The NIRSpec target acquisition (MSATA) process uses reference stars to center targets within the MSA, IFU or FS apertures for science observations. In cases where few reference stars are available, it is possible to use other compact sources with relaxed stellarity.
The NIRSpec target acquisition (TA) method MSATA1 will be used for most NIRSpec science observations performed with the micro-shutter assembly (MSA) in the MOS observing mode, and is optionally available for the NIRSpec fixed slits (FS) and IFU observing modes.
During the MSATA procedure, a set of TA reference objects (typically stars) are observed through the open micro-shutters. Their centroids are calculated and used to accurately correct the initial spacecraft pointing and orientation. The achievable TA performance depends significantly on the relative astrometric knowledge of both the reference stars and the science source positions used to plan the TA and the science. The MSATA procedure is designed to work best when the relative astrometric accuracy of the target field is in the range of 5 to 50 mas. The optimal TA accuracy of about 20 mas requires a relative astrometric accuracy of 5 mas in the planning catalog.
MSATA is not available for the bright object time-series (BOTS) observing mode because (1) the wide aperture TA available for this template in the JWST Astronomers Proposal Tool (APT) is equally accurate for a single target and takes less time to execute and (2) the required full frame detector readout would likely saturate on any BOTS science sources.
1 Bold italics font style indicates parameters, parameter values, and special requirements that are set in the APT GUI.
Operational sequence of MSATA
The MSATA procedure uses a set of reference stars distributed over the MSA quadrants, determines their observed centroids on the detector, and compares them to the desired positions to calculate a small-angle maneuver that adjusts the initial pointing and position angle. The process acquires two exposures to centroid reference stars in order to accurately align the science targets of interest within their apertures. The centroids of 5–20 reference stars will be measured in a full frame imaging exposure obtained through the "all open" MSA quadrants (or a protected configuration to block all operable shutters except those with reference stars) using both detectors NRS1 and NRS2.
The two exposures are offset by one-half of an MSA shutter "pitch" (the distance between adjacent shutter centers) in both coordinates. This “MSA half facet slew” is necessary to properly derive the location of the reference stars, minimizing the effect of the physical bars between micro-shutters that could otherwise bias the measured positions of the stars. The description of the micro-shutter assembly presents additional information on the MSA structure. The NIRSpec observation planning tool software will preferentially choose reference stars that are not affected by failed closed micro-shutters.
Both NIRSpec MSATA images are processed by the onboard software to determine precise locations of the reference stars. These locations are initially measured in pixel coordinates and must be transformed by the software into the distortion-free tangential coordinate system on the sky before they can be compared to the desired sky positions. This coordinate transformation requires precise knowledge of the combined distortion effects of the telescope (Optical Telescope Element; OTE) and NIRSpec, as well as the measured tilt of the NIRSpec target acquisition imaging mirror. The measured centroids of the reference stars will be transformed to sky coordinates and compared with catalog positions expressed in the same sky frame to determine offsets for each reference star and a combined mean pointing offset including a possible roll adjustment.
These quantities are calculated using a least squares methodology. After a sigma-clipping and threshold outlier rejection procedure to determine the mean offset and residuals, the telescope will be offset by the final derived mean slew value. The procedure may be repeated once if the overall offset exceeds a specified threshold (1.1") to improve centering accuracy. Subsequently, a reference image of the centered target field is automatically obtained to conclude the MSATA operational process. The overall timing to execute MSATA is estimated to be 1,440 s per science visit in observation planning. This overhead will be revised as new information becomes available.
The available filters for these acquisition images are F110W, F140X, or CLEAR. NIRSpec TA images are always acquired in full frame readout with Ngroups = 3 and detector readout pattern options of NRS, NRSRAPID, NRSRAPIDD1 and NRSRAPIDD2. The latter two are available for MSATA only.
Expected Accuracy of MSATA
Figure 2 shows the NIRSpec target acquisition planning uncertainty as a function of input catalog astrometric accuracy, derived and updated from the estimated error budget definition.
If target acquisition uses 5 or more reference stars, the estimated NIRSpec acquisition accuracy will be better than ~20 mas (<1/10 shutter width) if the input planning catalog astrometric accuracy is 5–10 mas. The estimated delivered TA accuracies of better than 25 mas (called optimal TA) are achieved only if planning catalog astrometry is better than about 15 mas for all reference stars and science sources. Delivered estimated TA accuracy of 25–50 mas is possible to achieve using input relative astrometry with 15–40 mas accuracy.
An estimated TA accuracy level of 50 mas is 1/4 of an MSA shutter width (or 0.2" FS width), and this would result in very significant flux calibration (i.e., slit loss) uncertainty and probable degradation in wavelength calibration accuracy. Hence, this TA regime (called NIRSpec “relaxed accuracy TA”) should primarily be used for MSA science on very extended sources and use cases that can tolerate a reduced flux calibration accuracy. A TA accuracy beyond 50 mas is recommended primarily for IFU mode observations. (Nevertheless, WATA is usually preferred over MSATA for IFU observations.)
Table 1 describes these MSATA accuracy classifications based on input astrometric accuracies for reference stars and science targets. The Astronomer's Proposal Tool (APT) planning interface with the MSA Planning Tool requests an astrometric catalog accuracy, and these MSATA classifications are presented to users when target acquisition is defined in visits.
Table 1. Description of the "optimal," "relaxed accuracy," and "IFU only" categories used to describe the accuracies achieved in MSATA
Approximate brightness limits for MSATA
The brightness limits for the reference stars in MSATA observations are defined by the instrument filter used to acquire the images, and the detector readout pattern used for the exposures (IRS2 readout patterns are not used for MSATA or WATA.) All NIRSpec TA exposures are acquired with Ngroups = 3 (specified using the Acq Groups/Int parameter in APT), which also defines the achieved brightness range. Table 2 provides the approximate limiting AB magnitudes of sources that might be used for MSATA, and the corresponding instrument settings. Values are provided for a source with a S/N equal to 20 and for a source near saturation, and for all available TA filters. These are estimated values based on ground test results and are subject to change.
In addition to the NRSRAPID and NRS readout patterns described in the NIRSpec detector readout patterns pages, MSATA has two additional detector patterns available called NRSRAPIDD1 and NRSRAPIDD2. These two patterns were added to more efficiently observe the magnitude range achievable for NIRSpec in the MSATA images, particularly for TA in extragalactic deep fields. The NRSRAPIDD1 and NRSRAPIDD2 patterns are available only with the CLEAR filter.
Table 2. Brightness ranges for NIRSpec MSATA filter and readout pattern options
Observation Planning for MSATA
In the MOS, IFU and FS science templates, MSATA is an option for target acquisition. MSATA is expected to be the common target acquisition method for the NIRSpec MOS science mode. In all cases, the NIRSpec science observations will be defined in a standard way via the Astronomer's Proposal Tool (APT). The MSATA parameters are defined using a planning catalog with reference stars, ingested in the MSA Planning Tool (MPT).
NIRSpec MSATA parameters, including reference stars and exposure specifications, can only be planned after fixed orients are assigned to a NIRSpec MOS program, or for FS or IFU programs using the MSATA method. Execution orients are assigned by the JWST long range plan (LRP) and communicated to teams after proposals are accepted by the JWST time allocation committee.
The observation planning parameters for observations that use NIRSpec MSATA are defined both at the observation and visit level in the APT program. In order to properly define NIRSpec MSATA, the catalog used to plan MOS science observations must include reference stars. Even IFU or FS science observations that use MSATA must have an available catalog of reference stars in the wider area covered by the MSA. In any case, the catalog must have columns containing the brightness in the NIRSpec target acquisition filters for each potential reference star. (To prevent catalog ingest errors, it is also advised to provide this information for all sources in the catalog.) After the science observation is planned, the observer must select a reference star bin for each visit. When a visit is highlighted in the hierarchical editor of APT, APT will create a representation of the MSA with selected reference stars. The reference stars have been grouped based on brightness bins defined for the various instrument filter and readout pattern combinations (see Table 2). Each selection, if multiple are presented, contains the brightest stars in the bin, up to a maximum of 20. The bins are ordered by those containing the most reference stars.