Optical Design#
ZShooter receives the Keck I beam at the Right Nasmyth (RNAS) platform. ZFront feeds two fixed-format slit spectrographs covering 0.31–2.45 µm and a three-channel optical imager, with selectable paths to neighboring instruments. ZShooter operates in natural seeing and will use STRATA seeing enhancement when available.
RNAS interface#
The deployable tertiary, K1DM3, directs the telescope beam to ZShooter. The nominal f/14.5 focus lies 386.5 mm in front of the elevation-bearing journal, whose inner diameter is 1.79 m. ZFront, ZImager, and the two spectrographs are separately supported assemblies for independent alignment and removal.
The interface must accommodate facility guiding and STRATA wavefront-sensing optics alongside the ZShooter and neighboring-instrument light paths.
ZFront#
The front-end selectors provide three observing routes:
ZImager: a full fold sends the field to the imager.
ZSpec + ZImager: a fold with a central approximately 30″ opening passes the central field to the spectrographs while reflecting the surrounding field to the imager.
Neighboring instruments: the telescope beam is directed to positions left or right of ZShooter on the RNAS platform, at nominal 60° feed angles.
The ZCal launch enters upstream of the ZImager/ZSpec selector. An integrating sphere fed by emission-line lamps and continuum sources provides common calibration illumination. The design includes extended illumination for arcs and flats and an on-axis point source for registration, focus, and steering measurements. Lamp selection, brightness ranges, and launch uniformity remain to be finalized.
The internal source must cover the noddable spectroscopic field on every imager detector and spectrograph slit. Full-field ZImager flats use dome or sky illumination.
The optical-path diagram shows the component connections.
ZSpec summary#
The visible and NIR spectrometers each have an independent 10″ slit, ADC, K-mirror field derotator, slit viewer, and steering mechanism. Each asymmetric white-pupil echelle feeds three cross-dispersers and f/1.8 cameras. The gratings, dichroics, and cameras remain fixed during observing; the design requires no detector or echelle mosaics.
Spectrometer |
Channel |
Passband (nm) |
|---|---|---|
VIS |
B |
310–430 |
VIS |
G |
430–650 |
VIS |
R |
650–980 |
NIR |
YJ |
960–1,340 |
NIR |
H |
1,470–1,830 |
NIR |
K |
1,990–2,450 |
The three VIS focal planes use 2,048×4,096 qCCDs; the three NIR focal planes use 2,048×2,048 LmAPDs, all with 15 µm pixels. The NIR optics downstream of the entrance doublet operate near 100 K, with a rotating cold Lyot stop matched to the Keck pupil.
The current performance model gives median resolving powers of approximately 18,000 with a 0.70″ slit and 38,000 with a 0.33″ slit. The detector scale is approximately 0.157″/pixel; the narrow slit spans about 2.1 pixels. Resolution varies across each order through anamorphic magnification.
See ZSpec for the integrated optical layout, prescription, and throughput, and Performance for resolution maps and updated sensitivity calculations.
ZImager summary#
The September optical design uses a CaF₂ field lens, a common ADC, two dichroics, and three f/1.7 cameras. Each camera feeds an ORCA-Quest 2 qCMOS detector with 4.6 µm pixels at approximately 0.05″/pixel. The detector heads rotate independently for field derotation; focus and the ADC are common to all three channels.
The optical channels span 300–400 nm, 400–820 nm, and 820–1,000 nm. The middle channel selects 400–550, 550–690, or 690–820 nm filters. These correspond to fixed u- and z-like channels and a selectable intermediate band.
The simulator uses 4,096×2,304 pixels per detector, giving an approximately 3.5′×2.0′ footprint. The illuminated field and common coverage through detector rotation remain optical design checks. ZImager supports acquisition, spectral flux normalization, and simultaneous multiband time-series photometry. See ZImager for the render, ADC, image quality, and field checks.
Guiding and seeing enhancement#
Facility guiding provides telescope tracking; ZSpec slit viewers interface with MAGIQ for fine acquisition and provide the reference for independent VIS/NIR slit-relative guiding and nodding. The guider, imager channels, and slit viewers require calibrated coordinate transforms across the 30″ common field.
With STRATA correction, telescope defocus will match the NIR EE80 to the Nyquist slit width. The VIS/NIR relative-focus mechanism then adjusts the VIS image, balancing U-band losses against R-band gains. This focus strategy replaces the flat tip/tilt-error allowance in the older simulations. The saved sensitivity calculations still use the notebook’s broader effective AO PSF; the sharper STRATA estimates and focus prescription are described under Seeing Enhancement.
Calibration and co-alignment#
The calibration and acquisition procedures define the measurements needed to register the optical assemblies and preserve alignment during operation:
Common point-source references and steering maps for the three imager channels and two slit viewers, including slit footprints and selector return repeatability.
Wavelength and line-spread response versus source position, slit selection, rotator angle, and ADC setting.
Telescope, imager, and relative VIS/NIR focus curves, followed by on-sky scale, orientation, and guider registration.
Extended-source illumination and flat-field maps, with dome/sky comparisons for ZImager.
Optical interfaces to close
RNAS focus and envelopes: establish the allowed secondary-focus range and its aberration or vignetting penalties, including the focus and clearances needed by neighboring instruments and STRATA hardware.
ZFront and ZCal: finalize selector apertures, calibration-field coverage, point-source image quality, and lamp brightness ranges against the acquisition and calibration procedures.
Requirement/design differences: resolve the slit inventory and the ZImager fixed i-band requirement versus the current z-like red channel. The detailed trades and field checks are on the ZSpec and ZImager pages.