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The JWST Exposure Time Calculator (ETC) offers target acquisition (TA) modes for all the JWST instruments, which allows users to estimate the exposure time required to obtain the required signal-to-noise for the TA source.


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Introduction

All spectroscopic and coronagraphic observations will require a science instrument assisted target acquisition procedure. Science observations that can tolerate absolute pointing errors of up to 0.45" to 0.30" (1-σ radial error), require no further TA procedures following the usual guide star acquisition by the FGS.

Target acquisition (TA) is used to place sources accurately in apertures or subarrays that are small when compared to the 3σ accuracy of the guide stars. The on-board system uses the measurement of the source position and the known central position of the aperture of interest to perform a small move of the telescope to put the source in the aperture.



Coronagraphic target acquisition

Coronagraphic target acquisitions are common to both NIRCam and MIRI.

Coronagraphic observations involve locating a target behind a coronagraphic mask with a precision of <5 mas. For MIRI and NIRCam this is a two-step process. The first step locates the target within a predefined TA area less than 20" from the desired coronagraphic mask. For MIRI each coronagraphic mask has a "sweet spot" located 0.5"–1.0" from the mask center. The second step offsets it to the center of the coronagraphic mask and iteratively centers the target on the mask area as needed.

The details of how to create a coronagraphy TA calculation in the JWST ETC are described in JWST ETC MIRI Target Acquisition and JWST ETC NIRCam Target Acqusition. Observers can select the desired filter for a given target acquisition procedure in the Instrument Setup tab. For MIRI the user should choose the neutral density filter (FND), this will prevent the star from saturating the detector during the subarray exposures used for the target locates. For NIRCam, the bright targets will be placed behind a neutral density square for the TA exposure.



Direct imaging acquisition

MIRI

In general, after the Observatory has entered fine guidance mode, MIRI will be able to commence direct imaging observations without the need to further refine the target position. However, high precision imaging photometery of bright sources, such as time-series observations (TSOs), require TA to ensure repeatable measurements. To ensure that the object falls on the same part of the subarray for every observation, a TA will be performed to center the object on the desired pixels. TA for MIRI imaging is not yet supported for cycle 1 observations.

NIRCam

In direct imaging mode, both NIRCam modules and all focal plane arrays (FPAs) will be operated simultaneously and synchronously to provide a 2 × 2.2' × 2.2' field and two wavelengths at a time. Filters are selected for the short and long wavelength arms of the instrument and subarrays may be selected. The time-series imaging TA is done using the sub-arrays on the long-wavelength detector arrays.

NIRISS

NIRISS uses target acquisition for the aperture masking interferometry (AMI) mode. While TA is strictly required only for the cases that use subarrays, it is recommended to perform a TA in full-frame readout mode as well, to ensure that the target is always placed on the same detector pixel. The TA is performed by taking 3 exposures with 64 × 64 pixel subarray, by offsetting the images from each other by small integer pixel offsets. The science aperture used for AMI is at a slightly different location on the detector to avoid persistence from the TA images. The JWST ETC allows to determine the exposure times required to achieve the desired signal-to-noise for the TA source as detailed in JWST ETC NIRISS Target Acquisition.

NIRSpec

Since the field of view of NIRSpec (3.4' × 3.5') is relatively large compared with the Observatory's pointing uncertainty , NIRSpec will be able to commence imaging observations for the pointing verification without the need to further the target position once the Observatory has entered fine guidance mode. However, in practice, the direct imaging mode of NIRSpec will primarily be used to obtain an image used by the observer after the fact to verify the correct location of the targets in the NIRSpec FOV, and thus will be obtained following a target acquisition used to set up for one of the spectroscopic modes.



Spectroscopic target acquisition

NIRSpec

NIRSpec will be able to commence spectral observations without the need to further refine the target position once the Observatory has entered fine guidance mode. However, TA will be necessary to place science targets with the required position accuracy within the slits of the MSA aperture mask or in one of the fixed slits. This accuracy critically depends on the number of reference targets used and on the precision of their coordinates. Details of how to perform NIRSpec TA ETC calculations are described in JWST ETC NIRSpec Target Acquisition.

The ETC currently supports two TA modes: WATA and MSATA (Single Object). The WATA mode, which is appropriate only for WATA, always uses the S1600A1 aperture. The MSATA (Single Object), which is appropriate for MSATA standard TA calculations, does a Full-Frame detector readout (no subarrays permitted) calculation using the MSA model. Both WATA and MSATA have a fixed number of groups (3), integrations (1), and exposures (1) and cannot be changed by the user in the ETC. In order to achieve TA requirements it is necessary for the on-board software to perform target locates on at least 5 reference stars, and therefore users should populate their scene as such (see JWST ETC Scenes and Sources Page Overview). The observatory is then able to locate the set of reference targets with respect to the fiducial slit locations and performs a small-angle maneuver to center the target field in the NIRSpec FOV and the TA procedure is concluded.


NIRISS

Target acquisition is required for the NIRISS single object slitless spectroscopy (SOSS) observing mode. The TA source is acquired at a 'sweet spot' location on the detector, such that the dispersed spectrum is projected onto a pre-defined location on the detector. The SOSS TA uses a 64 × 64 pixels array for TA, and the centroiding is done within 5 × 5 pixels. The recommended SNR is a minimum of SNR = 30 in the 5x5 pixels extraction box, to achieve a centroiding accuracy of less than 0.15 pixel for the TA source. The ETC setup for TA mode for NIRISS SOSS is described in JWST ETC NIRISS Target Acquisition.

MIRI

In order to acquire a target for observation with the fixed slit in the MIRI imager, the target is located using a 96 × 96 pixel subarray in the MIRI imager FOV, the required offset to place the target in the center of the fixed slit is computed, and a small angle maneuver to center the target in the slit is performed. When using the LRS in slitless mode, TA uses a 64 × 64 pixel sub-region of the SLITLESSPRISM subarray to perform TA.

For the MRS, not all science observations are expected to require TA given the size of the MRS fields of view. When TA is required, the baseline strategy for uses a 128 × 128 pixel sub-region of the MIRI imager field to locate the target since parts of the MIRI imager focal plane are within ~40" of the MIRI IFU spectrometer field center (See Figure 1 in MIRI MRS Recommended Strategies). First, the target is located within the FULL array, then a small angle maneuver is performed, moving the object to the center of the concentric IFU FOVs. The precision of the offset is less stringent than for the coronagraph, TA placement accurate to 100 mas in the IFU is acceptable. The JWST ETC can be used to determine the exposure times required to achieve the signal-to-noise required for accurate centroiding and is described in JWST ETC MIRI Target Acquistion.






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References

 JWST technical documents

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Last updated

Updated November 5, 2018

  • Updated for ETC v1.3.

Updated November 16, 2017

  • Corrected 30" to 40" in MIRI Spectroscopic TA required separation

  • Provided link to MRS Best Practices page for explanation for this separation

Published October 4, 2017



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Updated April 5, 2017

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Published March 2, 2017