Help is provided to select from the available JWST NIRSpec Target Acquisition (TA) options for a given observation. Practical advice for choosing the parameters is also given in a variety of science scenarios.
Pointing accuracy before target acquisition
For JWST instruments it is estimated that, without an instrument-level target acquisition, the pointing accuracy for cycle 1 will be between 0.3'' and 0.45'' (1σ radial error; post guide-star acquisition pointing accuracy), depending on the distance between the science instrument and the guide star. At the location of the S1600A1 aperture used for WATA, a more typical error is expected to be ~0.34''.
It is expected that most NIRSpec observations will require significantly better placement of the target in the NIRSpec aperture plane that can be supported by this expected initial pointing and so will need to include a WATA or an MSATA target acquisition.
It is anticipated that the coordinates for a majority of guide stars will be updated using the Gaia DR2 release prior to JWST launch. When one of these guide stars is used, it is likely that the blind pointing accuracy will be considerably better than described above. However, a substantial fraction of observations will still need to rely on guide stars, not in the Gaia catalog, and so all observations should be planned assuming the initial pointing performance described here.
Implications of this expected pointing performance for WATA observations and planning of IFU observations without a target acquisition are discussed below.
Typical time requirements for acquisition options
Detailed overheads for NIRSpec operations are discussed in the NIRSpec Overheads article, but applying those results in practice requires a detailed understanding of the operational sequences. When adding an acquisition, in addition to the time for the acquisition itself, there will often be additional overheads for instrument and detector changes between the acquisition and science observations. For a given observation, total overheads are best determined by using APT, but we give here approximate times from a few examples to illustrate typical costs for adding an acquisition.
Adding VERIFY_ONLY in place of TA = NONE will typically require 8 to 14 minutes or more, depending on the exact exposure parameters selected. Using MSATA will typically add between 24 and 30 minutes to the observation. Adding WATA to a Fixed Slit observation will typically require between 8 and 15 minutes depending on the exposure setup, while total overheads for adding WATA to an IFU observation tend to be about 3 minutes higher than for the Fixed Slit template.
WATA specific considerations
WATA is available with the Bright Object Time Series (BOTS), Fixed Slits (FS), and Integral Field Unit (IFU) templates, and with the Micro-Shutter Assembly (MSA) template when observations are created directly instead of with the MSA Planning Tool (MPT). For fixed targets, the acquisition target and science target may differ, but for moving targets, WATA must directly acquire the planned science target.
WATA compactness requirements
NIRSpec TA algorithms acquire the target by finding the flux-weighted center of the brightest 3 × 3 pixel, (0.3'' × 0.3'' on the sky) checkbox in the 32 × 32 pixel region that covers the footprint of the 1.6'' × 1.6'' aperture. If the acquisition target is extended, predicting the results of WATA will require understanding the spatial structure of the source in the TA bandpass at a resolution comparable to or smaller than the 0.3'' checkbox size. This may require pre-existing HST or JWST imaging of the field. Note that even given such information, there are currently no tools available from STScI to perform a detailed simulation of the acquisition.
Acquisition target isolation
To ensure that the correct object is acquired, in the bandpass used for the acquisition exposures the acquisition target should be the brightest compact object within a radius of ~2''. That is, no nearby 0.3'' × 0.3'' region should be comparable to or brighter than the acquisition target. If it is necessary to observe a faint target close to a brighter object, either an offset WATA acquisition or MSATA should be used; although in many cases observing both objects simultaneously with the IFU may be the best approach.
Required accuracy of WATA acquisition target coordinates
To provide a high (> 99%) probability that the target is sufficiently within the S1600A1 aperture slit at the initial blind pointing to allow WATA to succeed, it is recommended that the acquisition target’s coordinates, including any corrections for proper motion, be on the ICRS system and accurate to at least 0.15'' 1σ radial, (or equivalently about 0.1'' 1σ per axis).
The pointing improvements supplied by any instrument target acquisition only apply as long as the same guide star is used to control the pointing. This normally means that a new acquisition is needed for each visit and observation, although in some circumstances the use of target groups may permit multiple observations in a sequence using the same guide star and a single initial target acquisition. For WATA observations, this means that the desired science pointings must all be within visit splitting distance of the target acquisition exposure and that at least one guide star must actually be available that can support all the needed spacecraft pointings.
WATA using an offset acquisition target
For targets where a direct acquisition using WATA is not an option either because the source is not in the required brightness range or is too extended, acquisition of a nearby offset star may provide an alternative. Note that use of an offset target is only allowed for fixed targets; this option is not available for moving targets.
Impacts on schedulability
Adding an offset target may require an additional slew or a longer slew between the target acquisition and the science exposures. This may reduce the number of guide stars available to support the visit.
Additional errors sources when using an offset TA target
Any relative error in the coordinates of the acquisition target and the science target will directly affect the centering of the science exposure. In addition, errors in the spacecraft roll will add a pointing error that scales with the angular separation between the two targets, e.g., for an expected 1 σ 60'' roll error, a 30'' target separation would lead to offset of the science target location in the NIRSpec aperture plane equivalent to 9 milli-arcsec.
Practical advice for choosing an offset acquisition target
There are three points to keep in mind when choosing an offset target:
Ideally, an HST or JWST IR image would be available that allows the relative coordinates of the two targets to be measured in the same frame, direct determination of the IR flux of the offset star, and verification that the intended offset star is really a simple point source. In practice, it will often be necessary to use catalog information to supply some of this information and to check the targets for significant proper motion.
The Aladin interface in APT allows information for a number of catalogs to be over-plotted on images of the target field and the JWST aperture planes and approximate separations between objects to be easily estimated. Available catalogs in this interface that may be especially useful for choosing an offset target include the 2MASS Point Source Catalog, which includes JHK photometry and accurate coordinates, (but not proper motions), and the Gaia-DR1 catalog which should contain highly accurate coordinates for more than a billion stars as faint as 21st magnitude, although proper motions are currently only available for a subset of these. A release of Gaia-DR2 is anticipated in the spring of 2018, and this will provide highly accurate position and proper motion data for most of these stars.
The JWST guide star catalog is in the process of combining much of the best catalog information for JWST operational purposes, and in principle, it is also a good source for coordinates and magnitudes of potential acquisition targets, although the current interfaces available in APT are not designed to use the GSC for this purpose. Once the Aladin display tools have been used to pick a potential offset star, it may be useful to check against the latest GSC version to see if the GSC has updated coordinate information for your acquisition target.
Acquisition strategies for each template
Bright Object Time Series template
For the BOTS template, the allowed TA options are WATA or NONE. Because good aperture centering is essential to achieve the desired photometric performance, and because spatial offsets much larger than 20 milli-arcsec can shift the zero-point of the wavelength scale by more than the nominal wavelength accuracy, WATA should be used if at all possible. If the science target is too bright to be directly acquired with WATA, the observer should attempt to find a suitable offset star to use for the acquisition. It is recommended against using the BOTS template with TA = NONE, as the blind pointing has a high chance of putting the target close to the edge of the 1.6'' × 1.6'' aperture; this advice might be revised if the on-orbit blind pointing performance is demonstrated to be significantly better than the minimal requirements.
Fixed Slits template
For the FS template, WATA, MSATA, and NONE are the available TA options. Normally pointing accuracy of 20 milli-arcsec or better will be desired with the FS template, and this will require a WATA or MSATA target acquisition. If using TA = NONE for an observation of a point source, even for the wide 1.6'' x 1.6'' S1600A1 aperture there is a significant chance that the target will fall close enough to the edge of the aperture to significantly vignette the PSF wings especially at long wavelengths, while for the smaller 200–400 milli-arcsec wide apertures there is a good chance the target will miss the aperture altogether. In addition, with TA = NONE there is no direct way to measure the displacement of the target in the dispersion direction, and this will cause significant uncertainty in the wavelength zero-point. For extended targets, more relaxed pointing requirements may sometimes be applicable depending on the details of the science scenario, and it may make consider to TA = NONE when WATA or MSATA is not possible, but otherwise use of TA = NONE with the FS template is not advised.
Choosing between TA options for NIRSpec FS observations
If WATA is not practical for a particular FS observation, then MSATA should be considered. This does require that a catalog of reference stars visible through the MSA be provided, and very accurate planning coordinate precision between these reference stars and the science object. Advantages of MSATA include:
Depending on the exact scenario, observers of extended targets may be willing to accept a pointing accuracy coarser than the normal 20 milli-arcsec accuracies expected from WATA or from MSATA with a highly accurate reference catalog. This might allow the use of MSATA with a reference catalog of coarser accuracy, or perhaps even the use of TA = NONE. For moving targets that are unsuitable for WATA, TA = NONE will be the only choice with the FS template. In such cases, the observer might consider whether an IFU observation with its coarser pointing requirements might be a better choice. If use of a narrow 200 or 400 milli-arcsec wide fixed slit is required for an extended fixed or moving target observed with TA = NONE, a mosaic in the dispersion direction could be used to map out the object, but this would need to be padded on each side by an amount sufficient to cover the possible pointing error. We recommend adding at least 0.5'' on each side in the dispersion direction for this purpose.
Integral Field Unit template
The IFU template supports the MSATA, WATA, VERIFY_ONLY, and NONE options for target acquisition. Many science observations will not require precise centering of the target at an exact position in the IFU ~ 3'' × 3'' FOV, and this can lead to relaxed pointing requirements relative to other NIRSpec templates. However, if considering blind pointing without a TA, it is still necessary to take into account the pointing accuracy, target size and dither pattern used. While choosing between WATA and MSATA involves many of the same considerations discussed above for the FS template, there are a few differences that should be considered.
Considerations for WATA acquisitions with IFU:
For MSATA acquisitions with the IFU:
IFU Observations without a TA:
The source size, target coordinate accuracy, dither pattern, and the wavelength of the observation will affect the needed pointing margins when designing an IFU observation using either the VERIFY_ONLYor NONE options which do not perform a target acquisition.
Even with perfect pointing, the two and four-point nod patterns put the target as close as 0.71'' from the aperture edge, reducing the available margin for source size and coordinate or pointing errors.
Table 1 gives an estimate of the required target coordinate accuracy as a function of the selected dither pattern and the desired minimum margin between each targeted position and the edges of the IFU aperture. This table is based on preliminary estimates of the JWST blind pointing accuracy and if in doubt, users should select one of the smaller dither patterns.
Table 1. Required target coordinate accuracy for dither patterns
If relying on blind pointing without a TA to observe a source that is small compared to the IFU field of view, the selected NIRSpec IFU dither pattern needs to allow room for the PSF width, the source size, and the accuracy of the target coordinates. For the preliminary estimate of the blind pointing performance discussed above, the minimum 1 σ radial target coordinate accuracy needed to give a 95% chance that all four dither positions will fall within the desired margin from the IFU edge is provided here. For the Cycling patterns, it is assumed that the 1st four points in each pattern are being used. Requirements for the success of 2-point nods or the 1st two points of the cycling patterns are similar to those for the 4-point patterns.
Using VERIFY_ONLY with the IFU:
This mode does not perform any target acquisition. Instead, a verification image is taken at the final science pointing with the IFU aperture open and a user selectable MSA configuration. In the undispersed light, the MSA quadrants and the IFU slices project onto different locations on the NIRSpec detectors and in principle this allows the IFU field of view to be precisely aligned relative to field objects imaged through the MSA, (although currently there are no tools to support this alignment in the standard pipeline processing).
Using TA = NONE with the IFU:
Multi-Object Spectroscopy template
Visits generated from a plan
MOS visits not generated from a plan
It is also possible to specify MOS visits directly using the MOS template rather than generating them with the MPT. Currently supported TA options for this case include WATA, VERIFY_ONLY, and NONE, but not MSATA.
Near Infrared Spectrograph (NIRSpec)
NIRSpec Observing Guidance
JWST Proposing Tools of Interest to NIRSpec Users
Related JWST Observing Methods