JWST Pointing Performance

JWST's pointing performance for slewing accuracy and pointing stability, based on actual performance during observatory commissioning and early operations, is covered in this article.

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See also: JWST Slew Times and Overheads

JWST's pointing accuracy, target tracking capability, and pointing stability are all the result of complex interactions between the observatory's hardware performance and flight software. Pointing performance results from the interactions between star trackers, gyroscopes, reaction wheels, fine sun sensors, the fine steering mirror, the fine guidance sensors, multiple target acquisition modes (for different science instruments), flight software (both spacecraft and science instruments), ground software, guide star selection, and observation scheduling details. The spacecraft's attitude control system (ACS) controls the pointing and slewing of the JWST observatory. The Fine Guiding System works with the science instruments for the final location and tracking of science targets. This page summarizes the actual pointing performance, based on results from commissioning and post-commissioning operations. 


Definitions and units

Pointing accuracy is given in units of either arcseconds(") or milliarcseconds(mas), and expressed as 1-σ radial. 


Absolute pointing accuracy after guide star acquisition

The absolute pointing accuracy, without a science target acquisition, is generally better (smaller) than 0.19" (1-σ, radial). When systematic errors are removed between the FGSs and the science instruments, the residual scatter around the desired target position is usually below 0.10" (1-σ error, radial). This uncertainty is dominated by guide star catalog position errors, which continue to be reduced as guidelines for selecting guide stars are refined. Pointing repeatability is excellent; independent observations at the same target and same position angle usually result in pointings identical to within 0.10". These results confirm expectations that many NIRSpec IFU observations will be able to omit target acquisition, and rely only on guide star acquisition.


Pointing accuracy after target acquisition

Observations requiring better final pointing accuracy than 0.10" (e.g., coronagraphic spot (null) placement, repeatable position for NIRISS aperture masking interferometry (AMI), or single object slitless spectroscopy (SOSS)), will need to use onboard target acquisition. Although each instrument mode uses a different target acquisition process, all modes typically achieve target positions within a few mas rms per axis (NIR instruments), or slightly larger rms for MIRI, which has larger PSFs. For NIRCam coronagraphy, the recommended small grid dithers yield excellent coronagraphic contrast. 

Target acquisitions for NIRSpec multi-object spectroscopy are the most challenging, because they derive a roll correction for the observatory's position angle in addition to X, Y position offsets. Although the procedure requires careful attention to implement, it performs well with accurate input data. 

All modes requiring target acquisition meet or exceed their respective pointing requirements.


Guiding precision and pointing stability

For fixed targets, pointing stability is evaluated as the rms error in the guide star position in a 15 s interval, compared to the mean position over a 10,000 s observation. 

Line-of-sight (LOS) pointing stability under fine guiding control is superb. The Fine Guiding Sensor (FGS) sensing precision is parameterized as the noise equivalent angle (NEA, an equivalent jitter angle based on centroid position and SNR), and is usually ~1 mas (1-σ per axis), symmetric. The LOS jitter seen in the science instruments is similarly good, and high frequency sampling of the jitter in a NIRCam small subarray is the same, ~1 mas (1-σ per axis). Although a drift in observatory roll could theoretically generate image drift in a long duration observation, measurements during commissioning indicated such drifts are negligible. 

The below figure shows a typical jitter profile. This particular dataset was obtained from the first jitter analysis performed during commissioning (PID 1163, observation 2, reference JWST-STScI-008271).

Fig. 1.  Centroid measurements from a commissioning observation

Centroid measurements taken during a two-minute long commissioning observation using NIRCam's high-cadence 8x8 subarray, showing a symmetrical RMS jitter of 1.12 mas.
Fixed target guide stars are chosen with 2MASS J-band (Vega scale) brightnesses 12.5 < J < 18; brighter stars are preferred. Occasionally, guide stars are found to be fainter than predicted, leading to an increased NEA. This could be due to unflagged bright pixels, flagged bad pixels coincident with a guide star, mistakenly guiding on a slightly extended object, or a guide star at edge of the centroiding box in the FGS. Work is ongoing to reduce these variations.  

For Solar System (i.e., moving) targets, the line-of-sight pointing stability is evaluated as the rms value over a 1,000 s observation for a linear motion of 3.0 mas/s, and is estimated to be 6.2 to 6.7 mas (1-σ per axis, depending on the  science instrument). Moving target guide star faint limits are J > 16.5, and 17.0 in FGS 1 and 2, respectively.  



Dither precision

See also: JWST Dithering Overview

Dithers (small repointings) less than 60 mas are executed with the fine steering mirror while maintaining closed-loop guiding on the guide star. Dithers between 60 mas and 25" are executed by dropping closed-loop guiding, slewing, and re-entering guiding at the track mode stage. Dithers larger than 25" are executed by dropping closed-loop guiding, slewing, and then restarting guiding at the guide star acquisition stage. The accuracy for all 3 types of dither are usually 2–4 mas rms (1-σ per axis) on the sky. For larger dithers the offset accuracy may be a few mas larger due to residuals in the astrometric calibration of a particular science instrument.



Tracking moving targets

See also: Moving Target Acquisition and Tracking

JWST was launched with a requirement to track objects at speeds up to 30 mas/s. During commissioning, tracking was demonstrated at rates up to 67 mas/sec. All tests of moving targets were successful: centroids showed subpixel scatter in all instruments, jitter rms for the guider was typically <2mas (1-σ, radial), and image quality was comparable to that of fixed targets. Tracking at rates 30–70 mas/s had similar accuracy to that seen with lower rates. These tests verified science instrument performance for moving targets, including dithering and mosaicking. 

During Cycle 1, tracking has been successfully demonstrated at rates up to 110 mas, but with restrictions on exposure length. Beginning with Cycle 2, tracking at rates up to 75 mas/s will be supported by APT without special approval; shared-risk observations at rates 76–100 mas/s will be allowed, but only with careful coordination and approval from STScI moving target experts and operations support staff.



References

Lallo, M. & Hartig, G. F. 2022, JWST-STScI-008271 
JWST Line-of-Sight Jitter Measurement during Commissioning 

Gardner, J. P. et al. 2006, Space Sci.Rev., 123, 485 
The James Webb Space Telescope




Latest updates
  •   
    Updated with information for on-orbit performance from commissioning and early operations.

  •  
    In "Definitions and Units," conversion between 1-σ uncertainty per axis and 1-σ radial uncertainty corrected for Rayleigh criterion


  • Added radial uncertainties
Originally published