A walk through of the ETC for the JWST NIRISS AMI Example Science Program is provided, demonstrating how to select exposure parameters for this observing program.

See also: NIRISS Aperture Masking Interferometry, JWST Exposure Time Calculator Overview, Proposal Planning Video Tutorials

The JWST Exposure Time Calculator (ETC) performs signal-to-noise ratio (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 mode.

For the purpose of this calculation, assume that the flux ratio for the target HD 218396 and the planetary companion we wish to detect is ~10^-4. According to Ireland (2013), the number of photons necessary to detect this contrast is:

1.5 x N_hole² / (contrast ratio)², where N_hole refers to the number of apertures (holes) in a mask.

Since there are 7 apertures in the AMI NRM, this translates to:

73.5 / (contrast ratio)²

Considering the fact that NRM has not been used in space before, use a slightly more conservative value of:

100 / (contrast ratio)²= 100 / (0.0001)² = 10¹⁰

Therefore, the goal of our calculation is to detect 10¹⁰ photons from the target in the ETC simulations.

Below we discuss how to navigate the ETC to determine exposure parameters for this science program so that 10¹⁰ photons are detected from both the target star and PSF reference star. The optimal exposure specifications (e.g., numbers 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.

Define Sources and Scenes in ETC

See also:
JWST ETC Scenes and Sources Page Overview, JWST ETC Defining a New Source, JWST ETC Defining a New Scene, 
JWST ETC Source Spectral Energy Distribution

Define Source for "Target" Scene

We first defined a source in ETC that emulates the HD 218396 system, the science target of our observation. After selecting the source, we opened the Sources and Scenes tab and then updated the default source parameters in the Source Editor pane as follows:

• ID tab - we updated Source Identity Information to HD 218396;
• Continuum tab - we selected the Phoenix Stellar Models in the Continuum pull-down menu, and chose a star with spectral type F0V 7250 4.0;
• Renorm tab - we  chose the normalize in bandpass option, renormalizing the source to the observed magnitude of M = 5.26 (Vega) and choosing the normalization to be in the NIRISS F480M filter;
• Lines tab - we left this tab empty since we do not add any emission or absorption lines to the spectrum;
• Shape tab - we kept the default option of point source;
• Offset tab - we left this tab empty so the source will be at the center of the scene.

Define Source for "PSF Reference Star" Scene

We added a new source in the Select a Source pane to emulate HD 218172, the PSF reference star for this observation. We updated the default source parameters in the Source Editor pane as follows:

• ID tab - we updated Source Identity Information to HD 218172;
• Continuum tab - we selected the Phoenix Stellar Models in the Continuum pull-down menu, and chose a star with spectral type F8V 6250 4.0;
• Renorm tab - we  chose the normalize in bandpass option, renormalizing the source to the observed magnitude of M = 5.83 (Vega) and choosing the normalization to be in the NIRISS F480M filter;
• Lines tab - we left this tab empty since we do not add any emission or absorption lines to the spectrum;
• Shape tab - we kept the default option of point source;
• Offset tab - we left this tab empty so the source will be at the center of the scene.

We note that HD 218172 has a K-band magnitude of 5.85 (Vega), and we estimate the M-band magnitude with the color correction K - M = 0.02.

Assign HD 218396 to "Target" Scene

We highlighted Scene 1 in the Select a Scene pane and then renamed the Scene Identity Information entry in the ID tab of the Source Editor to Target.

Assign HD 218172 to "PSF Reference Star" Scene

We  defined a new ETC scene, added HD 218172 to this scene by highlighting the source HD 218172 and then clicking on Add Source in the Select a Scene panel. We then renamed the scene PSF Reference Star.

Select NIRISS AMI Calculation

See also: JWST ETC Creating a New Calculation, HCI NIRISS Limiting Contrast

Target

After selecting AMI from the NIRISS pull-down menu in the Calculation tab (Calculation #1), we specified the background parameters for the target scene. Since the JWST Background is position dependent, fully specifying background parameters are important. We entered the coordinates for HD 218396 (23:07:28.83 +21:08:03.53) in the Background tab, and selected Medium for Background configuration, which corresponds to the 50th percentile of the sky background.

PSF reference star

We selected another AMI calculation from the NIRISS pull-down menu in the Calculation tab and updated the scene to PSF Reference Star in the Scene tab (Calculation #2). We updated the background coordinates to those of HD 218172 (23:05:35.34 +20:14:27.69) and changed the Background configuration to Medium.

Select instrument parameters

For the AMI mode, the number of groups should be set such that the brightest pixel in the last group of an integration is slightly below the effective saturation value. We will then increase the exposure time by increasing the number of integrations until we detect the number of photons needed for our science goals (see below).

Target

Calculation #1 represents our initial calculation to determine the saturation threshold for our target star. We set the following parameters:

• Instrument Setup tab - we selected the F480M filter.
• Detector Setup tab -
  • subarray is set to SUB80; due to the brightness of the source, a subarray is required to avoid saturation, and only the SUB80 subarray is available in the AMI mode;
  • we chose the NISRAPID readout pattern (where there is 1 frame per group); this is the only permitted readout pattern when using a subarray in the AMI observing mode.
  • number of Groups per integration was kept at its default value of 10, and the number of Integrations per exposure and number of Exposures per specification were kept at the default values of 1.
• Strategy tab - we chose an aperture location centered on the source (HD 218396), an aperture radius of 2.5", and a noiseless sky background (see NIRISS AMI Recommended Strategies).

We clicked the Calculate button to perform the calculation with these parameters. The ETC Reports pane reports that the maximum number of groups before saturation is 9, which we use in our calculations below. The ETC Reports pane issues a warning since the default set up we used with 10 groups results in saturation.

PSF reference star

Calculation #2 represents our initial calculation to determine the saturation threshold for our PSF reference star. We set same parameters as the target star, discussed above. From the Reports pane, we see that the maximum number of groups before saturation is 16.

Adjust exposure parameters to detect required number of photons

See also: NIRISS AMI Recommended Strategies

Target

After determining above that the number of groups per integration prior to saturation is 9, we now calculate the number of integrations such that we will garner 10¹⁰ photons. From the ETC, we see that the extracted flux from the source is reported as 2264841.91 e^–/s in the Reports pane. The photon collecting time of an observation is given by:

t_{photon_collect} = ngroups x nint x tframe

where ngroups is the number of groups, nint is the number of integrations, and tframe is 0.07544 s with the SUB80 subarray and NISRAPID readout.

Please note that the exposure time reported by ETC includes reset time, equivalent to one tframe, between each integration and the time for full-frame reset of pixels outside the subarray, which occurs before every integration when the detector is in subarray mode. No photons are recorded during this reset time, so these reset times should not be included when calculating the total number of photons.

Since the total number of counts detected during this observation is:

photons = flux x ngroups x nint x tframe,

the number of integrations needed to detect 10¹⁰ photons is:

nint = 10¹⁰ / (flux x ngroups x tframe)

nint = 6503.1

The final number of integrations is thus 6504. Calculation #3 has these exposure specifications (Groups per integration = 9; Integrations per exposure = 6504) for the target star.

PSF reference star

We follow the same methodology to determine the number of integrations for the PSF reference star, recalling that the optimal number of groups found above is 16. The flux for this source, according to the Reports pane, is 1340907.46  e^–/s.

Using:

nint = 10¹⁰ / (flux x ngroups x tframe)

nint = 6178.5

The final number of integrations is thus 6179.  Calculation #4 has these exposure specifications (Groups per integration = 16; Integrations per exposure = 6179) for the PSF reference star.

Target acquisition

See also: NIRISS Target Acquisition, JWST ETC NIRISS Target Acquisition 

A target acquisition (TA) is required in order to precisely position the target on the detector. A signal-to-noise ratio (SNR) ≥ 30 is recommended to achieve a successful TA, which achieves a centroid accuracy of ≤ 0.15 pixel. Increasing the SNR to 100 improves the centroiding accuracy up to ≤ 0.05 pixel. 

The acquisition mode is either AMIBRIGHT, for sources with M-band magnitudes between 2.9 ≤ M ≤ 9.2 (Vega), or AMIFAINT, for sources with magnitudes between 9.2 ≤ M ≤ 14.4 (Vega). Since both sources are brighter than M = 8.5, we use the AMIBRIGHT acquisition mode. TA is always performed with the F480M filter. The only allowed readout pattern for a TA with the AMIBRIGHT acquisition mode is NISRAPID and the number of integrations and exposures is limited to 1; hence a user can not update these values in the ETC UI.

Target

Calculation #5, where we selected Target Acquisition under the NIRISS pull-down menu, shows our initial calculation to determine which parameters to specify for TA for the target star:

• Backgrounds tab - we entered the coordinates for HD 218396 (23:07:28.83 +21:08:02.53) and selected Medium for Background configuration;
• Instrument Setup tab - we updated the selection of Acq Mode to SOSS or AMI Bright;
• Detector Setup tab - there are no options for the Subarray, Readout Pattern, Integrations and Exposures parameters other than the default values. We left the number of Groups to the starting value of 3;
• Strategy tab - the only permissible option for target acquisition is Aperture centered on source, which for our case is the science target HD 218396.

From this calculation, we see the SNR is ~105, which ensures a successful TA with a centroid accuracy ≤ 0.05 pixel. 

PSF reference star

Calculation #6 shows our starting point for target acquisition for the PSF reference star, where we updated the Scene in the calculation from the default scene to the PSF Reference Star scene, and the coordinates for the Background tab to that of HD 217883 (23:05:35.33 +20:14:27.69). We otherwise set the same exposure parameters as above. With 3 Groups, the SNR is ~72, so we increased it to 5 Groups (Calculation #7), where we achieve a SNR of ~117. We thus choose 5 Groups for our Target Acquisition for the PSF reference star.

References

Ireland, M. J. 2013 MNRAS 433, 2
Phase errors in diffraction-limited imaging: contrast limits for sparse aperture masking