We present an approach to upgrading the beam transport system at the Navy Precision Optical Interferometer. These upgrades together will provide consistent beam transport, improve fringe contrast by preserving beam wavefront, reduce tracking errors by increasing the frequency response of the tracker, and automatically realign the entire transport train after thermal drift over the course of nightly observations. The beam transport system passively redirects stellar light from the telescope output to the fast delay line through a train of flat mirrors. This multi-mirror transport train reduces wavefront preservation due to stack-up of surface flatness errors. We demonstrated previously by using a contour-conformable mirror instead of one of the flats in the train that a 63% improvement in wavefront flatness is achievable. Also, the 25 Hz tracker is replaced by a 100 Hz tracker to further stabilize the trajectory during observations. Finally, we include an auto-aligner to systematically realign the entire beam transport system from thermal drifts. This is necessary for long baseline interferometry with short drift time constants. The beam transport system is common to all front ends (telescopes and siderostats), beam delay, and back-ends (beam combiners and detectors). These three upgrades expand the utility of the NPOI from a relatively short 97 m baseline interferometer to its full reconfigurable 437 m baselines and allow consistent beam transport with various potential experimental telescope front ends and beam combiner back-ends. In this paper, we describe our three-pronged upgrade approach, experimental method and results, and recommendations.
We present the results of analysis of a 6 in. diameter vacuum window. The design prevents contact between the metallic mounting cell and the glass window material, which can be a source of failure in the glass. The window transmits optical and infrared wavelengths for multi-baseline interferometry at the Navy Precision Optical Interferometer (NPOI). Our analysis investigates possible interactive contact with the mounting cell and consequent failure of the window. Our design philosophy is to avoid overconstraining the glass window and maintain integrity of the vacuum seal through diurnal and seasonal temperature changes. The objective is to create no additional crack nucleation sites or initiate cracking in the brittle material due to the mount design. The window is unconstrained laterally and free to expand radially. Furthermore, the glass is free to expand in the thickness direction over the expected temperature range. The lack of contact due to thermal expansion over a broad temperature range and bending stresses due to loading were calculated to ensure the integrity of the assembly. In this paper, we describe our design approach, method of analysis, results, and recommendations. Our analysis shows that a simply supported window can be designed to achieve no metal to glass contact.
The concavity of an initially flat wavefront typically increases after each reflection of the ten-reflection beam transport system at the Navy Precision Optical Interferometer (NPOI). Ideally, the exiting wavefront contour from the beam transport system preserves the original contour that enters. The beam transport system is common to and separate from the front-end, which includes primary light collectors such as siderostats or telescopes, and the back-end which includes major subsystems such as the optical delay lines, beam combiners and detectors. The beam transport system should have minimal influence on the interferometer. However, manufacturing tolerances and mount-induced deformations of each mirror collude to alter each reflected wavefront. All beam transport mirrors at the NPOI are slightly concave and each reflection adds to the concavity in the resultant wavefront. To improve the flatness of the resultant wavefront, we counter-deform a single mirror in the ten-reflection transport system. Previous analytical work using finite element analysis demonstrated the feasibility of this approach. In the present work, we have undertaken the task of verifying this approach experimentally. We set up a nine-reflection system of NPOI transport mirrors and measured the resultant beam wavefront contour. We applied a single actuator to the backside of one of the mirrors in the system and measured the contour of the exiting wavefront. Additionally, we compared the reduced concavity of the exiting wavefront to our finite element method results from the previous work, and excellent agreement was observed. In this paper, we describe our wavefront improvement approach, experimental method and results, and recommendations.
We describe a compliant static deformable mirror approach to reduce the wavefront concavity at the Navy Precision Optical Interferometer (NPOI). A single actuator pressing on the back surface of just one of the relay mirrors deforms the front surface in a correcting convex shape. Our design uses the mechanical advantage gained from a force actuator sandwiched between a rear flexure plate and the back surface of the mirror. We superimpose wavefront contour measurements with our finite element deformed mirror model. An example analysis showed improvement from 210-nm concave–concave wavefront to 51-nm concave–concave wavefront. With our present model, a 100-nm actuator increment displaces the mirror surface by 1.1 nm. We describe the need for wavefront improvement that arises from the NPOI reconfigurable array, offer a practical design approach, and analyze the support structure and compliant deformable mirror using the finite element method. We conclude that a 20.3-cm-diameter, 1.9-cm-thick Zerodur® mirror shows that it is possible to deform the reflective surface and cancel out three-fourths of the wavefront deformation without overstressing the material.
We compare a design innovation of an elliptically framed tip-tilt optical tracker with an existing circularly framed tracker for the Navy Precision Optical Interferometer. The tracker stabilizes a 12.5 cm stellar beam on a target hundreds of meters away and requires an increase in operational frequency. We reduced mass and size by integrating an elliptical mirror as one of the rotating components, which eliminated a rotating frame. We used the same materials as the existing tracker; however, light-weighted both the aluminum frame and Zerodur® mirror. We generated a computer-aided design model, converted it into a finite element model and performed modal analysis on two load cases. In load case 1, we tied down three points on the bottom surface of the tracker corresponding to the tie-down points of the comparison tracker. This reveals a first mode (lowest) frequency of 140 Hz, a factor of two over the baseline tracker’s first mode frequency of 67 Hz. In load case 2, we constrained four additional points inboard of the corners of the tracker base, for a total of seven tie-downs, simulating a firmly bolted and secured mount. The first mode of vibration for this case is 211 Hz, an increase over load case 1 by a factor of 1.5 and more than three times the fundamental frequency of the existing tracker. We conclude that these geometrical changes with the additional tie-down bolts are a viable solution path forward to improve steering speed and recommend a continuation with this effort.
We present our analysis methodology for a 20.3 cm prototype optical tracker to determine why instabilities occur below 50 Hz and suggest improvements. The Navy Precision Optical Interferometer makes use of six small optical telescope stations spaced along a Y-array to synthesize an equivalent single larger telescope. Piezoelectric-driven optical trackers steer 12.5 cm output beams from each station to an optics laboratory up to 700 m distant. A percentage of this starlight is split off and used in a closed-loop feedback to update the pointing of the telescope and steering of the tracker. Steering stabilizes atmospheric induced beam trajectory deviations, required for fringe generation. Because of closedloop feedback, we require all fundamental frequencies to be at least 3 times the desired operational frequency, or 150 Hz. These trackers are modified commercial aluminum gimbal mounts with flex-pivot axles and very small damping ratio. Steering is tip/tilt mirror rotation by push-only actuators and a return spring. It is critical contact be maintained between actuator, mirror mount and return spring. From our dynamic analysis, the 122 N return spring is 2.9 times that required, and has a natural frequency equal to 238 Hz. The range of steering, 140 microradian, is double that required and the 0.077 microradian precision is 2.6 times that required. The natural frequency of the tracker is 66 Hz and the tuned closed-loop operational frequency is only 22 Hz. We conclude the low fundamental frequency of the mount limits its performance below 50 Hz and stiffening the structure is required.
The Navy Precision Optical Interferometer, located near Flagstaff, Arizona, is a ground-based interferometer that collects, transports, and modulates stellar radiation from up to six primary flat collectors, known as siderostats, through a common vacuum relay system to a combiner. In the combiner, the modulated beams are superimposed, fringes obtained, and data recorded for further analysis to produce precise star positions or stellar details. The current number of observable stellar objects for the astrometric interferometer can increase from 6000 to at least 47,000 with the addition of full-aperture 20-deg down-tilting beam compressors in each optical train. Such an aperture increase, from the current 12.5 to 35 cm, opens the sky to many additional and fainter stars. Engineering analysis of our beam compressor primary mirror shows that the maximum allowable sag, 21 nm, occurs prematurely at 2.8-deg down-tilt angle. Furthermore, at the operational down-tilt angle of 20 deg, the wavefront deformation increases to 155 nm. We present a finite element analysis technique and design modification concept to reduce tilt-induced deformation on the mirror surface. This work is a first pass to determine the feasibility for a mechanical solution path forward. From this analysis, we found that four outwardly applied 17.8-N forces on the rear surface of the mirror could reduce sag from 155 to 32 nm at 20-deg down-tilt angle.
The Navy's ground-based optical interferometer requires 10 discrete reflections for each of its six stations that transport stellar radiation into a six-way beam combiner where the modulated beams are overlapped in a collinear fashion and fringes obtained for analysis. Wavefront aberrations, introduced at each reflection from non-perfect mirrors, reduce the quality of fringe contrast and adversely affect the final science results. In practice, mirror fabrication and mounting methods generate small surface irregularities that produce aberrations in the reflected wavefront beam. Under multiple reflection scenarios, these errors do not necessarily cancel one another, and can increase the resultant wavefront distortion. In a previous paper, we showed a single-force actuator acting on the back surface of an 8-inch diameter Zerodur® mirror will achieve a canceling deformation in the reflective surface that substantially reduces the combined wavefront aberrations resulting from a 7-reflection beam. Our finite element model demonstrated that the peak-to-valley difference can be reduced from 210 nm to 55 nm. In this paper, we extend our previous work to include a support structure to contain the deforming mirror and analyze its interaction and effect on the corrected wavefront. Our design used the mechanical advantage gained from a tuned flexure plate with a simple motorized screw actuator applied to the back mirror surface to achieve an 87:1 deflection ratio on the front mirror surface. A practical design is proposed, the support structure and mirror analyzed using the finite element method, and the results presented and discussed.
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.
The Navy Prototype Optical Interferometer (NPOI) array, located near Flagstaff, Arizona, transports 12.5 cm diameter
stellar radiation simultaneously from six primary collectors through a 9,000 cubic foot vacuum relay system prior to
entering a specialized laboratory where further manipulations of each beam occur. The relay system redirects each 12.5
cm beam 10 times. Ground-based optical interferometry requires very high quality, ideally flat, relay mirrors. The
mirrors used in the relay system have flatness deviation tolerance 32 nm peak-to-valley over the 18.3 cm clear aperture.
Once mounted in the 10-element optical train, errors from each mirror tend to stack up and increase the resultant
wavefront distortion for that path. This leads to reduced fringe contrast, fringe tracking, and number of observables. In
a previous paper, it was shown that it is possible to mitigate the resultant wavefront distortion by using a phase-shifting
interferometer combined with a single compliant static deformable mirror and control system. In that work, the mirrors
tested showed a fairly uniform, concentric concavity deformation, which a single, centrally located actuator may
significantly improve. In this paper, we extend the previous analysis to consider an off-center actuator acting on a mirror
to counteract an asymmetric wavefront distortion resulting from the superposition of several relay mirrors. The shape
applied to a single corrector mirror was determined from the resultant wavefront distortion of a 7-reflection optical relay
system using phase-shifting interferometer data. Finite element analysis results indicating how resultant wavefront error
from a collection of slightly deformed mirrors can be cancelled are presented and discussed.
The Navy Prototype Optical Interferometer (NPOI) near Flagstaff, Arizona, makes use of separate smaller optical
elements spaced along a Y-array and used simultaneously to simulate an equivalent single large telescope. The
instrument is useful in generating and upgrading existing astronomical catalogues and investigating synthetic aperture
optical imaging techniques. The NPOI is a joint collaboration between the US Naval Observatory and Naval Research
Laboratory in collaboration with the Lowell Observatory. Stellar radiation (visible light) reflects off 35 cm diameter flat
mirrors, also known as siderostats, toward a tilt-tip mirror, which reflects a 12 cm diameter beam through a multi-reflection
relay transport system. To maximize the reflective area of the siderostat optics and achieve an increase by a
factor of 8.5 in light collecting area, a beam compressor is to be installed between the siderostat and fast tip/tilt mirror.
However, the present configuration of a prototype beam compressor mount (BCM) vibrates at unacceptable amplitudes,
which makes it nearly impossible to optically align the mirrors. This paper presents the results of finite element analyses
conducted to quantify the design limitations of the prototype beam compressor mount. The analyses indicated that the
current configuration is too soft, with very low fundamental frequencies, which verified the difficulties encountered
during alignment tests. Based on these results, design modifications have been proposed to increase the overall
structural stiffness of the mount and increase its fundamental frequency of vibration. These modifications will
mechanically stabilize the structure for the alignment of the optics, and allow integration of the compressor into the
interferometer. The interferometer will then have the capability to capture more light from each siderostat and allow
observations of fainter stellar targets. More generally, the results can be useful as a guide for engineers and scientists
involved in the design of similar optomechanical structures.
For ground-based optical interferometry, the simple specification of high surface quality flat relay mirrors is not the end
of the story for obtaining high quality fringes. The Navy Prototype Optical Interferometer array transports the stellar
radiation from six primary collectors through a 10-reflection vacuum relay system, resulting in six separate combinable
wavefronts. The surface error of each of the 60 relay mirrors is specified to be no greater than 32 nm peak-to-valley for
fabrication purposes. However, once mounted in the 10-element optical train the errors from each mirror do not
necessarily cancel one another, but can add and increase the resultant wavefront distortion for that path. This leads to
fringe contrast reduction, reduced ability to fringe track, and a reduction in the limiting magnitude of observable objects.
Fortunately, the total wavefront distortion for each train can be measured, calibrated, and nullified by using a phaseshifting
interferometer combined with a compliant static deformable mirror and control system. In this paper we
describe a system that mitigates the resultant wavefront distortion.
Surface flatness of 6-inch diameter mirrors at the Navy Prototype Optical Interferometer is specified to be within 32
nanometers over a 5.4-inch diameter circle centered on the mirror. The current mounting technique is to use three spring
plungers applied to the back surface of the mirror, near the perimeter edge, thereby pressing the front surface against
three small diameter Teflon® pads directly opposite the plungers. The pads have the effect of dissipating the deformation
effects within the 5.4-inch diameter region. This paper describes the effects of varying the size of the pads, from a 7/32
inch diameter pad to a point-type contact such as a ball bearing. Experimental results using a phase shifting
interferometer are presented, as well as finite element analysis results.
The Navy Prototype Optical Interferometer (NPOI) in Flagstaff, Arizona, makes use of separate smaller telescopes
spaced along a Y-array and used simultaneously to simulate an equivalent single large telescope. The performance of
the NPOI can be improved by increasing the steering response of the 8-in. diameter Narrow Angle Tracker (NAT). The
mirrors of the NAT correct the image position for atmospherically induced motion. The current tracker has a slow
response due to the low fundamental frequency of the mount and limits the quality of the data. A higher frequency will
allow a faster servo feedback to the steering mirror, which will enhance the tracking performance on stellar objects
resulting in final fringe data of higher quality. Also, additional and fainter objects could be observed with a faster
response system, and the interferometer as a whole would be less sensitive to fluctuations in atmospheric quality.
Improvements in the NAT performance over the current cast aluminum frame and glass mirror were achieved by the use
of advanced composite materials in the design of the frame and mirror. Various design possibilities were evaluated
using finite element analysis. It was found that the natural frequency of the NAT can be increased from 68 to 217 Hz,
and the corresponding weight decreased by a factor of 5.6, by using a composite mount with a composite mirror.
The preservation of mirror surface quality and figure are of paramount importance at the Navy Prototype Optical
Interferometer. There are on the order of 108 eight-inch optical flats mounted in the interferometer's optical train, 102 of
which are permanently mounted inside the 9000 cubic foot vacuum feed system. The flats are specified for manufacture
at λ/20 peak-to-valley surface variation (λ = 633 nm) over a 7.2 inch clear aperture. Silver coating with a dielectric
overcoat is subsequently applied to the reflecting surface. The objective when mounting the mirror is to preserve the
surface quality and figure of the coated flats as much as possible. Surface deflections occur due to pressure points
inherent in the mount. The mount consists of a modified commercially available tangent-arm gimbaled-type structure.
In order to minimize the mounting effects and allow for a wider thermal operational range, modifications were made to
the primary mirror cell in the following areas: edge support region, front face tabs, rear face loaders, and diameter. In
this paper we describe the detailed cell modifications, a finite element analysis (FEA) of the mounted flat, the free-standing
and as-mounted surface figure of a typical eight-inch diameter flat as measured with a phase-shifting
interferometer, the resulting mount-induced deflections, a comparison between the measured and FEA model, and
The Navy Prototype Optical Interferometer (NPOI) in Flagstaff, Arizona, makes use of separate smaller telescopes
spaced along a Y-array and used simultaneously to simulate an equivalent single large telescope. Each telescope is
mounted on a massive reinforced concrete pier tied to bedrock. The mass of the pier dampens most, but not all, of the
unwanted vibration in the required spectrum. The quality and resolution of a stellar image depends on minimizing
movement of the mirrors due to vibration. The main source of pier vibration is due to the soil-pier interaction.
Surrounding environmental and man-made vibration propagates through the soil as body and surface waves, and forces
the pier to move. In this paper, a novel concept based on a sleeve/air gap system to isolate the soil from the pier is used
to minimize the vibration input to the telescope. An example of the concept is presented with respect to the future
implementation of a 1.4-m diameter composite telescope at the Navy Prototype Optical Interferometer.
The use of composite materials in the fabrication of optical telescope mirrors offers many advantages over conventional
methods, including lightweight, portability and the potential for lower manufacturing costs. In the construction of the
substrate for these mirrors, sandwich construction offers the advantage of even lower weight and higher stiffness.
Generally, an aluminum or Nomex honeycomb core is used in composite applications requiring sandwich construction.
However, the use of a composite core offers the potential for increased stiffness and strength, low thermal distortion
compatible with that of the facesheets, the absence of galvanic corrosion and the ability to readily modify the core
properties. In order to design, analyze and optimize these mirrors, knowledge of the mechanical properties of the core is
essential. In this paper, the mechanical properties of a composite triangular cell core (often referred to as isogrid) are
determined using finite element analysis of a representative unit cell. The core studied offers many advantages over
conventional cores including increased thermal and dimensional stability, as well as low weight. Results are provided
for the engineering elastic moduli of cores made of high stiffness composite material as a function of the ply layup and
cell size. Finally, in order to illustrate the use of these properties in a typical application, a 1.4-m diameter composite
mirror is analyzed using the finite element method, and the resulting stiffness and natural frequencies are presented.
Engineering specifications for O-ring seal surfaces are well documented. However, when seal surfaces are located
on asymmetrically loaded vacuum end-plates, consideration must be given not only to surface finish and
flatness, but also to load-induced deflections. When deflections are significant, O-ring compression can relax
and potentially cause vacuum leaks. Large vacuum systems, such as the 9000 cubic foot system at the Navy
Prototype Optical Interferometer (NPOI), cannot afford costly vacuum leaks due to improper end-plate design.
The NPOI employs vacuum end-plates that serve both as structural members, and as vacuum system entrance
and exit ports for stellar light. These ports consist of vacuum components attached directly to the end-plate via
static O-ring sealing techniques. Optical geometry dictates off-center port locations, which create asymmetric
end-plate loading. This paper details the behavior of a 22 inch diameter, multi-port, end-plate for the NPOI Fast
Delay Line subsystem. In depth CAD modeling and finite element analysis techniques were used to determine
load-induced stress distributions and deflections in the end-plate. After several design iterations, an end-plate
design was substantiated that maintains vacuum seal integrity under loading, exhibits a conservative factor of
safety, and is readily manufacturable.
Many aspects must be considered in the design of telescope enclosures. One critical aspect is the floor sensitivity to
movement. The floor moves due to floor-foundation interaction, floor-wall interaction, soil-floor interaction, and
internal enclosure loads. This paper presents the details of the design of an environmental enclosure floor having
minimum rotation due internal laboratory equipment loads, which can have a significant effect on the deformation of the
floor. Floor analysis is presented by finite element methods. An example of a floor design is presented in the context of
a future Navy Prototype Optical Interferometer (NPOI) environmental enclosure.
Reconfigurations of the original optical mounts are required to facilitate the expanding capabilities and diverse science
programs at the Navy Prototype Optical Interferometer. The mounts of current interest are tangent-arm gimbaled mounts
located in vacuum chambers, remotely controlled, and precisely aligned through a narrow range of motion. In order to
achieve the desired large changes in pathway reflections, the articulated range of the mount was increased from 4 to 45
degrees in elevation and from 4 to 90 degrees in azimuth. This increase was achieved on the elevation axis by fashioning
and attaching a worm gear device, and a direct-drive type mechanism was used on the azimuth axis. The original
alignment resolution and stability were preserved by retaining the high precision tangent-arm actuators. In this paper, we
present the design modifications that achieved the form, fit, and function required for remote-controlled reconfiguration
and alignment. The mechanical modifications, modes of operation, test results, and reconfigurations are described in
A 0.4 meter lightweight telescope has been developed as a prototype for a future 1.4 meter telescope to be implemented at the Naval Prototype Optical Interferometer (NPOI). Using carbon fiber construction for all components, including optics, an order of magnitude reduction in weight is easily obtainable, with the estimated weight of the 1.4 meter telescope being less than 300 pounds. However, lightweight composite materials traditionally offer certain drawbacks, such as different material behavior and vibration characteristics from conventional materials and difficulty in obtaining optical surface quality. This paper describes the characterization of the mechanical properties of the advanced materials used in the construction of these telescopes and includes measurements of the optical figure obtained with carbon fiber construction.