Pre-Cooling of Infrared Instruments Now Automatic
October 23, 2008
Modern astronomy at Subaru and other world-class observatories uses sensitive instruments (e.g. imagers and spectrographs) to detect objects fainter and further than ever before. To optimize observations, engineers and astronomers are continually upgrading instrument systems, and, recently, an automatic pre-cooling system (APS) was introduced at Subaru for its large infrared instruments. This revolutionary system introduces innovative design to Subaru’s system of instrument engineering and management.
Astronomers and engineers at Subaru are continuously working to develop ever more sensitive sensors for the telescope’s instruments to catch even the weakest signals. Many of these instruments must be cooled well below room temperature to have the necessary precision. The summit of Mauna Kea at 14,000 feet (4,200m) is on average a cool place (32°F, 0°C); nonetheless, our instruments work best hundreds of degrees colder. Why so? First and foremost is that today’s instruments are extremely sensitive, and, in turn, are very receptive of thermal and electrical “noise”. Essentially, because the instruments are looking for astronomical objects at extreme proportions (i.e. dimmest, smallest, furthest, etc.), any unnecessary light and/or heat around the instrument can negatively effect observations. For infrared observations, unwanted heat is called “thermal background”, and this heat comes from everything, including machines, electronics, and people. The only way to reduce disruption by heat or electronic noise is to have infrared instruments operate under extreme ultra-cold temperatures and be self-contained in temperature-controlled vacuum containers called dewars or cryostats.
To achieve extreme cryogenic temperatures, the instruments are cooled using liquid nitrogen (N2) and helium (He) gas in closed cycle coolers. The precise operating temperatures vary between instrument systems and depend on engineering specifications. For instance, Subaru’s Multi-Object Infrared Camera and Spectrograph, MOIRCS, uses liquid nitrogen to pre-cool various internal points down to 90K (-297°F/-183°C), and then uses mechanical cooling with compressed helium gas to further cool its detectors down to operating temperatures at 77K (-321°F/-196°C). At these temperatures, stray electrons that may cause interference are slowed to insignificant levels and thermal radiation by the instrument itself is negated.
The MOIRCS instrument served as the platform for APS development, and APS began as an enhanced version of the conventional instrument pre-cooling system. Although MOIRCS operates in a very cold environment, it has several warm-up and cool-down periods throughout the year for routine maintenance. The MOIRCS instrument has a large heat capacity and standard cool-down was found to be impractical due to the significant temperature range and huge amount of time required. To address this problem, Subaru engineers created a mechanism to allow pre-cooling of the instrument via liquid nitrogen, allowing for a much faster cooling process. However, even with liquid nitrogen, pre-cooling was still very lengthy, requiring nearly half a dozen nitrogen tanks and almost a week of continual on-hand monitoring in order to reach the desired target temperature. To resolve the issues of intermittent cooling, wasted liquid nitrogen, and excessive man-hours, Subaru embarked on automating pre-cooling operations.
Subaru developed APS to work continuously and remotely at the summit to reduce the issues of concern noted above. The development of APS was also motivated by the task of cooling large infrared instruments (e.g. MOIRCS) more cost-effectively, safely, and quickly using existing technologies. The APS team consists of about a dozen Subaru engineers, scientists and technicians.
The strategy for APS is to have basic functionality for pre-cooling and user friendly interface. This goal was achieved by: 1) continuous cooling until the target temperature is reached by automated liquid nitrogen tank exchanges and precision temperature control by automated changes to the liquid nitrogen flow; and 2) remote monitoring and control of all parameter setting by web browser. Temperature control is an important issue addressed by APS, because in the past, the pre-cooling of many instruments was made in some cases without much control of the temperature rate. Without careful control, temperature decrease done too quickly may cause damage to the optics or infrared detectors through thermal stress or condensation (i.e. frost). The Subaru instruments usually have several thermometers, a closed cycle cooler or two, liquid nitrogen cooling pipeline, and electric heaters inside the dewar. The pre-cooling process is done by taking advantage of these sensors and actuators and advanced control software.
There are relevant technologies in aerospace, chemical, and other high-tech industries controlling cryogenic fluid flow. The APS team investigated commercial liquid nitrogen cooling systems with precise temperature rate control but did not find any system meeting their exact demands. In turn, the group decided to build APS by combining varied commercial cryogenic components (i.e. valves and switches) to come up with a suitable system. To achieve proper pre-cooling automation, several goals were established: 1) complete remote control of pre-cooling with liquid nitrogen tank transportation and connection to the system at most once a day; 2) instrument cool down with a given temperature rate and smooth tank switching; and 3) establish procedures and easy operation for those with minimum training. The improvements by APS took a few years to complete because the testing opportunity during maintenance of MOIRCS and other instruments is only two or three times per year. Ultimately, APS was fine-tuned to control liquid nitrogen temperature and flow control, liquid nitrogen tank switching, and vacuum pumping after pre-cooling.
The current APS is divided into a control system and liquid nitrogen supply system. The control system contains almost all the active control devices, monitoring sensors, control computer, and connections to the instrument. The system is controlled by operators logged into the control computer from base headquarters in Hilo, from the summit, or remotely from anywhere using the Internet. The liquid nitrogen supply system includes coolant tanks and pressure tank that supply nitrogen gas and maintain a constant pressure in all tanks. Electric valves to each supply tank allow them to be isolated from the pressure system, and a floor scale is used to continuously monitor the remaining nitrogen in the pressure tank.
APS has four main operating modes – Cooling, Pumping, Tank Replacement, and User. The Cooling mode automatically supplies the liquid nitrogen to the instrument and cools it at a set rate of -5K per hour. The control system watches the coolant supply pressure to detect an empty tank and will automatically switch to a new tank. The system maintains the status of all tanks and goes to Pumping mode when all tanks are empty. In turn, the inlet and outlet valves are closed, the pump valve is opened, and the vacuum pump is started. When the empty nitrogen supply tanks are replaced, an operator puts the system into Tank Replacement mode. The operator dialogues with the system specifying the new tank configuration, and the system returns to Cooling mode. User mode allows for complete manual override of the system, and is used for testing and initial set up of the system before automatic cooling.
The automatic temperature control maintains the instrument at a set temperature using a classic feedback loop controller. The instrument is cooled steadily down to target operating temperatures between 110K and 90K, before switching over to mechanical cooling using compressed helium gas. For pre-cooling, MOIRCS usually stabilizes between 116K and 88K using liquid nitrogen. Note that the temperature control is only used for cooling, and cannot actively warm the instrument. The only source for warmth is natural, ambient heat.
A benefit of APS is that automatic tank switches followed by a short venting period are now fast and efficient and cause little increase in temperature or delays in pre-cooling time. Although tank replacements still cause larger increases in temperature, the delays are small in relation to pre-APS schedules and to the overall pre-cooling time. Management of the liquid nitrogen supply is critical, and APS improvements include using only full tanks and better scheduling of tank replacement. The APS team identified that it is important to avoid Pumping mode and the significant delays that can result from interruptions in cooling. They also found that maintaining constant pressure in the liquid nitrogen supply is critical for steady, trouble-free operation. After a cooling run in October 2007, a dedicated, pressure building tank was installed to maintain a constant pressure in all supply tanks.
The original goals (precision temperature control, worker safety, labor reduction) of APS have been attained through several pre-cooling and software/hardware modifications. In Summer 2008, APS cools two large infrared instruments (MOIRCS and IRCS) with an accurate and continuous rate of temperature change without relying on skilled operators and without exposing them to unnecessary hazards. This efficiency is achieved by reducing temperature rise during supply tank switching and tank replacement and operating continuously for 24 hours a day. With improved supply tank management, APS very efficiently cools an instrument in the shortest possible time. Remote monitoring and control over a local network or the Internet give the operational team a high level of flexibility and comfort during a pre-cooling run. Finally, work at the summit has been reduced to one team member performing liquid nitrogen tank replacement once per day.
he details of APS and initial operating results were first presented in June 2008 at the SPIE conference in Marseilles, France. A published article summarizing APS may be found in the Proceedings of SPIE, Volume 7018. Further information on MOIRCS and IRCS can be found within this website.