CARMENES. III: an innovative and challenging cooling system for an ultra-stable NIR spectrograph

The CARMENES project, which is currently at FDR stage, is a last-generation exoplanet hunter instrument to be installed in the Calar Alto Observatory by 2014. It is split into two different spectrographs: one works within the visual range while the other does it in the NIR range. Both channels need to be extremely stable in terms of mechanical and thermal behavior. Nevertheless, due to the operation temperature of the NIR spectrograph, the thermal stability requirement (+/- 0.07 K in 24 hours; +/- 0.01 K (goal)) becomes actually a major challenge. The solution here proposed consists of a system that actively cools a shield enveloping the optical bench. Thus, the instability produced on the shield temperature is further damped on the optical bench due to the high mass of the latter, as well as the high thermal decoupling between both components, the main heat exchange being produced by radiation. This system -which is being developed with the active collaboration and advice of ESO (Jean-Louis Lizon)- is composed by a previous unit which produces a stable flow of nitrogen gas. The flow so produced goes into the in-vacuum circuitry of the NIR spectrograph and removes the radiative heat load incoming to the radiation shield by means of a group of properly dimensioned heat exchangers. The present paper describes and summarizes the cooling system designed for CARMENES NIR as well as the analyses implemented.