The age of large space optical systems began in the early 1960s. With very little precedent but driven by necessity, there emerged a series of new mechanical and structural technologies including dramatically lightweighted mirrors, ultra-stable low thermal expansion structures, large-scale testing thermal and vacuum environment testing, and robust alignment sensing and control systems. We will review in this monograph the growth of these and other technologies, beginning with a heritage that had its origins in amateur telescope making. We will also address a newer design technology, design-to-cost, which has become a dominant system discriminator in the past decade. This is not intended as a how-to technical paper. Instead it is intended to surface some of the questions and considerations that need to be answered in the development of any new systems concept, rather like a checklist, to ensure that low cost and a high probability of initial success are achieved.
Today’s optical instruments must be lightweight, rugged, and economical. In the past, these three requirements were usually considered to be mutually exclusive. These demanding requirements are now being satisfied in a new class of optomechanical instrument designs. Creating rugged, yet lightweight optical instruments requires the use of new design techniques. New optomechanical design techniques include new materials, structural optimization, insensitive optical configurations, isolation of systems from environmental effects, and active control.
Rugged design implies a severe service environment. Typical severe service environments include mechanical shock, high inertial loading, mechanical vibration, temperature extremes, nuclear radiation, environmental contamination, and user abuse. Effective design requires precise knowledge of both the actual service environment and the effects of the service environment on the system.
Lightweight is a controversial word, but is usually taken to mean systems that are lower in mass than conventional systems. For example, a lightweight telescope primary mirror is defined as being lighter than a solid mirror of the same diameter and stiffness. A key parameter in lightweight systems is the ratio of stiffness to weight. This parameter affects response to both inertial and dynamic loading.
New materials include both newly developed materials, such as the metal matrix composites, and non-traditional metallic materials, such as titanium. Structural optimization is applied to improve stiffness to weight. Telescope primary mirrors are optimized by contouring the back of the mirror; support structures are optimized so that parts of the structure serve more than one purpose. Insensitive design includes traditional structural practice and clever optical design. An Airy point supported beam is highly resistant to dynamic and inertial loading. Tube bending is compensated by passive optical design in relay systems. Isolation from environmental effects includes the use of enclosures and vibration isolation systems. Finally, active control is used to reduce weight and stiffness of systems, while still maintaining optical alignment through the use of position sensors and actuators.
Design examples of rugged and lightweight systems include cryogenic space optics, ultra-lightweight telescopes, and large astronomical astrographic lenses. A 0.5 m, 15 kg, fused silica mirror for the Space Infrared Astronomical Facility operated successfully at 9 K, demonstrating the use of materials selection, structural optimization, and isolation from temperature effects. A series of ultra-lightweight telescopes, up to 0.4 m aperture, employs new metal matrix composite materials, and structural optimization. Passive optical compensation for temperature effects, non-traditional metallic materials, and isolation are effectively employed in a 2 m focal length, f/10 astrometric objective.
The design and fabrication of components comprising ultraviolet and infrared optical systems require the same general engineering considerations to be given. In actuality, a variety of significant differences exist in the magnitude of considerations for optical systems in these two spectral regions. Among these are the optical materials available for use, surface finish, mechanical and optical fabrication techniques, housing and mounting methods, alignment and test, and cost. This paper contrasts and compares the practical issues facing the designer, fabricator, and assembler dealing with optical systems for the ultraviolet and infrared spectrums.
The focus of this paper is concerned with the practical aspects of designing optical instruments which are intended for production in large or even small quantities. Certain aspects of performance are required of an instrument which is its reason to exist. It is usually expedient, plus fiscally and ecologically sound, to strive for an overall efficient use of resources in the life cycle of the instrument (i.e., keep the net cost down). This includes the processes from concept through design, prototypes, production, use, and disposal. The design and tolerancing aspects of the process have a major effect on the life cycle cost and efficiency of the system and that is the principal subject of this paper.
We discuss what makes up the cost of a lens and the effects of tolerances and other factors on that cost. This results in a new lens cost estimation formula. We describe the interactions of lenses and lens cells from the tolerance viewpoint. We then explain the principles whereby the system tolerances can be determined which will give the minimum cost system which meets the performance requirements. We conclude with an example from real life of the preliminary application of the principles.
Thermal design and analysis play an integral role in the development of spaceborne cryogenic infrared (IR) instruments. From conceptual sketches to final testing, both direct and derived thermal requirements place significant constraints on the instrument design. Although in practice these thermal requirements are interdependent, the sources of most thermal constraints may be grouped into six distinct categories. These are: (1) Detector temperatures, (2) Optics temperatures, (3) Pointing or alignment stability, (4) Mission lifetime, (5) Orbit, and (6) Test and Integration.
In this paper, we discuss these six sources of thermal requirements with particular regard to development of instrument packages for low background infrared astronomical observatories. In the end, the thermal performance of these instruments must meet a set of thermal requirements. The development of these requirements is typically an ongoing and interactive process, however, and the thermal design must maintain flexibility and robustness throughout the process. The thermal (or cryogenic) engineer must understand the constraints imposed by the science requirements, the specific hardware, the observing environment, the mission design, and the testing program. By balancing these often competing factors, the system-oriented thermal engineer can work together with the experiment team to produce an effective overall design of the instrument.
Once the temperature distribution is known for a large optical system, there are various methods to predict its effect on the optical performance. The thermal distribution is assumed known by measurement, heat-transfer analysis, or supposition. A system consisting of reflective and refractive elements, their supporting structure, and the surrounding medium will all be affected. The reflective optical elements and structure are usually analyzed for their thermo-elastic response, while the refractive elements are subject to both elastic distortions and refractive index changes.
While it might appear almost hopeless to look for theoretical (closed form) solutions, there are some available that are both powerful and practical. Most finite difference and finite element elastic solutions can incorporate temperature effects and are used for a wide range of opto-mechanical structures. In addition to the temperature, one must also know the corresponding material parameter (for example, the coefficient of thermal expansion) These parameters are ofter themselves temperature dependent and are not constant either throughout the structure or even within a single (non-homogeneous) component.
Since temperature distributions can be irregular, variable and difficult to predict exactly, orthogonal functions can often be analyzed. Then the thermal distribution can be approximated by a sum of these functions thereby predicting the whole response.
The refractive properties and physical dimensions of optical components change with temperature, and hence also do the characteristics of optical systems. Athermalization is the principle of stabilizing the optical performance with respect to temperature, either by designing the optical elements and mounts to be mutually compensating, or by including movable corrective mechanisms. For refractive materials, two coefficients can be defined which characterize the thermooptical sensitivity, one applicable to uniform temperature changes, and the other to spatial temperature gradients. For normal optical glasses, the effects are small, but for plastics, infrared materials, and liquids, the thermal effects can be so great as to limit their usefulness. Passive athermalization is analogous to achromatism and optical systems can be designed simultaneously achromatic and athermal. The use of composite, and high expansion mounts employing plastics or fluids, gives greater control of thermo-optical effects. Active athermalization uses auxiliary power to drive compensating elements to maintain optical performance. Commercial optical design programs can be used to model and analyze thermally perturbed systems accurately.
Dimensional instability exists to some extent in all components no matter what the materials may be. So the question is not "how can we eliminate instability?" but rather, "how can we reduce it to a tolerable level?" The maximum allowable dimensional instability will vary with application and depends on the particular component and its role in the optical instrument. The purpose of this paper is to provide the basis for deciding how much can be tolerated and for making intelligent choices in the selection of materials and processes for components that will achieve stability design goals with which to meet optical instrument performance specifications. This basis is a better understanding of the causes of instability and methods for minimizing instability.
After a discussion of tolerable levels of instability, four types of dimensional instability are defined: thermal, temporal, cycling and hysteresis, with examples given for each. The principal causes of these instabilities: external stress, changes in internal stress, microstructural changes and inhomogeneity/anisotropy of properties, are explained in some detail along with a discussion of material types and properties. Most importantly, methods for minimizing the instabilities are shown. This discussion includes specific recommendations for commonly used materials including: processing techniques to minimize instability, specific problems observed in some materials and how to avoid the problems, and some general guidelines regarding the effects of fabrication methods on stability.
It is most important to realize that increasingly tighter specifications for optical instruments mean that the optomechanical designer must work concurrently with other engineering disciplines, particularly materials and processes engineers, to insure the desired thermal and temporal stability of the product.
As optics increase in size, so increases the difficulty in achieving optical performance goals. Such challenges, however, can be accomplished by careful consideration of design, mounting, and material characteristics to lead to a selected concept of large ground or space based optics with high aspect ratios.
This paper outlines some of the large mirror design methodology and understanding required to meet specified performance criteria for both monolithic lightweight and thin solid meniscus design approaches. These criteria are many. The optic must be shown capable of meeting performance goals during a changing gravity environment, for ground based telescopes, often looking from horizon to zenith, or be tested for the zero-g environment of space. It must further be insensitive to, or provide accommodation for, thermal swings about the nominal fabrication temperature. At temperature, it must preclude excessive error causes by real time thermal gradients. It must resist vibratory and wind loadings as well, and be of adequate strength to withstand all phases of handling and transportation in various stages of completion.
The mirror must be generated, cut, ground, polished, tested, mounted, and coated. During these stages, the effects of residual stress, temperature, assembly tolerances, mount force errors, tool pressure, bimetallic distortion, surface flaws, and humidity must be duly considered to meet the stringent criteria of fractional visible wavelength performance and fractional arc second encircled energy requirements.
The body of this paper presents an overview of the detailed analyses required to meet such criteria. Discussed are methods of supporting, mounting, and controlling such optics. The analytical tools of finite element math modeling are presented which assist in determining effects of material inhomogeneities, thermal strain, initial stress conditions, lightweighting, and mirror curvature, which play an important role as the diameter to thickness ratio increases. Material phenomenon and comparative trades are developed to aid in the choice of ideal candidates. Design examples from experience are finally given to illustrate the sensitivity of proven optics to the design criteria which have met stringent performance goals. These include lightweight monolithic optics with diameter to thickness ratios in excess of 15:1 and thin meniscus designs with ratios in excess of 100:1. In the latter case, active control is demanded, and correctability to both fabrication and operational errors is discussed, including actuator count determination.
In the context of this review, an optical window is a solid barrier with a principal function of transmitting some portion of the electromagnetic spectrum between the short wave atmospheric cutoff of 180 nanometer wavelength and the 12 micrometer thermal infrared. Uses range from protection of optical instruments against hostile natural environments to containment of exceptional man-made environments for study (e.g., combustion) or for production of special forms of energy (e.g., lasers). Flat, uniform thickness panels are most common. Spherical segments and conical elements have found use. Other curved elements and multiple flat glazings are often used to better conform to the shape of a vehicle or other enclosure.
Application of such barriers requires that we examine the effects, other than the desired unimpeded and undistorted transmission, which may be introduced.
The various optical elements in a sophisticated optical system must be precisely aligned to each other to obtain an aberration-free image. In optical systems with very tight alignment tolerance requirements, the optics and their mounts are usually manufactured to rather loose tolerances, and adjustment mechanisms are then used to align the optics relative to each other at assembly. Another class of adjustment mechanisms is employed to move one or more optical elements of a system in real time to compensate for the image degradation due to environmental effects. This paper discusses the three basic types of adjustment mechanisms namely: linear, rotary and tilt mechanisms. Each mechanism consists of a number of parts such as the actuator, locking and preloading components. The selection criteria for these components of the adjustment mechanisms are presented in detail. Finally, some design guidelines for applications of the adjustment mechanisms in complex optical systems are presented.
In the last 10 to 15 years, a considerable body of international standards literature has been published on both mechanical and optical design. We discuss the influence of these internationally developed standards on the design and fabrication of optical systems. We conclude that while there are large benefits to be gained from using these international standards, there will have to be a substantial educational effort at all levels from project scientist to worker on the shop floor to take advantage of the benefits. Many sources to help in this education process are outlined.
The most frequently used lens-to-mount interfaces involve direct contact of shaped shoulders, spacers and/or retainers onto the polished lens surfaces or onto ground bevels. In some cases, the lens rim may contact a machined surface of the mount. Elastomeric suspension of the lens in the mount is a possible alternative design. Surfacecontact mount types reviewed here include "sharp corner", tangent and toroidal interface versions using retaining rings to hold the lenses against shoulders or spacers. Means for conservatively estimating stress build-up with-in the lens due to axial preload at assembly and at extreme temperatures are suggested. Examples showing the influences of different material characteristics (such as coefficients of thermal expansion and Young's modulus) and of both positive and negative changes of temperature from that existing at assembly are discussed.
Modem Single Point Diamond Turning (SPDT) technology offers new opportunities to address a large number of traditional problems in optical assembly and alignment. Traditional optical manufacturing rules-of-thumb have resulted from a lack of control over the Optomechanical Interface (OMI). Because of the extreme accuracy associated with SPDT, conventional alignment methods can be replaced with deterministic and cost effective methods. These SPDT techniques have been demonstrated at OCA to be compatible with athermalization strategies, and with design-to-unit-production-cost (DTUPC) considerations.
In its most general form the finite element method is a computational procedure used for resolving tensor states in three dimensional continua based upon the conditions at the boundary. The procedure provides approximate solutions for continua of any shape or configuration, some of which may be very difficult or impossible to solve in closed form. This facility is provided by the fact that the procedure divides a continuum into smaller units, often many units, and the shape of the units are selected to be shapes for which solutions are either known or can be solved by approximate numerical methods. The elastic equations for each of the small units are assembled for simultaneous solution by matrix methods, thereby solving for the boundary conditions (displacements) of each small unit that are consistent with the conditions at the boundary of the continuum. The final step of the analysis is to apply a modified Ritz analysis method to determine the tensor state inside each small unit based upon the boundary conditions for the unit.
In spite of the widespread and mushrooming use of FEM (Finite Element Method) based modeling and analysis by the optical community in recent years, other methods of analysis and modeling are still viable, useful, cost-effective and sometimes necessary in some areas of opto-structural mechanics. This paper provides a review of some of this writer’s applications using closed-form solutions and mathematical modeling based on the equations of elasto-mechanics to three topics in opto-structural mechanics; namely, (1) the distortion of circular mirrors under operational and environmental loads, (2) design/analysis of deformable mirrors and related performance prediction, and (3) polishing of optical surfaces (analysis/design of flexible polishing laps, material wear rate and smoothing). Solutions of Kirchoffs flat plate equation and E.Reissner's spherical shell equation were derived and used for the first two topics of application. A modified flat plate equation, which included, besides bending, the transverse shear and lap compressibility, was solved for the flexible lap pressure distribution. Using this, Preston’s wear rate equation was solved for material wear and smoothing during optical polishing.
Where available, comparative data are presented in this paper for finite element analyses and test measurements corresponding to the closed-form solutions. Excellent correlation is observed among the three. The paper also includes reviews of (1) the flexural rigidity characteristics of open and closed back light weighted mirrors, (2) the meshing characteristics of curved deformable mirrors, and (3) related publications of other writers in the field of opto-structural mechanics.
How stable are structures made of fused silica, Zerodur, ULE, Invar or Superinvar? We review what is known about dimensional changes with time or temperature for these important materials, including some recent findings.
What are the variations in commercially available materials? How does one go about obtaining the best Invar and Superinvar?