Seeing Enhancement#

STRATA, the planned adaptive-optics system using Keck I’s adaptive secondary, will concentrate light for ZShooter spectroscopy and imaging. ZShooter will also operate in natural seeing.

Performance overview#

Science path

Image-quality estimate

Field used in the AO model

ZSpec

Approximately 0.15″ FWHM in R–K, before deliberate defocus to match spectrograph sampling

On-axis correction

ZImager

Field-averaged FWHM of approximately 0.35–0.4″ at 0.31 µm, improving to approximately 0.2″ at 1 µm

A 3′-diameter circular correction field

The ZImager values are approximate readings of the September simulation curves below. The correction field differs from the camera’s detector footprint.

The baseline uses four laser guide stars in a square asterism: 15.2″ in diameter for ZSpec and 50″ for ZImager. It includes facility guiding without requiring a bright star for fast, on-axis tip/tilt sensing. Six lasers give little improvement in these cases. Fast tip/tilt correction gives its largest gain with an on-axis star in the narrow ZSpec field.

Matching the PSF to the spectrograph#

  1. Set the NIR reference with telescope focus. Defocus the telescope so the PSF’s EE80 matches the NIR Nyquist slit width.

  2. Tighten the VIS image with relative focus. Adjust the VIS/NIR relative-focus mechanism for the VIS slit, balancing U-band losses against R-band gains.

  3. Calibrate focus and slit coupling. Measure focus curves and wavelength-dependent slit throughput through the telescope and relative-focus calibration.

This replaces the flat tip/tilt-error allowance used in the older SPIE simulations. The AO plots below report EE50; the focus prescription needs EE80 evaluated directly from the PSFs for the slit geometry.

Considerations for ZShooter#

  • Simulation scope: median turbulence, 30° zenith angle, 600 Hz sampling, and 10-second simulations averaged over four turbulence seeds. Windshake and vibration are included; instrument implementation errors are not.

  • Guide stars: establish brightness limits, truth-sensing needs, and sky coverage for the optional fast tip/tilt correction.

  • Laser/WFS: 25 W-class lasers suffice at the modeled sodium return, wavefront-sensor throughput, and read noise. More lasers or faster sampling give limited gains when residual tip/tilt dominates.

  • RNAS dichroic: the modeled laser pickoff avoids vignetting the science field and helps reject Rayleigh backscatter. Its transmission, chromatic effects, wavefront error, and ghosts remain inputs to the ZShooter budgets.

  • PSF mapping: evaluate correction across ZImager’s actual bands and rotating detector footprint, and evaluate the VIS focus trade between U-band losses and R-band gains.

Simulation results#

ZSpec#

STRATA ZSpec simulations comparing laser return and read noise, four versus six lasers, and tip/tilt configurations across wavelength.

ZSpec on-axis predictions. Rows show FWHM, the square width enclosing 50% of the energy, and the slit width enclosing 50%. Columns compare laser signal/read noise, laser count, and tip/tilt configuration. Dashed black curves show the uncorrected case.

ZImager#

STRATA ZImager simulations showing field-averaged PSF widths and their variation across the modeled field, with weaker correction at shorter wavelengths.

ZImager predictions averaged over the 3′-diameter field, with the same metrics and comparisons as ZSpec. Error bars show variation across the field.

Sources

  • Lianqi Wang, STRATA Simulation Configuration and Results, 26 September 2026, version 2: slides 2–3, 11–17, and 34–35. Figures reproduced from slides 11 and 15.

  • STRATA MAOS Simulation Baseline Configuration and Trade Studies, 27 August 2026: performance evaluation, ZSpec and ZImager configurations, conclusions, and future work.

  • ZShooter instrument paper, SPIE 2026, Section 7 and Table 3: the earlier seeing-enhancement and natural-guide-star comparison. See Publications.

  • The 0.15″ ZSpec working estimate and telescope/VIS focus strategy follow the current instrument direction. The Performance numbers still use the saved notebook’s broader effective AO PSF.