Step-by-Step PandExo Guide for NIRCam Grism Time-Series Observations of GJ 436b
The JWST PandExo (Pandeia ETC for Exoplanets) for the NIRCam Grism Time Series Science Use Case is provided, demonstrating how to select exposure parameters for this observing program.
PandExo is a version of the ETC optimised for spectroscopic observations of transiting exoplanets with JWST. It forms part of the ExoCTK package of tools to aid users with transiting exoplanet observations with JWST and HST. While it uses the same calculation engine ('Pandeia') as the ETC, it allows the user to provide more detailed inputs, including known planet parameters, and offers output that is more optimised for these types of observations. We recommend that the regular ETC is used to determine the required parameters for a single integration, but for more detailed investigations of spectroscopic time series observation to use PandExo.
For the "NIRCam Grism Time-Series Observations of GJ 436b", we focus on selecting stellar, planetary, and exposure parameters to detect the exoplanet transit at the desired signal-to-noise ratio (SNR), for each wavelength. We start by defining the stellar, planetary, observational, and instrument modes specific to the proposal observations. The result of these calculations will be both the expected SNR on the host star and exoplanetary atmospheres, as well as necessary parameters to input into the Astronomer's Proposal Tool (APT) observation template, which is used to specify an observing program and submit proposals.
Observing goals of time series observations
The scientific measurement for an exoplanet transit is the "transit depth," which is a temporal measurement. The spectroscopic result is therefore a relative comparison between a contiguous sequence of time-series measurements, (i.e., transit depth over wavelength). It is equivalent to measuring variations in the stellar spectrum over time.
Our goal in this example is to achieve a relative precision of ~50 parts per million (ppm) on the transit depth per "spectral bin" or "channel," after subtracting the primary transit from the out-of-transit data. Atmospheric models predict that this should provide a useful signal-to-noise on the exoplanet atmospheric spectroscopic signal (~100–250 ppm). A "spectral bin" or "channel" is a set of pixels across the spectrum that we will combine ("bin") to maximize the temporal precision per spectroscopic channel.
To maximize the relative precision between the in-transit and out-of-transit flux (over time), we need to observe the science target long enough to observe the transit, plus a window of time before and after the transit. For this source, the transit duration (T14) is 0.76 hours. We will therefore choose a transit window of 3.27 hours (11,772 s), which is three times the transit duration + 1 hour—for "detector settling" (see TSO Noise Sources).
After opening the PandExo website, select "New Calculation" under the JWST menu option. This produces a form for entering observation data. Start by entering a name for your calculation.
Setting star and planet parameters
Under "Star/Planet properties", select the radio button for "User Defined Properties". The "Load Target Properties" feature is under construction. We enter the following in the fields that appear:
- Stellar temperature = 3350 K
- Stellar metallicity = -0.32 ([Fe/H])
- Stellar log g = 4.843 (dex)
- Stellar magnitude = 6.073 in K band.
- Stellar radius = 0.46 Rsun (solar radii; optional entry)
- Planet radius = 0.38 RJ (Jupiter radii; optional entry)
- Transit duration = 0.76 hrs
Setting the stellar model
In the "Stellar Model" panel, we select the option "Get default from Phoenix" to load stellar model spectra. There is also an option to upload a spectrum file.
Setting the planet model
Here we supply the exoplanetary parameters for GJ 436b, using the "Select From Grid" option. We enter the following:
- Temperature (K) = 1,000 K (The equilibrium temperature for GJ436b is 712 K)
- Select Chemistry Type: Equilibrium Chemistry
- Clouds or Scattering: Weak Rayleigh
- Planet Mass = 0.0727 MJ (Jupiter masses)
Setting transit observational parameters
We need to supply the observational parameters for GJ 436b and encompass the entire transit and necessary out of transit time to make the relative measurement.
- Under "Baseline", we can select the length of the total observation, in seconds or hours, or specify the ratio of observing time in and out of the eclipse. In our case, we want to observe a single eclipse event with equal time spent out of eclipse, so we select the "Fraction of time: In/out" option from the drop-down list, and enter 1 in the text box.
- Under "Number of transits" we enter 1. Note that we assume throughout that the duration of the secondary eclipse is the same as that of the primary transit.
Set instrument parameters for the F322W2 filter
To supply the instrumental parameters, we will select the NIRCam Grism Times Series option and sub-array selections as follows:
Instrument: NIRCam Grism Time Series
Mode: "F322W2, 2.7–4 μm" and "SUBGRISM64, 4 outs (tframe = 0.34)"
Number of Groups per Integration: optimize (the default: This scans a range of options and selects the one that sustains half the saturation limit)
Saturation Limit: 80% Full Well. This will assume saturation occurs when a pixel reaches 80% of the full well count. This is the value also assumed by the JWST Exposure Time Calculator.
Noise Floor: 10 ppm. This is most useful with multiple transit/eclipse/rotation observations; it sets the lowest possible uncertainties per wavelength
Set instrument parameters for the F444W filter
To supply the instrumental parameters, we will select the NIRCam Grism Times Series option and sub-array selections as follows (Figure 5)
Instrument : NIRCam Grism Time Series
Mode: "F444W, 4–5 μm" and "SUBGRISM64, 4 outputs (single-frame read time = 0.34 s)"
Number of Groups per Integration optimize (the default: This scans a range of options and selects the one that sustains half the saturation limit)
- Saturation Limit: 80% Full Well. This will assume saturation occurs when a pixel reaches 80% of the full well count. This is the value also assumed by the JWST Exposure Time Calculator.
- Noise Floor to 10 ppm—this is most useful with multiple transit/eclipse/rotation observations; it sets the lowest possible uncertainties per wavelength
To compute all of the necessary—and useful—values for planning and proposing for JWST TSO observations, select the "Submit" button at the end of the page. This operation could take a few minutes. The following page will provide a rotating symbol and the label "Running" if the calculations are ongoing. Moreover, the buttons to the right (a box and an eye) will be grey and unusable.
After the calculation has finished processing on STScI server, the rotating dial will stop and the label "Running" will change to "Finished". Moreover, the buttons to the right are now useful.
Select the EYE symbol (), to view the JWST TSO planning and proposing calculations.
After selecting the EYE symbol, the first image that we can see is the raw, unbinned planetary spectrum as it would be resolved by fitting a transit model (see Kreidberg 2015) to the synthetic JWST NIRCam simulated observations, which include photon noise, background noise, and read noise as is expected from both Pandeia and field testing of the instruments (Figure 6).
The native resolution can provide a useful transmission/emission spectrum, if the atmospheric signal is large enough to overcome the intrinsic, temporal noise, which is dominated by the photon noise and read noise. It is much more common to "bin" the native spectrum into what are called "channels", which are higher SNR sets of pixels that improve the SNR on the temporal signal. In our case, we will bin 20 pixels together to form ~100 "channels" (i.e., 2,048 pixels / 20 (pixels per bin)), which results in a binned resolution of R ~ 80. Sliding the dial to 1.70103 (=log10 (1.6/80)), is equivalent to binning by 20 pixels per channel. The plot itself will update to reveal what this binning should look like from our synthetic observation. This dial allows the user to visually determine ithe theoretical spectrum (here: clear, equilibrium chemistry) with the precise stellar model for the GJ 436 system.
Figure 7 shows a zoom-in of the binned spectrum using the Bokeh tools on the right side of the interactive plot.
1D stellar and error spectra for GJ 436
Scrolling down on this page, we come to the 1-D stellar and error spectra for GJ 436. The first tab on the top figure defaults to "Total Flux" (Figure 8), which shows the integrated electrons per wavelength, integrated to form the 1D stellar spectrum (top) and 2-D spectral image (bottom).
Transit/Eclipse depth uncertainty over wavelengths
The final tab to select on this figure is likely the most important. Select the "Error" tab to see the predicted ppm uncertainty as a function of wavelength (Figure 10).
Determining number of groups and number of integrations
APT requires the user to enter the number of groups (NGroups) and number of integrations (NINT) to define the length of the exposure. We want to observe a balanced number of groups per integration to maximize both temporal resolution and spectral precision. Previous experience has led the community to nominally fill the detector pixel wells to an average of half full. Because an H2RG pixel can hold ~65k electrons, we nominally choose ~30k electrons per pixel per integration. In the context of number of groups for JWST, PandExo derives the number of groups corresponding to the onset of saturation (NGroupssat) (we defined this at 80% well depth), and then selects the number of groups per integration to be NGroupssat/2 (rounding up).
After the "Table of Original Inputs"—which matches the values we entered at the beginning—there is a second table named "Timing Info" (Figure 12).
APT: Num Groups per Integration = 7
APT: Num Integrations per Occultation = 4338
Both of these values will be entered in the APT NIRCam Grism Times Series Observation template as described in the NIRCam Grism Time-Series Observations of GJ 436b.
The final table provided by PandExo is named "Warnings" (Figure 13). If the target is not too bright for the minimum number of groups (NGroupsmin = 2), and the given instrument/detector setup, then all "values" should be listed as "All Good." If the any pixels on the detector are saturated, or experience strong non-linearity, then one of these warnings will list further information.
"Group Number Too Low" warns if the target is too bright for the minimum number of groups.
"Group Number Too High" warns if the target is too faint for the maximum number of groups.
"Non linear?" warns if any pixels are expected to sustain significant non-linearity, which could happen if the target is not too bright to saturate pixels, but bright enough to sustain non-linearity.
"Saturated?" warns if any pixels are expected to be saturated.
"% full well high" warns if the definition of full well provided on the previous page is useful to achieve nominal integration parameters.
"Num groups Reset" is a catch all warning that warns if the "something else went wrong", which might have required PandExo to reset the number of groups to the minimum value of 2.
PandExo provides a "Download Data" button at the bottom of the results page, which allows the user to download a `pickle` file for the user to use Python to plot and interact with all of these figures. The follow code snippet would allow user to create the "Error Spectrum" figure above – with Python 3.6.1 :: Anaconda 4.4.0 (×86_64):
PandExo does not yet compute expectations for target acquisition (TA). Please see the "Target Acquisition" section of the NIRCam Grism Time-Series Observations of GJ 436b for guidance on how to select observing parameters to ensure a successful TA.