A Detailed Thermal Analysis of the Binospec Spectrograph

Refractive optics in astronomical instruments are potentially sensitive to temperature gradients and temperature transients. This sensitivity arises from thermally dependent refractive indices, lens spacings, and lens dimensions. In addition, thermal gradients in the instrument structure can cause undesirable image shifts at the detector that degrade instrument calibration. We have therefore undertaken a detailed thermal analysis of Binospec, a wide-field optical spectrograph under development for the converted MMT. Our goals are to predict the temperature gradients that will be present in the Binospec optics and structure under realistic operating conditions and to determine how design choices affect these gradients. We begin our analysis by deriving thermal time constants for instrument subassemblies to estimate the magnitude of temperature gradients in the instrument and to determine where detailed thermal models are required. We then generate a low-resolution finite difference model of the entire instrument and high-resolution models of sensitive subassemblies. This approach to thermal analysis is applicable to a variety of other instruments. We use measurements of the ambient temperature in the converted MMT’s dome to model Binospec’s thermal environment. In moderate conditions the external temperature changes by up to 8 C over 48 hours, while in extreme conditions the external temperature changes by up to 17 C in 24 hours. During moderate conditions we find that the Binospec lens groups develop ∼0.14 C axial and radial temperature gradients and that lens groups of different mass develop ∼0.5 C temperature differences; these numbers are doubled for the extreme conditions. Internal heat sources do not significantly affect these results; heat flow from the environment dominates. The instrument must be periodically opened to insert new aperture masks, but we find that the resulting temperature gradients and thermal stresses in the optics are small. Image shifts at the detector caused by thermal deflections of the Binospec optical bench structure are ∼0.1 pixel hr. We conclude that the proposed Binospec design has acceptable thermal properties, and briefly discuss design changes to further reduce temperature gradients. Subject headings: instrumentation: spectrographs, methods: numerical, conduction, convection, radiation mechanisms: thermal