Proceedings Article | 24 September 2011
Proc. SPIE. 8125, Optomechanics 2011: Innovations and Solutions
KEYWORDS: Observatories, Mirrors, Interferometers, Wavefronts, Wavefront aberrations, Adaptive optics, Relays, Finite element methods, Off axis mirrors, Prototyping
The Navy Prototype Optical Interferometer (NPOI), located near Flagstaff, Arizona, is a ground-based interferometer
that collects and transports stellar radiation from six primary flat collectors, known as siderostats, through a common
vacuum relay system to a beam combiner where the beams are combined, fringes are obtained and modulated, and data
are recorded for further analysis. The current number of observable stellar objects can increase from 6,000 to
approximately 47,000 with the addition of down-tilting beam compressors in the optical train. The increase in photon
collection area from the beam compressors opens the sky to many additional and fainter stars. The siderostats are
capable of redirecting 35 cm stellar beams into the vacuum relay system. Sans beam compressors, any portion of the
beam greater than the capacity of the vacuum transport system, 12.5 cm, is wasted. Engineering analysis of previously
procured as-built beam compressor optics show the maximum allowable primary mirror surface sag, resulting in λ/10
peak-to-valley wavefront aberration, occurs at 2.8° down-tilt angle. At the NPOI operational down-tilt angle of 20° the
wavefront aberration reduces to an unacceptable λ/4. A design modification concept that reduces tilt-induced sag was
investigated. Four outwardly applied 4-lb forces on the rear surface of the mirror reduce the sag from 155 nm to 32 nm
at 20° down-tilt and reduce peak-to-valley wavefront deviation to λ/8.6. This preliminary effort indicates that this
solution path is a viable and economic way to repair an expensive set of optical components. However, it requires further work to optimize the locations, magnitudes, and quantity of the forces within this system and their influence on
the mirror surface.
