Especially for space applications, the problems of seeing are being superceded by the problems of
dimensional changes in remotely located optical systems. Conditions of operation, in addition to launch
considerations of acceleration and weight, often include
. vacuum environment
. thermal cycling over wide temperature ranges
. passage of considerable time without replacement
. fixed focus.
In choosing materials, the usual figures of merit, shown in Table I, are a good starting point, but ignoring
other considerations can lead to disaster. We discuss below a number of these considerations.
In this paper we report measurements of the dimensional stability of samples of brass,
beryllium copper, and tellurium copper taken over an 18 month time span. Of the
materials, brass was the most stable, decreasing slightly in length at the rate of
1 part per million per year (ppm/y) with an uncertainty (3a) of about 1 ppm/y.
Tellurium copper shrank at an average rate of 2.Li ppm/y and beryllium copper, the
least stable, at the rate of 5.8 ppm/y. To measure the instrumental uncertainty 4
samples of each material were measured, and the measurement scheme was designed to
detect and correct for thermal drift ,during measurements. The experiment design
problems associated with these measurements and the associated uncertainties are
Deviations from the circular shape of the cutting edge of a single-point turning tool cause form errors in the workpiece during contour cutting. One can compensate for these tool-form errors by determining the size of the effective deviation at a particular instant during cutting and then adjusting the position of the cutting tool accordingly. An algorithm for the compensation of tool-nose-radius errors in real time has been developed and implemented on a CNC turning center. A previously developed computer-vision-based tool- inspection system is used to determine the size of the deviations. 1 Information from this system is fed to the error compensation computer which modifies the tool path in real time. Workpieces were cut utilizing the compensation system and were inspected on a coordinate measuring machine. Significant improvements in workpiece form were obtained. 1.
Dimensional stability is the time dependent strain response to mechanical thermal chemical or physical loads whether internal or externally applied. This paper presents recent work on moisture induced strains and viscoelastic behavior of polymer matrix composite materials. Topics covered include determination of lamina coefficients of moisture expansion (CME) or /11 and P22 and the microyield strength (MYS). It is shown that the creep recovery part of a MYS test can be a sensitive indicator of moisture level and/or matrix damage. 1.
A major problem in the use of standard linear position encoders is that the etched glass scales they use have a coefficient of thermal expansion (C. T. E. ) of about 1 1 . Oppm/ C. This means that their position measurements drift with changes in environmental temperature proportional to their C. T. E. . A new low cost dimensionally stable composite scale was made for use in a new absolute linear position encoder. The unidirectional Polyphenylene Sulfide/AS4 carbon fiber composite material that this scale is made of was tested for dimensional stability with respect to changes in temperature humidity and creep. The C. T. E. of the scale material was measured to be nominally 0. 29 ppm/ C with a standard deviation of 0. 12 over the operating temperature range of -2 to 62 C. The overall displacement strain due to 98 moisture absorption relative to 0 moisture absorption was measured to be 9 ppm. The strain due to stress-relief creep was found to be a maximum of 3 ppm over a period of 173 days. These results show that a linear position encoder scale made of this material is superior to those made of the standard glass currently being used and more cost effective than fused silica for most applications. 1.
A single-point turned (SPT) beryllium surface has been evaluated
for manifestations of surface damage which could lead to dimensional
instability. Surface fissures, residual stress and crystallographic
changes have been observed. Measurements of the residual , stresses
have been made using a variation of the Treuting-Read Method wherein
deformations associated with the progressive removal of surface
layers are used to estimate the residual stresses. In this case, the
deformations have been measured interferometrically on the reverse
surface of the single-point turned disk. Residual stresses of 40,000
psi were observed at the machined surface. At a depth of 120
microinches, the residual stresses are negligible.
This paper will describe many recent aerospace applications where beryllium was successfully used in different types of optical support structures. It discusses how several manufacturing methods including machining metallurgical brazing epoxy bonding and near-net-shape processing were used to produce the desired overall configurations. The installation of non-beryllium inserts is also discussed in detail. Typically made of titanium these inserts allow the end user to final finish mechanical attach points and optical lapping points thus circumventing potential toxicity hazards associated with airborne beryllium particles. Data is also presented about the stress and stability characteristics in some of the more specialized applications as well as weight and stiffness.
In order to fabricate dimensionally stable components the causes of dimensional instability must be understood and controlled to the level required by the specifications. This paper reviews four types of dimensional instability and the sources that cause dimensional change including the effects of external and internal stress on both macro- and micro-levels. Stress relief and stress relaxation as well as the use of low expansion materials are discussed. Examples are given of both unstable components and methods employed for fabricating stable components from materials such as aluminum beryllium glass and composites. 1.
The microdeformation characteristics of a new instrunent grade of beryllium 1-250 are presented and compared to previously available conunercial grades. The importance of iuicroyield strength to mirror stability under mechanical and inertial loads as well as under temperature excursions is discussed and related to the HCP crystal structure. It is concluded that 1-250 may provide significantly improved dimensional stability over 1-70 or 0-50. 1.
Comprehensive Purchase Specification(s) must not merely define a generic type of material by chemistry and mechanical properties. It must be capable of specifying the method of material formation (i. e. rolled cast forged vacuum hot pressed etc. ) it''s grain size preferred orientation homogeneity etc and the method of material removal to minimize surface damage and/or work hardening. Starting out with heavily stressed material will in many instances negate the possibility of fabricating components which can be subsequently processed and heat treated to eliminate the residual stresses which cause components to change dimensionally and/or creep or experience premature micro-yielding - the anisotropy of work hardening Bauschinger Effect. . MATERIAL AND FORM SELECTION In order to produce parts or precision assemblies with maximum stability one must recognize the various forms that are available and the selection of alloys to choose from. The next step may be selecting a form with the best homogeneity or a form which will permit the subsequent processing to a condition of useable stability (minimum residual stress). Another vexing problem is the fabricating of parts from bar or plate for prototypes and the subsequent purchase of cast or powder compacted parts for production. Two very diverse material conditions. A brief familiarization (See Figure 1 and 2) with the forms available from a partial list is a starting point in the ultimate selection of form alloy condition and subsequent
Dimensional instability in electroless nickel (EN) coatings is the result of two effects related to thermal expansion mismatch between the substrate and the EN coating: a continuous reversible contribution from the coefficient of thermal expansion (CTE) mismatch between the EN coating and the substrate and one or more one-time irreversible thermally-induced volume contractions of the EN. This paper presents a description of both mismatch components how each effects the ultimate stability of an EN-plated optic and which processing parameters can lead to improved stability.
Structural stability of mounts and housings is critical to the success of high performance optical designs. Often however stability is at odds with the multiple degrees of freedom required on the mounts for accurate alignment. Here we describe the technique used to accommodate these two factors for an eccentric pupil collimator. 1.
Reaction bonded silicon carbide (RB SiC) is a two phase mirror material which is readily formable to near-net-shape but can be difficult to polish to a high quality optical surface. The usual solution is the addition of a thick layer of silicon (Si) which may be polished to very high optical quality but which may have a thermal distortion problem due to the mismatch in thermal expansion of the two materials. The second solution is the application of a thick layer of chemically vapor deposited (CVD) SiC which can be polished to high quality but not as readily as the Si. The CVD SiC can also have a mismatch since it is deposited at high temperature and is beta SiC compared to the alpha in the substrate. We have chosen to develop a low temperature method for depositing amorphous SiC which should provide both a polishable surface and a better match of properties. To determine the levels of thermally induced distortion in SiC mirrors we have cryogenically tested 6-inch diameter spherical RB SiC mirrors bare and with polished coatings of amorphous SiC Si and CVD SiC. Results from this program are presented which show that all but the CVD SiC coated mirror are thermally stable. 1.
In recent years a large effort has been made to protect electro-optic systems from various laser threats. One approach has been to use multilayer dielectric rejection filters as part of the system. This paper will discuss the effects of temperature humidity and UV irradiation on the optical performance of these filters. 2.
The unloaded dimensional stability and thermal expansivity of a single furnace melt of superinvar have been measured at room temperature using interferometric techniques. Thermal expansivity has been determined with an uncertainty of several parts in 108 per degree centigrade while dimensional stability has been determined with an uncertainty of order one part in i09 per day. Samples subjected to plastic deformation in their processing history displayed a stability improvement from 20. 5 x 1O/day to 5. 5 x 109/day and a reduction in thermal expansivity from 0. 56 x 1O/C to 0. 23 x 106/C associated with the increased mechanical work in the material. 1. MATERIAL SELECTION AND PROCESSING Modern scientific instruments place increasingly stringent demands on the dimensional stability of materials used for the construction of precision structures. The dimensional change of a material due to a change in temperature is characterized by its coefficient of thermal expansion (CTE). Dimensional changes may also occur over time in a fixed environment. In considering the selection of a material for demanding applications both the CTE and temporal stability must be considered. Indeed with reasonable control of the ambient temperature pressure humidity and magnetic field temporal instability may be the factor limiting performance. Superinvar the material studied here is one alloy in a class of materials of practical interest for the construction of precision systems. The invars are relatively inexpensive materials (by comparison other low-CTE materials) and may be fabricated using conventional metalworking techniques. Superinvar is of particular interest because it exhibits a lower CTE than the other invars and because of an early report by Jacobs Bradford and Berthold indicating that it possessed a particularly high degree of temporal stability. 1 This paper reports a series of CTE and temporal stability measurements performed on superinvar samples taken from the same melt but subjected to different degrees of processing. One of the experimental aims of this study was to learn about the conditions leading to the single high stability result reported by Jacobs. Accordingly the chemistry of the material was specified as a high purity superinvar with 31 Ni and 5 Co. This alloy lies at the minimum CTE point of the Fe-Ni-Co system2 and is the nominal alloy used by Jacobs. Table I shows the composition which was specified the analysis performed at the mill at the time of melt and an independent analysis of the material after receipt at Livermore. For reference purposes the composition of the sample measured by Jacobs is also listed. 3 The material used in the samples was cast in an air induction furnace as part of a large order. Although the same steel mill produced both this material and Jacobs'' samples the furnaces used for the two melts were different: Jacobs'' sample came from a melt in
The choice of a substrate material for large mirrors is a complex engineering task that must account for structural and thermal properties as well as mirror blank fabricability polishability and surface scatter. Nuclear hardness is also a consideration in some applications. Cost is almost always a concern. The standard material for mirror substrates has always been glass. Beryllium technology however is well developed and offers distinct advantages over glass in many applications. Reaction-bonded silicon carbide is a relatively new material that has matured to the point where it can now be considered as an alternative to either beryllium or glass in some large optics applications. The availability of these three different substrate materials offers the system designer a great deal of flexibility in optimizing the material for each particular application. In this paper we present a methodology for comparing the structural properties of mirror substrate materials and lightweighting designs. This methodology is used to compare glass beryllium and silicon carbide. 1. LIGHTWEIGHT MIRROR DESIGN PARAMETERS Basic lightweight mirror design parameters are illustrated in Figure 1. Sandwich mirrors comprise two faceplates with a structured core in between. Although the faceplates are shown as having equal thicknesses tf they can in general be unequal. Open-back mirrors have a single faceplate. For both sandwich and open-back mirrors the mirror core is characterized by its height hc the web thickness tw of the core elements
This paper presents the fabrication and assembly techniques for high resolution lens assemblies such as the projection lenses used for submicron photolithography. In order to meet the stringent precision and dimensional stability requirements for such lens assemblies the aluminum lens cells are machined and heat treated in several steps to achieve the required accuracies. Special optical tools and mechanical gages are then used to assemble the lenses into the cells on an airbearing table. 1.
We have conducted a series of measurements to evaluate the dimensional stability of carbon fiber-plastic laminates based on a new Lockheed-developed hydrophobic resin system. The test specimen was 10 inches long and was in the form of a hollow truncated right circular cone. To aid the metrology the tube was fitted with stainless steel end flanges that were lapped to be flat and parallel to within 0. 0001 in. The design of the fabric lay-up of the tube was chosen to maximize the sensitivity of the test specimen to length changes induced by varying the moisture content of the laminate. A second test specimen prepared with an epoxy resin system was included in the measurement program as a control. The absolute length of each specimen was periodically measured with respect to laboratory grade gage blocks using a digital electronic height comparator. Moisture content changes were induced by exposing the specimens either to a high humidity environment (90 R. H. 1OO F) or to a vacuum over (iO torr 1500 F). Maximum length differences observed in the control sample were of the order of e51 0. 003 in while those in the hydrophobic sample were of order 51 0. 0002 in more than an order of magnitude smaller. The Lockheed hydrophobic resin system shows great promise as a structural material for lightweight telescope tubes optical benches and other structures requiring a high
The properties of four candidate mirror materials--beryllium, silicon carbide,
a silicon carbide/aluminum iretal-matrix carposite and aluminum--are corrpared.
Because of its high specific stiffness and dirrensional stability under changing
mschanical and thermal loads , beryllium is the best choice . Berjllium mirrors have
been made irore cost-conpetitive by new processing technologies in which mirror
blanks are isostatically pressed to near-net shape directly fran beiyllium pc1ers.
Isostatic pressing also improves material properties and mskes it possible to
develop mirror rraterials with superior properties.
A lightweight graphite/epoxy sandwich mirror design has been developed with an aim toward reducing the traditional sources of dimensional instability throughout the mirror structure. The mirror manufacturing process attempts to neutralize the hygroscopic behavior of the epoxy matrix and also minimize the tendency for micro-cracking at the fiber-matrix interface. The process was used to fabricate several mirrors as part of a program to develop a spacebased laser radar beam steering flat for use at 10. 6 rim. The mirror construction consists of two facesheets of ulira-high modulus (UHM) graphite/epoxy prepreg bonded to a triangular-cell core of UHM graphite/epoxy ribs. Aluminum cladding is utilized on each facesheet surface to: a) prevent mairix dimensional changes due to hygroscopic effects such as moisture absorption and outgassing b) tailor the in-plane laminate coefficient of thermal expansion (CTh) yielding a facesheet with near-zero CTh and c) provide a surface suitable for diamond-turning. The core is also clad with a moisture barrier which additionally tailors the CTE to match the facesheets. This paper describes the design analysis fabrication and testing of graphite/epoxy mirrors from the viewpoint of enhancing dimensional stability. Testing and evaluation of mirrors ranging in size from 14 cm square to 65cm in diameter were performed. The evolution of the fabrication process is described in detail. Surface figure data after temperature cycling are presented for several mirrors illustrating the ability to achieve and maintain figures