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 you 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

Use a slightly more conservative value of:

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

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

The sections below 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

First, define a source in ETC that emulates the HD 218396 system, the science target of the observation. After selecting the source, open the Sources and Scenes tab and then update the default source parameters in the Source Editor pane as follows:

• ID tab - update Source Identity Information to HD 218396;
• Continuum tab - select the Phoenix Stellar Models in the Continuum pull-down menu, and choose a star with spectral type F0V 7250 4.0;
• Renorm tab - choose the normalize in bandpass option, renormalize the source to the observed magnitude of M = 5.26 (Vega) and choose the normalization to be in the NIRISS F480M filter;
• Lines tab - leave this tab empty since you are not adding any emission or absorption lines to the spectrum;
• Shape tab - keep the default option of point source;
• Offset tab - leave this tab empty so the source will be at the center of the scene.

Define Source for "PSF Reference Star" Scene

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

• ID tab - update Source Identity Information to HD 218172;
• Continuum tab - select the Phoenix Stellar Models in the Continuum pull-down menu, and choose a star with spectral type F8V 6250 4.0;
• Renorm tab - choose the normalize in bandpass option, renormalize the source to the observed magnitude of M = 5.83 (Vega) and choose the normalization to be in the NIRISS F480M filter;
• Lines tab - leave this tab empty since you are not adding any emission or absorption lines to the spectrum;
• Shape tab - keep the default option of point source;
• Offset tab - leave this tab empty so the source will be at the center of the scene.

Note that HD 218172 has a K-band magnitude of 5.85 (Vega), and you've estimated the M-band magnitude with the color correction K - M = 0.02.

Assign HD 218396 to "Target" Scene

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

Assign HD 218172 to "PSF Reference Star" Scene

Defined a new ETC scene, add HD 218172 to this scene by highlighting the source HD 218172 and then clicking on Add Source in the Select a Scene panel. Then, rename 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), specify the background parameters for the target scene. Since the JWST background is position dependent, fully specifying background parameters are important. Enter the coordinates for HD 218396 (23:07:28.83 +21:08:03.53) in the Background tab, and select Medium for Background configuration, which corresponds to the 50th percentile of the sky background.

PSF reference star

Select another AMI calculation from the NIRISS pull-down menu in the Calculation tab and update the scene to PSF Reference Star in the Scene tab (Calculation #2). Update the background coordinates to those of HD 218172 (23:05:35.34 +20:14:27.69) and change 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. You'll therefore increase the exposure time by increasing the number of integrations until you detect the number of photons needed for the science goals (see below).

Target

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

• Instrument Setup tab - select 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;
  • Choose 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 is kept at its default value of 10, and the number of Integrations per exposure is kept at the default value of 1.
  • Update the number of Total Dithers to 5 to emulate the recommend 5-point SUBPIXEL dither pattern.
• Strategy tab - choose 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).

Click the Calculate button to perform the calculation with these parameters. The ETC Reports pane reports that the maximum number of groups before saturation is 8, which is used in the calculations below. The ETC Reports pane issues a warning since the default set up used with 10 groups results in saturation.

PSF reference star

Calculation #2 represents the initial calculation to determine the saturation threshold for the PSF reference star. Set the same parameters as the target star, discussed above. From the Reports pane, you'll see that the maximum number of groups before saturation is 14. 

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 8, you can now calculate the number of integrations such that you'll will garner 10¹⁰ photons. From the ETC, you can see that the extracted flux from the source is reported as 2516102.01 e^–/s in the Reports pane. The photon collecting time of an observation is given by:

t_{photon_collect} = ngroups x nint x ndithers x tframe

where ngroups is the number of groups, nint is the number of integrations, ndithers is the number of dithers, 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 ndithers  x tframe,

the number of integrations needed to detect 10¹⁰ photons is:

nint = 10¹⁰ / (flux x ngroups x ndithers x tframe)

nint = 1317.1

The final number of integrations is thus 1318. Calculation #3 has these exposure specifications (Groups per integration = 8; Integrations per exposure = 1318; Total Dithers  = 5) for the target star. 

PSF reference star

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 14. The flux for this source, according to the Reports pane, is 1489419.40  e^–/s.

Using:

nint = 10¹⁰ / (flux x ngroups x ndithers x tframe)

nint = 1271.4

The final number of integrations is thus 1272.  Calculation #4 has these exposure specifications (Groups per integration = 14; Integrations per exposure = 1272; Total Dithers = 5) 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, 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 you selected Target Acquisition under the NIRISS pull-down menu, shows the initial calculation to determine which parameters to specify for TA for the target star:

• Backgrounds tab - enter the coordinates for HD 218396 (23:07:28.83 +21:08:02.53) and select Medium for Background configuration;
• Instrument Setup tab - update 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. Leave 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 this case is the science target HD 218396.

From this calculation, you'll see the SNR is ~109, which ensures a successful TA with a centroid accuracy ≤ 0.05 pixel. 

PSF reference star

Calculation #6 shows the starting point for target acquisition for the PSF reference star, where you 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). Set the same exposure parameters as above. With 3 Groups, the SNR is ~76, so increase it to 5 Groups (Calculation #7), where you'll achieve a SNR of ~122. Therefore, choose 5 Groups for the 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