Although analyses tell much about how a given paper design will function in use, a more direct indi- cation is derived from tests of hardware made to that design. This hardware may be called a func- tional mock-up, breadboard, brassboard, engineering model, or preproduction prototype, depending on the degree of approximation allowed. The decision about the appropriate form of the model may depend on allowable costs and schedule limitations. It would also be influenced by the degree of maturity of the technology utilized in the design. A very sophisticated design involving state-of-the- art technology and materials would demand a closer representation than the one involving well- understood technology and materials.
Thorough testing of models is especially important if the total cost of developing the system and placing it in operation is large. For example, before grinding and polishing began on the pri- mary mirror that is now part of NASA’s HST, a subscale mirror of 60 in. (1.52 m) diameter (see Figure 1.18) was built and evaluated. The materials, fabrication and test techniques, and support (metrology mount) configuration were similar to those that would later be used to make the 94.5- in. (2.4-m)-diameter flight version. Details of this preliminary activity, which resulted in an aspheric
To interferometer in vertical tunnel Tangential constraint (3 pl.) 52-point metrology mount Mirror transport carriage on rails
FIGURE 1.18 The 60-in. (1.52-m)-diameter λ/61 rms figure quality aspheric mirror fabricated and tested as a subscale experimental model of the larger mirror to be built later for the Hubble Space Telescope. (From Montagnino, L.A., Arnold, R., Chadwick, D., Grey, L., and Rogers, G., Proc. SPIE, 183, 109, 1979.)
mirror of the required (and unprecedented) λ/61 rms figure quality at λ⫽ 0.6328 µm wavelength, were given by Babish and Rigby (1979) and Montagnino et al. (1979). Successful completion of this preliminary experiment provided a sound technical basis for building the full-size mirror. That the null lens to be used to test the latter mirror would accidentally be misaligned prior to use was certainly not anticipated during the building and testing of this model.
Another reason for building experimental models is to permit hardware evaluation in operational and storage environments before the commitment is made to mass produce an optical instrument to a
given design. An example of this occurred during the development of the 7 ⫻ 50 Binocular M19 by
the U.S. Department of Defense. The size of the experimental design (known then as the Binocular
T14) is graphically compared with the prior standard 7 ⫻ 50 M17 version (of World War II vintage)
in Figure 1.19. The T14 design was unique in that it featured a modular design for improved low-cost maintainability (Brown and Yoder, 1960). Prototypes of this design (see Figure 1.20) were extensively evaluated by military personnel and subjected to rigorous environmental testing in the laboratory and in the simulated battlefield. Although found to be excellent in optical performance and to provide the
required improvements in size, weight, and maintainability over prior 7 ⫻ 50 instruments, its dura-
bility in the military environment was judged to need improvement.
A new version of this binocular with a more rugged mechanical design and only very slightly increased size and weight was developed at the U.S. Army’s Frankford Arsenal during the 1960s. The highly favored modular design feature was retained. This improved binocular, called the Binocular T14EI, was successfully tested by the intended users and adopted in the 1970s as the standard binocular to replace the M17. With a few further changes, it was produced in large quan- tity as the Binocular M19 (Trsar et al., 1981). This instrument is shown in Figure 1.21 and is dis- cussed from a structural viewpoint in Chapter 14. Although the total time span (1956 to 1975) of the cycle from initiation of development to initial production was unusually long in this case (owing primarily to the then adequate inventory of M17 equipment), the design evolution was greatly facil- itated by the availability of two generations of experimental models for evaluation. The M19 Binocular is described in detail in Section 14.3.2.
Before concluding the subject of experimental models, a few words are appropriate regarding the use of catalog optics instead of optics custom fabricated for the purpose. Many suppliers offer lenses,
Binocular M17 (WGT. 53 OZ.) Binocular T14 (WGT. 25 OZ.) 7.3 8.3 2.2 3.0 5.4 7.2 72 mm
FIGURE 1.19 Size comparison of the experimental Binocular T14 developed as a reduced-size, lightweight
7 ⫻ 50 military instrument to replace the military standard Binocular M17. Dimensions are in inches except as
noted. The Binocular M19 produced later was essentially the same size as the T14. (From Yoder, P.R., Jr., J. Opt.
prisms, mirrors, windows, filters, etc., of fine quality at competitive prices. The radii of singlet elements with equal radii or one plano surface can be easily computed from the catalog focal lengths and thicknesses if the design wavelengths and materials are known. Equations from texts such as Smith (2000) are useful for this purpose. Exact designs for simple elements and doublets such as achromats from suppliers, for example, Edmund Optics, Opto-Sigma, Melles Griot, or Spindler and Hoyer, are available as catalog designs in the resident libraries of some lens design programs. The designs for more complex commercial lens assemblies are generally not available to the public, so exact performance computations are virtually impossible. In some cases, standard designs from sources such as Smith (1992, 2004) or Laikin (2001) can be used to approximate the design of an off-the-shelf assembly of similar type. Such a design may be adequate for computer modeling of a system containing the assembly or to serve as a starting point for custom design of a new assembly.
In order to build a working model of some types of optical systems, such as the periscope shown in Figure 1.5(b), a group of cemented doublets and singlets selected from commercial suppliers’
FIGURE 1.21 The modular 7 ⫻ 50 Binocular M19 developed as a ruggedized production version of the pro-
totype Binocular T14 shown in Figure 1.20.
catalogs on the basis of focal length, aperture, a 90º prism, and a flat mirror would probably suf- fice. A suitable eyepiece could probably be purchased as a subassembly. These components could be mounted in a crude or more elaborate mechanical surround, depending upon the degree of real- ism to be provided. Performance of the optical system so created would, of course, not be optimum, but should allow for demonstration, preliminary evaluation, and approximate packaging study.
It would be impractical to construct even a functional mock-up of any but the very simplest of photographic objectives using anything other than customized parts because of the intricate depend- ence of aberrations on lens configurations. A commercial objective of focal length and relative aper- ture approximating the desired version may be available. It may suffice for preliminary evaluation purposes while a customized version is developed.
Among the factors that must be considered in making the choice between catalog and custom optics are the availability of parts of the appropriate dimensions and materials, the adequacy of the quality of the catalog parts, and the types and quality of coatings available. In some cases, uncoated optics will suffice. For lenses, it is usually important to know the wavelengths and conjugate dis- tances for which the designs have been optimized. A lens achromatized for the “F” to “C” (blue- green to red) spectral region will probably work fairly well for many visual applications or with red helium-neon laser light. Other factors deserving consideration when deciding between catalog and custom optics are the conjugates for which the off-the-shelf lens has been designed. For example, a photographic objective type lens designed for an object at infinity may not work well with finite conjugates whereas an enlarging lens assembly would perform better at finite conjugates than with an object at infinity. In some applications, such as in a telescope, field lenses may be needed in the experimental system to locate the entrance and exit pupils properly.