For JWST NIRISS target acquisitions (TAs), sources are positioned with subpixel accuracy on a specific part of the detector. TAs are required for subarrays, and strongly recommended for full frame readout with SOSS and AMI modes.
An onboard target acquisition (TAs) positions a source with subpixel accuracy on a specific location on the detector. TAs are supported for two NIRISS observing modes: single object slitless spectroscopy (SOSS) and aperture masking interferometry (AMI). While a TA is only required for those observing modes when a subarray is used, it is also strongly recommended for the full frame detector readout to ensure that the target is always placed on the same detector pixel.
Steps in the NIRISS TA procedure are described below:
- The target is located:
- Three 64 × 64 pixel subarray exposures are taken; images are offset from each other (dithered) by a small integer pixel in X and Y to allow for removal of bad pixels.
- Onboard TA software processes the images (aligns the images, flags bad pixels, removes cosmic rays, and subtracts the background level) and applies a centroiding algorithm to determine the target coordinates (see Target location algorithm below).
- The spacecraft is moved to place the target at the nominal center of the TA subarray being used, which is listed in onboard tables.
- The target is re-imaged with a single (undithered) exposure. This is the so-called "TA verification" exposure.
- If the observing mode is AMI, the spacecraft is moved to the science aperture which is located on a separate (but nearby) subarray. This is done to avoid any persistence from the TA images. (This step is not performed for the SOSS observing mode, for which the spectra are projected onto a different location on the detector, so that the telescope does not have to be moved.)
A TA exposure sequence produces scientific data—images of the target produced in steps 1 and 3. These data will be returned to the observer along with the other science data.
Target location algorithm
To locate the target, the onboard software first passes a square checkbox of 3 × 3 pixels (0.196 × 0.196 arcsec2) over the TA subarray image and determines the intensity contained within the checkbox at each pixel in the subarray. The software then selects the checkbox with the maximum intensity and determines the flux-weighted centroid of the target using a 5 × 5 pixel window centered on that checkbox (see Figure 1 below).
Target acquisition modes
TAs are performed with either the CLEARP or non-redundant mask (NRM) element in the pupil wheel and the F480M filter in the filter wheel. In general, the CLEARP element is used for faint TA targets while the NRM element, which limits throughput to 15% of the CLEARP element, is used for bright targets. There are additional considerations at play in selecting the best TA mode for a given target brightness for science observations in a given observing mode (i.e., SOSS vs. AMI), namely the sensitivity of the TA procedure to the presence of saturated pixels in a given TA mode, and differences between the CLEARP and NRM elements in terms of sampling of the telescope pupil. Table 1 lists the target magnitude ranges recommended for the various TA modes.
Table 1. Recommended magnitude ranges for NIRISS TA Modes
|TA mode||PW element|
|SOSSBRIGHT 1||NRM||3 ≤ M ≤ 6.1|
|SOSSFAINT||CLEARP||6.1 ≤ M ≤ 14.5|
|AMIBRIGHT||NRM||3 ≤ M ≤ 9.3|
|AMIFAINT||CLEARP||9.3 ≤ M ≤ 14.5|
A TA exposure time, one that achieves a signal-to-noise ratio (SNR) of at least 30, is best determined using the Exposure Time Calculator (ETC). This enables a centroid accuracy of ≤0.15 pixel. However, increasing the TA exposure time to get a higher SNR should be considered while planning observations for which accurate centroiding is deemed crucial. For example, the centroiding accuracy improves to about ≤0.10 pixel at SNR = 50 and to about ≤0.05 pixel at SNR = 100. More information on the use of the NIRISS TA ETC is given in the JWST ETC NIRISS Target Acquisition article.
Exposure Time Calculator
Astronomer's Proposal Tool
Goudfrooij, P., 2016, Technical Report JWST-STScI-004784: "NIRISS Operations Concepts: Target Acquisitions".
Goudfrooij, P., 2017, Technical Report JWST-STScI-005934: "NIRISS Target Acquisition: the sensitivity of centroid accuracy to the presence of saturated pixels".
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