.
PandExo: A Community Tool for Transiting Exoplanet Science with JWST & HST

Husser, T.-O., et al. 2013, A&A, 553, 6, 9.
A new extensive library of PHOENIX stellar atmospheres and synthetic spectra

Kurucz, R. L., 1979, ApJS, 40, 1-340.
Model atmospheres for G, F, A, B, and O stars. http://kurucz.harvard.edu/grids/




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