APT Step-by-Step Instructions for Gliese 1214b

Step-by-step APT observation definition instructions for the JWST NIRSpec BOTS mode observation of GJ 1214b (Gliese 1214b) are presented and discussed.

Specifying target acquisition and science exposure parameters

  1.  Add target by using the fixed target resolver option. 
  2.  Add appropriate category keyword - stars - and description M star and exoplanet.
  3.  In Observations, click ‘create observation folder'.
  4.  Click on the created Observation. Change the observation label. Select the NIRSpec instrument and the “Bright Object Time Series” template. Add GJ 1214b as the selected target for the observation. 
  5.  Fill out the target acquisition parameters following the findings on signal-to-noise from the  ETC calculation. TA option is “WATA” (wide aperture TA). The target is the same as the observation because we will do TA on GJ 1214 itself. Since GJ 1214 is very bright, we choose the SUB32 subarray and the F110W filter. In NRSRAPID, the exposure time will be 0.08 seconds for the TA. In the ETC, these choices show some saturation in the central pixel.
  6.  Next comes filling out the exposure parameters. First, we use the same subarray as for the ETC calculation - SUB2048. Next, we select the G140H/F100LP grating filter option, which will allow us to acquire 1–1.9 micron spectra at one time. We select 1 exposure for the observation. The constraint on the NIRSpec BOTS template is that the exposure duration must be less than 48 hours.
The exposure time parameters are tuned using the calculation information determined in the ETC - the NRSRAPID readout pattern with 13 groups will give a good S/N across the 1.15 micron spectrum with no saturation. Binning multiple integrations and/or multiple spectral elements can gain S/N.

The total exposure time is set by the duration over which we want to observe. The total phase period of GJ 1214b is 1.58040482 days, and the transits of GJ 1214b have a 1 hour duration. We will acquire spectra for an even amount of time before, during and after the transit for benchmark measurement plus1 hour of padding. Hence, the exposure time needs to be 4 hours in total, with 2 hours before the transit and 1 hours after the transit.  An optimal number of integrations is found to be 1142, which gives an exposure time of 14445 seconds (4.012 hrs) to cover the transit of GJ 1214b.

A warning is added to the Exposure time definition. Hovering the computer mouse over the warning icon shows: “Warning: Exposure duration exceeds the limit of 10000 seconds. Above this limit it is possible that a High Gain Antenna move may move during the exposure”.

Observations in the NIRSpec BOTS template are allowed to have individual exposure times beyond the 10000 second limit, thus exposing right through the repoints of the High Gain Antenna (HGA). When the HGA moves to repoint for ground communication contact, the guide star drops out of fine lock. This results in movement of the guide star and hence the science source within the NIRSpec aperture. After the HGA repoints, the FGS re-establishes lock and the guide star (and science source) are repositioned to within less than 5 milli-arcsec of their previous location. 

The observation described here requests an exposure time of 14421 seconds, and it is likely that the HGA will repoint during the course of the exposure time. The HGA repoints last approximately 60 seconds, and the source position and thus the position of the spectra will slightly change during the HGA movement, resulting in short excursions the flux in the science spectra. The integrations affected by HGA repoints can be removed from the measured time series during data processing and analysis.

Figure 1. NIRSpec BOTS APT Interface

The filled-out APT template for the BOTS observation of GJ 1214b.


Adding special requirements

Timing special requirement

  1. The timing special requirements are needed to define when the transit observation of GJ 1214b should start. The Heliocentric Julian Date of the zero phase (transit time) is 2455391.65402. The orbital period of GJ 1214b is 1.58040482 days. In order to capture the exposure at the right starting point to measure the eclipse, the recommendation is to include a 45 minute pad time and 15 minutes of settling. Hence, we will start each observation at least 2 hours prior to the transit start, acquire spectra during the 1 hour of transit, and observe for an hour afterward to establish the baseline. The total observation time per transit is 4 hours. In order to start the transit observation in the window of 1 to 2 hours before a transit occurs, the phase range start and end will be 0.93408 to 0.960453. These values, when multiplied by the period and added to the zero phase HJD, define the range of times that the exposure will start prior to a transit. The selected values ensure that the observations will start with at least 1 hour of time at the beginning of the transit. The values discussed here are input as observation special requirements for the PHASE RANGE (START) to (END), PERIOD and ZERO PHASE (HJD). (Note that any transit following the initial one defined by the phase special requirement could also be selected for the execution window.)

Figure 2. APT phase constraints

APT phase constraints filled out for the GJ 1214b transit. The phase range describes the one hour timing window for starting the exoplanet transit observations. A window of less than one hour will result in an excess slew overhead charged on the observation.


Time-series observation special requirement

The NIRSpec BOTS mode observation will automatically have a Time Series Observation Special Requirement added. This shows up as an implicit special requirement for all NIRSpec BOTS mode observations in APT. The purpose of this special requirement is to ensure that the NIRSpec BOTS data gets processed through a particular pipeline calibration workflow specific for time series exposures. Users do not need to add this special requirement, but it's useful to take note that it is automatically set.

Copying and completing the observations

  1. The observation definition for GJ 1214b in the G140H/F100LP spectral configuration is complete after the addition of the timing special requirements.
  2. The observation for GJ 1214b is copied into a second and third observation and altered for the spectral setting and necessary integration times found from the ETC. Copying the observation preserves all of the special requirements on the transit timing, so all that needs to be changed is the grating+filter combination and the exposure parameters.
  3. This program’s 19.65 hrs of requested observations are now complete. These values are summed across all observations (and visits), and presented on the Proposal Information summary page.




Last updated

Published January 23, 2018