Astronomical optics may encounter a wide range of nighttime temperature (～-10 ℃ to +20 ℃) at mountaintop observatories. Complex refractive optics used in high-performance spectrographs and focal reducers may perform poorly at temperature extremes unless special care is taken in their design. Refocusing will not always restore image quality, and thermally induced focal length changes may introduce troublesome image motion. We describe the techniques that we have used to predict the thermal behavior of the Binospec Spectrograph, an instrument under development for the converted Multiple Mirror Telescope. This thermal analysis must account for (1) the change in optical power due to thermal expansion of the optical elements, (2) thermal variations in the refractive indices of the optical elements, and (3) element respacings due to the interplay between the thermal expansions of the optical elements and cell materials. Binospec's multiplets are fluid couped to reduce the reflection losses hat would occur at glass-air surfaces; to athermalize the Binospec optics, we form weak lenses in the optical coupling fluid within multiplets. The large variation of the coupling fluid's refractive index with temperature allows these weak fluid lenses to correct thermal changes in the optical elements and cell. This athermalization technique is attractive because it introduces no additional mechanical complexity.
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