High-Resolution Fourier-Transform UV-Visible Spectrometer
Flex-pivot-supported interferometer for atmospheric trace species measurement
Richard P. Cageao, Jean-Francois Blavier, James P. McGuire, Yibo Jiang, Vassilii Nemtchinov, Frank P. Mills, Stanley P. Sander — Jet Propulsion Laboratory, Caltech
Overview
A compact, high-resolution Fourier-transform spectrometer for atmospheric near-ultraviolet spectroscopy has been installed at JPL's Table Mountain Facility. Designed with an unapodized resolving power near 500,000 at 300 nm, it provides high-resolution spectra from 290 to 675 nm for quantifying column abundances of trace atmospheric species — particularly the hydroxyl (OH) radical, a key intermediate in catalytic ozone destruction.
Key Highlights
Resolving Power 500,000
Unapodized at 300 nm — spectra from 290 to 675 nm
Flex-Pivot Interferometer
Parallelogram with eight Bendix flex pivots on 152-mm rolled Invar tubes
10 nm Position Accuracy
Two-sensor laser interferometer with 10 nm resolution
Active Tilt Compensation
Servo-controlled piezoelectric actuators maintain 0.1 mrad tilt over 5 cm travel
Heterodyne Laser Lock
Zeeman-split He-Ne reference — alignment without mechanical dithering
Solar Tracking
Heliostat with closed-loop CCD tracking of the Sun or Moon
Instrument System
The FTUVS system at Table Mountain Facility (34.4°N, 117.7°W, elevation 2290 m) comprises three subsystems: a heliostat for tracking the Sun or Moon, a beam-defining Gregorian telescope with aperture and field stops providing closed-loop tracking feedback, and the interferometer that records spectra. The telescope uses confocal off-axis parabolic mirrors coupled to an Invar metering rod for thermal stability, free from spherical aberration, coma, and astigmatism.
Flex-Pivot Interferometer
The interferometer is a flex-pivot-supported, voice-coil-actuated, active-aligned plane mirror design with an etendue of 2×10⁻⁴ cm² sr. Plane mirrors minimize reflections and wave-front distortion. The moving mirror mechanism is a parallelogram of four tubes constructed from 152-mm-thick rolled Invar for thermal and mechanical stability, connected by eight Bendix flex pivots. A voice coil drives the lower arm along an essentially frictionless porch-swing path, moving the mirror horizontally 5 cm.
Active Alignment & Calibration
Intrinsic mechanical alignment is better than 1 mrad over 1 cm of travel. To achieve 0.1 mrad tilt stability over 5 cm of travel (maintaining high fringe contrast at high spectral resolution), an active tilt compensation system uses a servo-controlled piezoelectric actuator on the fixed mirror. Three miniature p-i-n photodiodes sense phase mismatch in the He-Ne reference beam; a phase comparator drives piezostacks to null the error. The heterodyne (Zeeman) laser system achieves alignment lock without mechanical dithering. A He-Ne laser fringe marker triggers interferogram sampling.
OH Column Measurements
For hydroxyl column abundance measurements, a narrow-bandpass interference filter (75 mm diameter, centered at 307.8 nm, 45% peak transmission, 3.8 nm FWHM) is placed at the interferometer entrance. The instrument targets the A²Σ ← X²Π electronic transition of OH within the solar Fraunhofer structure. Measurements of terrestrial OH column abundances have been conducted since July 1997.
High-Resolution Fourier-Transform UV-Visible Spectrometer — PDF
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