The detector that is being used in this band is a Transition Edge Sensor (TES), specifically a bolometer operating at 450 mK (or -272.7 degrees Celsius). Optics and filters within the receiver as well as the telescope mirrors themselves direct sources of 90GHz radiation to an array of bolometers, sensors that detect heat incident on their surface. This array has eight by nine pixel elements. All bolometers in the array are read using an electronics system (Mark III) developed by the National Institute of Standards and Technology (NIST) and modified by the National Aeronautics and Space Administration (NASA). Part of the electronics system and the bolometers themselves are kept within a cryostat designed and built by the Experimental Cosmology Group at Penn that cools to about 250 mK in a single chamber.
The goal of this computer science engineering project was to design and implement algorithms that can be used to tune a three stage amplification and multiplexing system, each stage comprising of one or more Superconducting QUantum Interference Devices (SQUIDs). SQUIDs are circuit elements that respond to a magnetic flux through the device by changing the flow of electrical current out of the device’s two terminals. These SQUIDs, working with inductors that produce magnetic flux, act to transfer signals from one stage to the next and also create the gain. The amplification chain of SQUIDs is what is used to read out the changes of the bolometer detectors as they sense incoming radiation. “Tuning” this system consists of properly setting voltages through digital to analog converters associated with each of the three stages of SQUIDs such that the output is as close to linear as possible with respect to the input. This is desirable because only after a response that is close to linear is achieved can digital feedback be applied so as to make the amplification system completely linear. A secondary requirement of the tuning algorithm is to place each of the TES detectors onto their transition from normal to superconducting.
The algorithm that has resulted from this project is necessary because the instrument, as a whole, is to be used by astronomers who are unfamiliar with the system, and may only have a basic knowledge of the technology involved. Thus, a user friendly interface that allows the observation of the tuning process as well as the control of parameters must also accompany the algorithm. Tuning the SQUID amplification system will result in different voltage settings to the stages due to changes in the overall system such as temperature of different components. Therefore, it must be done before any observation is attempted. Creating an algorithm to do this previously human task has sped up the process to limits that are dependent on the data acquisition and system commanding latencies.