Exoplanet spectroscopy at the diffraction limit.

Injecting a point source into a few microns of glass

On the Nasmyth platform behind the upgraded HAKA AO, the FEI acquires the target, stabilizes it with a fast steering mirror and infrared tracking camera, reshapes the beam with PIAA optics, and couples it into single-mode fibers — on-axis, on a faint companion, or through a nulling mask.

Light arrives through the FEI pickoff and meets a compact train: an atmospheric dispersion corrector keeps all four bands stacked on the fiber tip, the fast steering mirror closes its loop against the tracking camera overhead, and a selector wheel drops pupil masks — including the nulling masks — into the beam. PIAA lenses then remold Keck’s segmented pupil into a near-Gaussian beam before the coupling lenses and fiber alignment mechanisms take over.

An internal light source and retro-illuminated fiber beacons let the whole chain be aligned and calibrated without touching the sky — the key to fast, automated target acquisition once the unit is installed behind HAKA.

• Throughput 57–62 % measured y–K — beats the 55 % spec
• Coupling 68.7 / 73.0 % (y / H) — PIAA boost, near theory
• ≤ 2 mas RMS tip/tilt — FSM + tracking camera
• ADC + mask selector — wide-band correction, nulling

Sixty-five meters, summit to basement

A ~65 m run of single-mode fibers carries the light from the Nasmyth focus down to the basement: silica for y and J, custom ZBLAN fluoride fiber for H and K. Mechanical switchers — carrying Subaru-IRD heritage from the Japanese collaboration — plus MEMS switchyards and a photonic lantern reconfigure science, sky and calibration paths.

Every connection is engineered to stay pristine over thousands of reconfigurations: the sealed, pressure-regulated switcher units carry their own stick-cleaner banks, an inspection microscope and 3D-printed fiber guides inside. From the Nasmyth platform, the bundles thread the telescope’s cable wraps and the Coude tunnel before landing at the trunk interface box in the basement lab.

• < 2 % loss per connector on the science bundles
• ZBLAN HK fiber meets its 65 m transmission requirement
• 48 switches (26 silica + 22 ZBLAN) received & tested
• 3-port photonic lantern — > 90 % throughput, y & J

The backbone of centimeter-per-second velocimetry

Two laser frequency combs, gas absorption cells and a U/Ne hollow-cathode lamp anchor the absolute wavelength solution, while two astro-etalons track drift continuously. Split into BCAL (yJ) and RCAL (HK), the references inject at the spectrometers or all the way up at the front end — tracing the full optical path.

Each rack pairs an absolute reference with a continuous one: the frequency combs and lamps pin the wavelength solution, while temperature-stabilized etalons — kept under vacuum by ion pumps, held by precision temperature controllers and cooled through a glycol heat exchanger — track instrumental drift between comb exposures. Running on sky at Palomar against PARVI is retiring the risk long before the racks reach Maunakea.

• 0.14 – 0.34 m s⁻¹ etalon uncertainty per order (H / J)
• 2 LFCs + 2 etalons · gas cells · U/Ne hollow-cathode lamp
• Racks integrated — on sky at Palomar for validation
• Dual injection points — spectrometer and front end

Cryogenic echelles on Invar

Split at the FEI dichroic upstairs, the yJ and HK channels arrive on separate fibers at BSPEC and RSPEC in the basement laboratory: each an Invar-36 bench carrying a three-mirror-anastigmat collimator and camera, a gold-coated R4 single-crystal germanium echelle and a cross-disperser — cooled by a PT-60 cryocooler behind multi-layer insulation and an actively controlled radiation shield.

Inside each vessel the fiber outputs are collimated by a three-mirror anastigmat, dispersed by the echelle — orders 59 to 97 in RSPEC — cross-dispersed by a −2 to +2-order grating, and reimaged by a matching TMA camera onto the detector. Everything bolts to an Invar-36 bench on G10 A-frames, thermally isolating the optics inside the gold-shielded vacuum space.

• All optics & cryostats fabricated — integration under way
• R4 echelle — single-crystal Ge, direct-ruled by Canon, gold-coated · 13 nm rms WFE · 96 % of theoretical diffraction efficiency
• Invar-36 benches · TMA collimator & camera
• PT-60 cryocoolers · MLI · active radiation-shield control

From photons to spectra

Each camera focuses the cross-dispersed orders onto a Teledyne H4RG-10 — an engineering spare from the Roman Space Telescope — read through light-tight feedthroughs by Archon controllers. Up front, a JWST-heritage H2RG tracking camera closes the tip/tilt loop at up to 1 kHz.

The two science arrays were characterized side by side before installation — 4.95 and 7.02 e⁻ CDS read noise, 93 and 99 % quantum efficiency for the BSPEC and RSPEC devices — sampled up the ramp for jump-free, low-noise integrations. The tracking camera is a small instrument in its own right: a four-channel H2RG behind a cold snout and K-blocking filter, windowing on the target at up to 1 kHz over its 6″ field.

• 2 × H4RG-10 arrays at Caltech — < 7 e⁻ noise spec
• ATC: 13 e⁻ CDS · QE 80 % · 75 K · 1 kHz windows
• ATC cold since July 2025 — final characterization
• Science-cryostat verification — fall 2026

From 3.2 TB of raw reads to radial velocities

The Python pipeline turns raw up-the-ramp detector reads into science-ready spectra and radial velocities in three stages — with a calibration database synced to the Keck Observatory Archive and quick-look feedback in the dome while you observe.

Stage 0 alone runs a full detector-calibration chain — saturation and jump detection, bias, reference-pixel and 1/f correction, non-linearity, dark, gain and flat — before the maximum-likelihood ramp fit turns each read cube into a rate map. Downstream, wavelength solutions accurate to tens of m/s, simultaneous-etalon drift correction and telluric treatment deliver spectra ready for cross-correlation radial velocities.

• Quick-look L2 in ≤ 2 min of readout — goal 10 s
• ML ramp fitting — ~9.5 % over least squares
• 3 fibers × 40 + 39 orders extracted per exposure
• Full processing < 24 h — products archived at KOA