This article provides instructions for filling out the JWST NIRISS aperture masking interferometry APT template.
Aperture masking interferometry (AMI) is one of the four observing modes for the Near Infrared Imager and Slitless Spectrograph (NIRISS). The AMI mode uses a non-redundant mask (NRM) in the pupil wheel of NIRISS. It has 7 "holes" (apertures) that produce an interferogram in the image plane, and samples 21 unique ("non-redundant") baselines. AMI provides the highest effective angular resolution of any instrument mode on JWST, enabling the detection of planetary or stellar companions at a distance of 70–400 mas from their host stars. This mode also enables high-resolution mapping of extended sources through near-infrared aperture synthesis.
The NRM is used in conjunction with one of three medium-band filters (F380M, F430M, F480M) or a wide band filter (F277W) in the Filter Wheel (FW). A typical observation with AMI will include a Target Acquisition (TA) image, which is required if the science exposures use a subarray, followed by several exposures with the NRM in the PW with one of the four allowed filters in the FW, and possibly a direct image to help associate features in the NRM images with morphological features of the target. Direct images may help constrain or stabilize image reconstruction methods applied to extended object NRM data. Direct images can also be used on fainter multiple point source targets, especially when recently-developed “kernel phase” data analyses are used.
NRM data analysis of a target observation requires one or more PSF reference stars. The PSF reference is used to calibrate out the instrumental contribution to closure phases and visibility amplitudes. To minimize the impact of detector effects, the PSF reference star should be observed with the same dither pattern, especially for more demanding observations. It is likely that lower contrast targets may not need dedicated PSF reference stars, but could be calibrated with PSF reference data taken by other programs. Ideally, for higher contrast needs, target and reference observations should be scheduled close in time, so that the telescope is in a similar state, thermal or otherwise, for all the observations. In this case, the science target(s) and PSF reference star(s) should be observed using the same telescope optical configuration, so no wavefront correction should occur between any of the observations. We recommend specifiying GROUP NON-INTERRUPTIBLE or SEQUENCE NON-INTERRUPTIBLE in the Timing Special Requirements window in APT. The APT requires justification if an observer does not request any reference star observation.
Step-by-step APT instructions
Target acquisition parameters
Target Acquisition (TA) places the centroid of the brightest object in the TA field-of-view on the location of the AMI aperture used for the science exposure. An observer can specify one of the 3 options for ACG TARGET: NONE, SAME TARGET AS OBSERVATION, or any source within the target list that is within 60" of the science target. If a subarray is chosen for the science observation, a TA must be performed. TA is performed with the F480M filter in the filter wheel.
There are 2 options for ACQ MODE, based on the M-band magnitude of the target which is specified in the AMI Template Parameters article: AMIBRIGHT and AMIFAINT.
Acquisition exposure time
Main Article: NIRISS Detector Readout Patterns
The only permitted ACQ READOUT PATTERN for AMIBRIGHT is NISRAPID. Both NIS and NISRAPID are available readout patterns when the ACQ MODE is AMIFAINT. The observer can specify the ACQ NO. OF GROUPS (3, 5, 7, 9, 11, 13, 15, 17, 19), where a group is defined by the number of frames in an integration (for NISRAPID) or by the number of frames in an integration divided by four (for NIS).
The NIRISS AMI dithers article discusses AMI dithering choices and the drawbacks and potential benefits of dithering. Typically primary and subpixel dithering are not recommended.
NRM imaging will include both primary and secondary dither patterns.
Specify numer of dithers:
PRIMARY DITHERS: NONE, 2, 3, 4
SUBPIXEL DITHERS: NONE, 5, 9, 25
Main article: NIRISS Detector Subarrays
Observations of bright and compact targets can make use of an 80 × 80 subarray (SUB80). Target acquisition is required if observing with a subarray. Allowed options for SUBARRAY are FULL, SUB80. See the NIRISS AMI article for the limiting Vega magnitudes accessible with the SUB80 and FULL subarrays for the various filters.
To set up a science observation or PSF reference star observation, Add an observation to include it in the Filters dialog box. Select the FILTER for the observation, where allowed options are F277W, F380M, F430M, F480M.
The only supported READOUT PATTERN is NISRAPID. Specify:
NO. OF GROUPS: the number of groups during an integration, equal to the number of frames read per integration. Up to 800 groups are supported for this mode.
NO. OF INTEGRATIONS: the number of integrations during an exposure, where an integration is the time between detector resets.
Exposures can be Added, Duplicated, Inserted Above existing exposures, and Removed.
Direct imaging parameters
Main Article: NIRISS imaging
Specify TRUE or FALSE depending on whether a direct image is to be taken in the observation.
If TRUE is selected, then specify:
IMAGE DITHERS: refers to primary image dithers. Options are NONE, 2, 3, 4
NO. OF GROUPS
NO. OF INTEGRATIONS
The direct image will be performed with the same filter as the corresponding NRM image specified in AMI Parameters. The only supported READOUT PATTERN is NISRAPID.
Note: the Exposure Time Calculator mode for AMI does not include calculations for direct imaging. To run simulations for direct imaging to determineNO. OF GROUPS and NO. OF INTEGRATIONS, use the NIRISS imaging mode in ETC.
PSF reference observations
This is a PSF reference observation
If this observation is the PSF reference star, then THIS IS A PSF REFERENCE OBSERVATION must be selected. Note that PSF reference star observations have a 0 month exclusive access period.
PSF reference observations
If a PSF reference observation has already been specified, then that observation can be selected in the PSF REFERENCE OBSERVATIONS dialog box. The PSF reference star must be observed with the same FILTER and SUBARRAY as the science object. Note: if APT issues a warning that the PA offset between the reference star and science target must be within 1°–14°, this warning can be safely ignored.
If no PSF reference observation is required, then ADDITIONAL JUSTIFICATION must be selected. In the current implementation of APT (v. 25.4), an error is issued if no PSF reference star is specified and the target is not marked as the PSF reference observation, even if this box is checked. It is still possible to submit the proposal through APT. In a future version of APT, this error will be replaced by a warning.
Verify observation set-up
Create Target Confirmation Charts to verify that the input target coordinates will position the telescope in the correct place.
Run APT Visit Planner to check schedulability of observations, check constraints, and see whether guide stars are available to support the observation. Then run APT Smart Accounting to update time accounting and to remove excess overhead charges. APT can generate various reports through the "Reports" option in the Visit Planner view. Observers can access additional reports through File → Export option in the top menu bar. For example the .times file provides a break-down of individual exposures and associated exposure times, including overhead. These reports may help observers to tweak their observations to increase efficiency.
Observers can also visualize their observations onto the celestial sphere using the APT Aladin Viewer.
Main article: JWST APT Special Requirements
Target and reference star observations can be temporally grouped by using the TIMING→GROUP/SEQUENCE OBSERVATIONS LINK option in the EXPLICIT REQUIREMENTS section in the Special Requirement tab. We recommend specifiying GROUP NON-INTERRUPTIBLE or SEQUENCE NON-INTERRUPTIBLE.
Extended object imaging may need observations at more than one position angle in order to fill in gaps in uv-plane coverage, which can be achieved by invoking the POSITION ANGLE option in the EXPLICIT REQUIREMENTS section in the Special Requirement tab.