Choosing appropriate glasses and athermalizing a lens system with only one metal by James Wu, jamesw@optics.arizona.edu, ywuumme99@excite.com

Figure 1 shows a typical opto-mechanical design for athermalizing a lens system using two metals with dissimilar CTE’s. However, with the vast numbers of glasses to choose from, an athermal technique using only one metal as the metering structure is presented. A look-up table is provided for choosing the glass combinations that yield βlens = αmetal.

Introduction
In designing a lens system, it is common to start with a known design form and to use familiar glass types. After the optical design evolves to a mature stage, then the mechanical structures are “wrapped around” the prescribed optics. Different opto-mechanical athermal techniques have been developed for this serial approach. However, the mechanical design is strongly influenced by the near-finished lens prescription. For example, Figure 1 shows the opto-mechanical designs suitable for athermalizing some already prescribed lens systems. In order to maintain focus over some temperature fluctuations, two structural materials with dissimilar CTE’s and lengths are chosen to match the change in the focal length. The complexity in this opto-mechanical design adds volume, different materials, additional parts, handling, and assembling to the overall optical system.
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(1a) (1b)
Figure 1: Athermalizing a triplet and a doublet by matching (α1L1 ± α2L2) of the metering structure to the dF/dT of the lens systems.

Ideally, one would like to use only one structural material, aluminum for example, for both mounting and athermalizing the optical system. A systematic approach for designing such lens system is presented here. Specifically, the emphasis is placed on the doublet, triplet, and their variations (doubling the doublets and triplets) are presented.