CARMENES is a high resolution spectrograph to detect planets through the variation of radial velocity, destined for the
Calar Alto Observatory in Almeria, Spain. The optical bench has a working temperature of 140K with a 24 hours
stability of ±0,1K; goal ±0,01K. It is enclosed with a radiation shield actively cooled with thermalized nitrogen gas that
flows through strategically positioned heat exchangers to remove its radiative load. The cooling system has an external
preparation unit (N2GPU), which provides the nitrogen gas through actively vaporizing liquid nitrogen with heating
resistances and a three stage circuit flow, each one controlled by an independent PID.
Since CARMENES is still in the construction phase, a dedicated test facility has been built in order to simulate the
instrument and correctly establish the N2GPU parameters. Furthermore, the test facility allows a wide range of
configurations set-ups, which enables a full characterization of the N2GPU and the cooling system.
The N2GPU has been designed to offer a wide temperature range of thermally stabilized nitrogen gas flow, which apart
from CARMENES could also be used to provide ultra-high thermal stability in other cryogenic instruments. The present
paper shows the testing of the cooling performance, the hardware used and the very promising results obtained.