The world’s largest optical telescope, the 10.4m Gran Telescopio Canarias (GTC) on the Canary Island of La Palma, is equipped with a powerful new astronomical camera built by the University of Sheffield, UK. This camera, called HiPERCAM, was commissioned on the telescope in 2018 and can take over 1000 pictures every second simultaneously in five optical colours. The person behind HiPERCAM, Professor Vik Dhillon of the University of Sheffield, states that “HiPERCAM is the most sensitive astronomical camera in the world, and has been used to study black holes, neutron stars, white dwarfs, exoplanets, active galactic nuclei and the outer regions of our Solar System”.
The Problem: The five light sensors used in HiPERCAM are ultra-sensitive charge-coupled devices (CCDs), which require cooling to 183K (-90degC) to minimise noise in the images. This cooling is achieved using very high performance thermo-electric (peltier) coolers, which in turn require a liquid coolant (a water-glycol mixture at +5degC) to remove the heat they extract. If the flow of coolant fails, the heat input from the peltier power supplies would destroy the CCDs, which would cost over £500,000 and take years to replace. With so much at stake, the Sheffield team have developed a fail-safe system, where any disruption to the flow of coolant past the detector causes the power supply to the peltiers to switch off automatically, thereby protecting their expensive equipment. Clearly, the most important element in this fail-safe system is an accurate and reliable flow sensor for the coolant.
This is where Titan Enterprises entered the story. Our 1000-series mini-turbine flow sensors were initially selected for HiPERCAM. Six such flow sensors were required, one for each of the 5 detectors and a sixth for the control electronics (which are also cooled). These six flow sensors were attached to the 6-way input manifold mounted on HiPERCAM that is used to feed the 6 parallel cooling circuits, as shown in the photo below. The Titan 1000-series flow sensors were first tested in the lab in the UK and then commissioned on the camera on La Palma in 2018.
All seemed to work well at first, with each sensor typically reporting a flow rate of 1.5 litres/minute. However, over time (days-weeks), the sensors started to report zero flow rate, which caused the peltier power supplies to switch off and the detectors to warm up, making HiPERCAM inoperable. Given that each night on the telescope costs 50,000 euros, this was a serious problem.
On inspecting the faulty sensors, it was discovered that the magnets in the Hall-effect propellers had accumulated a slimy, metallic substance over time, which had eventually prevented them from spinning on their sapphire bearings, thereby reporting zero flow rate. Clearly, despite filtering, the coolant supply provided by the observatory was not clean enough to use this type of flow sensor.
The HiPERCAM team therefore recontacted Titan Enterprises for advice. We recommended a change to ultrasonic sensors, as these are impervious to the metallic sludge present in the coolant. We provided custom housings for our Process Atrato ultrasonic sensors and a special calibration to emulate the 1000-series sensors, so they were “plug and play” replacements. Once again, the flow sensors were tested extensively in the lab at Sheffield prior to shipping to La Palma for commissioning on HiPERCAM at the start of 2019.
Unfortunately, once the flow sensors were installed in HiPERCAM on La Palma, they behaved erratically, exhibiting zero or highly variable flow rates, something that had not been evident in the lab during testing in Sheffield, UK. This problem was traced to the presence of tiny bubbles in the coolant supply of the telescope, which attenuated the ultrasonic signal and made the flow sensors unreliable. Since the telescope’s coolant supply is provided by an observatory-wide chiller system housed in a separate building, something which the HiPERCAM team have no access to and no control over, there was no way of removing either these micro bubbles or the metallic sludge.
Finally a working solution
As a result the HiPERCAM team turned again to Titan Enterprises for further advice. Is there another type of sensor that could be used that is impervious to the undesirable (but unavoidable) characteristics of the telescope’s cooling fluid? The answer was yes – the Titan FT2 optical flow sensor that uses an LED and photo-diode to measure flow. When the rotor spins the photo-diode can detect the LED with a frequency proportional to the flow rate. When the rotor is static, the photodiode cannot see the LED, indicating no flow.
While this is an old and infrequently requested technology, fortunately for the HiPERCAM team, they are still available, and six were delivered in October 2019. The optical flow sensors have just been successfully tested in the lab at Sheffield and are now on their way to La Palma for final commissioning on HiPERCAM. Everyone involved is now hopeful that this will be a successful end to the story, and Titan Enterprises will have enabled HiPERCAM to finally achieve its full scientific potential.
For further information on the HiPERCAM camera being used on the world’s largest optical telescope, the 10.4m Gran Telescopio Canarias (GTC) on the Canary Island of La Palma – please visit www.gtc.iac.es/instruments/hipercam/hipercam.php