Step-by-Step ETC Guide for NIRCam Time-Series Imaging of HAT-P-18b
A walk-through of the JWST ETC for the NIRCam Time-Series Imaging Example Science Program is provided, demonstrating how to select exposure parameters for this observing program.
The JWST Exposure Time Calculator performs signal-to-noise (SNR) calculations for the JWST observing modes. Sources of interest are defined by the user and assigned to scenes which are used by the ETC to run calculations for the requested observing modes.
For the "NIRCam Time-Series Imaging of HAT-P-18 b" Example Science Program, we focus on selecting the exposure parameters for NIRCam Time-Series Imaging.
We start by defining a scene relevant to this science case. We show how to run ETC calculations to achieve the desired SNR for a single integration and how to convert this to the SNR over the secondary eclipse observation and assess how it will be detected. An accompanying ETC workbook on which this tutorial is based can be downloaded as a sample workbook from the ETC user interface.
The optimal exposure specifications (e.g., number of groups and integrations) are the input needed for the Astronomer's Proposal Tool (APT) observation template, which is used to specify an observing program and submit proposals.
The ETC woorkbook associated with this Example Science Program is called "#29: NIRCam Time-Series Imaging of HAT-P-18 b" and can be selected from the Get a Copy of an Example Science Program dropdown on the ETC Workbooks page to get the read only version. The nomenclature and reported SNR values in this article are based on ETC v.1.5. There may subtle differences if using a different version of ETC.
Define Sources and Scenes in the ETC
In the "Scene and Sources" tab, you can edit the sources within your scene. In the "Scene Editor" box, there are tabs for setting the source continuum, shape, and flux normalization. Below are the specification for this case.
Table 1. Input Source Parameters
(host star for the planet HAT-P-18 b)
Phoenix Stellar Models: K2V
|K (Johnson) = 10.23 (Vega mag)||point source|
Select NIRCam target acquisition Calculation
All NIRCam TSOs require target acquisition (TA) to place the target at the appropriate pointing location. It is recommended that the TA achieves a SNR ≥ 30, which enables a centroid accuracy < 0.15 pixel. Increasing the SNR to 100 improves the centroiding accuracy up to ≤ 0.05 pixel.
We selected "Target Acquisition" in the NIRCam pull-down menu to determine the exposure parameters we need to specify in order to achieve the desired SNR. For Calculation #1, we set the following parameters:
- "Backgrounds" tab - we entered the coordinates of GSC 2594-00646 (17:05:23.151, 33:00:44.97) and selected "Low" for "Background configuration," which corresponds to the 10th percentile background.
- "Instrument Setup" tab - we selected "Time Series or Grism Time Series" for Acq Mode. The filter (F335M) is fixed for TA.
- "Detector Setup" tab -
- We choose the RAPID readout pattern, which is optimal for bright targets.
- Subarray (Sub32 Time Series TA) is fixed for TA.
- Number of integrations and number of exposures are fixed to 1 for TA.
- Among the possible choice, the number of "groups per integration" is set to 9.
- "Strategy" tab - the only permissible option for target acquisition is "Aperture centered on source".
With number of groups per integration set to 9, we achieve SNR = 145 for the TA.
Select NIRCam time-series imaging Calculation
In APT, the NIRCam Time-Series imaging template, include both the short-wavelength (SW) and long-wavelength (LW) detector setups. In ETC, however, this is split in two modes: SW Time-Series and LW Time-Series. Hence, we need to setup two different calculations for the SW and LW with the same parameters (i.e., same subarray, readout pattern, number of groups per integration, etc.) to verify that the SNR is well chosen for both channels. From the NIRCam pull-down menu, we select both "SW Time Series" (Calculation #2) and "LW Time series" (Calculations #3).
As we did for the TA calculation, we entered the coordinates of GSC 2594-00646 in the "Background" tab and selecting "Low" for "Background configuration" for both calculations.
Select instrument parameters
We entered the following parameter for the NIRCam Time-Series SW calculation (Calculation #2) and LW calculation (Calculation #3):
- "Instrument Setup" tab -
- SW Time-Series (Calculation #2)
- LW Pairing: LW Imaging Time Series
- SW Pupil: CLEAR
- Filter: F210M
- LW Time-Series (Calculation #3):
- LW Pupil: CLEAR
- Filter: F444W
- SW Time-Series (Calculation #2)
- "Detector Setup" tab, for both SW and LW calculations -
- subarray is set to SUB64P. This is the smallest subarray available for NIRCam Time-Series imaging. For this observation, it is advisable to choose the smallest possible subarray to avoid data volumes exceeding the capacity of the Solid State Recorders (SSRs).
- we choose the RAPID readout pattern for which there is no averaging of reads into groups.
- number of "groups per integration" is set to 10 and the number of "integrations per exposure" is kept at 1.
- "Strategy" tab -
- We selected the "centered on source" option for "Aperture location" from the drop-down menu, so that the SNR is calculated for the source.
- "Aperture radius" is set to 0.180" (0.280") for Calculation #2 (Calculation #3), which represents the ~ 80% encircled energy fraction in the F210M, (F444W) filter (see NIRCam Point-Spread Function and NIRCam Imaging Recommended Strategies for filter-dependent choices for the aperture extraction radius and background annulus radii for point sources).
- We sample the background from an annulus around the source, choosing an inner radius and outer radius that is 2x and 4x the source extraction radius, or 0.36", (0.55") and 0.72", (1.12") for Calculation #2 and #3, respectively.
Examine signal-to-noise ratio
Main article: TSO Saturation
For the Time-Series Imaging Calculation, the ETC predicts that one F444W integration will have SNR ~ 480 (Calculation #3); in the short-wavelength channel, we expect to achieve a SNR in the F210M filter of ~580 in this same integration time. The risk to observations of such a bright star with this spectral time is saturation at the shorter wavelength. The "Groups before saturation" panel in the image view of the ETC results shows that the short-wavelength setting allows NGROUPS = 13 before saturation occurs, so our choice of NGROUPS = 10 will not saturate the detector.
We note that the integration time for a single integration of 10 groups, with the RAPID readout pattern and the SUB64P subarray, is 0.55 seconds.
For these observation we plan to monitor the star's brightness through 3 full lengths of the secondary eclipse duration (3 x T14 = 3 x 2.71 hrs = 8.13 hrs), assuming the length of the secondary eclipse is the same as the total transit time T14. Given the time taken for a single integration, we compute the required number of integrations to be 8.13 * 3600 / 0.55 = 53,200 integrations. In the Poisson limit, the SNR increases with the square root of the number of integrations, i.e. if we co-added the integrations we would achieve a SNR in F444W of 480 x √53,200 ≈ 110,700.
By observing three full T14 durations, we have the same amount of observing time on the star alone as on the star + planet dayside. The secondary eclipse signal is the difference between these two periods, which we estimate will be measured with SNR ≈ 110,700 / √2 ≈ 78,000; giving a photometric precision of 1/78,000 ≈ 13 parts per million (ppm). This is sufficient for detecting the signal change from the eclipse with high accuracy. We note that this calculation does not fully account for systematics. This is sufficient for determining whether the planet day-side temperature is close to its predicted equilibrium value, and this result will constrain the efficiency of day–night circulation on the planet.
To account for detector settling time, we add an extra hour (9.13 hrs), giving a total number of integrations of 59,759, which we round up to 60,000.
With the exposure parameters now determined for this program, we can populate the observation template in APT. See the Step-by-Step APT Guide to complete the proposal preparation for this example science program.