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Dewars of liquid nitrogen (LN2) installed on the two Nasmyth platforms provide each TMT instrument’s cooling needs and maintain a steady state of cooling for all instruments. The total wet mass (including full cryogen load) of the CRYO components on each Nasmyth platform shall be less than 3,000 kg.

Thirty Meter Telescope’s Cryogenic Cooling System Passes Conceptual Design Review

The conceptual design of TMT’s Cryogenic Cooling System (CRYO) has passed its conceptual design review. Engineers from the Technical Institute of Physics and Chemistry (TIPC) of the Chinese Academy of Sciences (CAS) joined TIO engineers at the project office to present the system that will be used to maintain parts of TMT’s first-light science instruments at cryogenic temperatures of 77 K. The CRYO team was recognized for its dedication in providing an excellent CRYO System for TMT, and the system is now ready to proceed to preliminary design phase after a few minor revisions are fulfilled.

The Thirty Meter Telescope requires an instrument cooling system designed to keep parts of the science instruments at cryogenic temperatures. This system serves two main functions:

  • To maintain the instrument detector at low temperatures, which is crucial to obtain high quality science data with the lowest possible levels of background noise.
  • To maintain the TMT instruments’ optical elements in stable temperature-controlled cryostats, to reduce thermal background.

The TMT Instrumentation Cryogenic Cooling system uses a concept of open cycle of liquid nitrogen (LN2). It is the most straightforward, reliable and upgradable approach to meet the telescope requirements for first-light science instruments, and beyond.

A liquid nitrogen plant will be deployed at the telescope summit facility building, adjacent to the TMT enclosure that houses the telescope. The liquid nitrogen will be piped via vacuum-jacketed transfer lines to storage containers located on the Nasmyth platforms. Individual dewars of liquid nitrogen in each instrument will be refilled from these storage tanks during daytime. Because the liquid nitrogen plant is in the facility building, and transfers only run during daytime, this system will generate no vibration during nighttime observation.

Cooling must be provided without dissipating significant heat inside the telescope enclosure and without producing vibration that could impact image quality. At the extreme levels of resolution that are achieved by the TMT Adaptive Optics system, the telescope itself is very sensitive to vibration forces. Even a force of only a fraction of a Newton can generate relative motion of the telescope optics sufficient to degrade the optical performance of the system.

The CRYO Conceptual Design Review’s objectives demonstrated the completeness of the requirements and the interfaces between CRYO and other subsystems: the TMT observatory and the summit facilities building, the telescope structure and the TMT client subsystems, which include the Infrared Imaging Spectrograph (IRIS), the Infrared Multi-Slit Spectrometer (IRMS), the Wide Field Optical Spectrograph (WFOS), and the Alignment and Phasing System (APS).

TMT’s Telescope Structure has already completed its Final Design Review, so it is important that the telescope-mounted components of CRYO can be implemented without requiring design changes. Similarly, the cold heads and other components of CRYO located within the summit facilities building must respect their space and power allocations. Finally, some of the science instruments that are cooled by CRYO have also reached design phases where the necessary interfaces must be defined.

Design and analysis work accomplished by the study team showed that the planned cooling operational system complies with TMT’s overall requirements for the first light instruments. A simple open-loop liquid nitrogen (LN2) system can meet all requirements, including low maintenance, and at relatively low costs and risks. Environmental, functional and operational requirements were well documented during the review meeting. An upgrade path was also described to serve future instruments that will require cooling below the liquid nitrogen temperature of 77 K.

 


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