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Balloon-borne telescopes present unique thermal design challenges which are a combination of those present for both space and ground telescopes. At altitudes of 35-40 km, convection effects are minimal and difficult to characterize. Radiation and conduction are the predominant heat transfer mechanisms reducing the thermal design options. For long duration flights payload mass is a function of power consumption making it an important optimization parameter. SuperBIT, or the Super-pressure Balloon-borne Imaging Telescope, aims to study weak lensing using a 0.5m modified Dall-Kirkham telescope capable of achieving 0.02 stability and capturing deep exposures from visible to near UV wavelengths. To achieve the theoretical stratospheric diffraction-limited resolution of 0.25, mirror deformation gradients must be kept to within 20nm. The thermal environment must thus be stable on time scales of an hour and the thermal gradients must be minimized on the telescope. SuperBIT plans to implement two types of parameter solvers; one to validate the thermal design and the other to tightly control the thermal environment.
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Balloon experiments are an economically feasible method of conducting observations in astronomy that are not possible from the ground. The astronomical payload may include a telescope, a detector, and a pointing/stabilization system. Determining the attitude of the payload is of primary importance in such applications, to accurately point the detector/telescope to the desired direction. This is especially important in generally unstable lightweight balloon flights. However, the conditions at float altitudes, which can be reached by zero pressure balloons, could be more stable, enabling accurate pointings. We have used the Inertial Measurement Unit (IMU), placed on a stratospheric zero pressure balloon, to observe 3-axis motion of a balloon payload over a fight time of 4.5 hours, from launch to the float altitude of 31.2 km. The balloon was launched under nominal atmospheric conditions on May 8th 2016, from a Tata Institute of Fundamental Research Balloon Facility, Hyderabad.
We present the second generation BLASTbus electronics. The primary purposes of this system are detector readout, attitude control, and cryogenic housekeeping, for balloon-borne telescopes. Readout of neutron transmutation doped germanium (NTD-Ge) bolometers requires low noise and parallel acquisition of hundreds of analog signals. Controlling a telescopes attitude requires the capability to interface to a wide variety of sensors and motors, and to use them together in a fast, closed loop. To achieve these different goals, the BLASTbus system employs a flexible motherboard-daughterboard architecture. The programmable motherboard features a digital signal processor (DSP) and field-programmable gate array (FPGA), as well as slots for three daughterboards. The daughterboards provide the interface to the outside world, wi
An attitude determination system for balloon-borne experiments is presented. The system provides pointing information in azimuth and elevation for instruments flying on stratospheric balloons over Antarctica. In-flight attitude is given by the real-time combination of readings from star cameras, a magnetometer, sun sensors, GPS, gyroscopes, tilt sensors and an elevation encoder. Post-flight attitude reconstruction is determined from star camera solutions, interpolated by the gyroscopes using an extended Kalman Filter. The multi-sensor system was employed by the Balloon-borne Large Aperture Submillimeter Telescope for Polarimetry (BLASTPol), an experiment that measures polarized thermal emission from interstellar dust clouds. A similar system was designed for the upcoming flight of SPIDER, a Cosmic Microwave Background polarization experiment. The pointing requirements for these experiments are discussed, as well as the challenges in designing attitude reconstruction systems for high altitude balloon flights. In the 2010 and 2012 BLASTPol flights from McMurdo Station, Antarctica, the system demonstrated an accuracy of <5 rms in-flight, and <5 rms post-flight.
The E and B Experiment (EBEX) was a long-duration balloon-borne instrument designed to measure the polarization of the cosmic microwave background (CMB) radiation. EBEX was the first balloon-borne instrument to implement a kilo-pixel array of transition edge sensor (TES) bolometric detectors and the first CMB experiment to use the digital version of the frequency domain multiplexing system for readout of the TES array. The scan strategy relied on 40 s peak-to-peak constant velocity azimuthal scans. We discuss the unique demands on the design and operation of the payload that resulted from these new technologies and the scan strategy. We describe the solutions implemented including the development of a power system designed to provide a total of at least 2.3 kW, a cooling system to dissipate 590 W consumed by the detectors readout system, software to manage and handle the data of the kilo-pixel array, and specialized attitude reconstruction software. We present flight performance data showing faultless management of the TES array, adequate powering and cooling of the readout electronics, and constraint of attitude reconstruction errors such that the spurious B-modes they induced were less than 10% of CMB B-mode power spectrum with $r=0.05$.
We present the technology and control methods developed for the pointing system of the SPIDER experiment. SPIDER is a balloon-borne polarimeter designed to detect the imprint of primordial gravitational waves in the polarization of the Cosmic Microwave Background radiation. We describe the two main components of the telescopes azimuth drive: the reaction wheel and the motorized pivot. A 13 kHz PI control loop runs on a digital signal processor, with feedback from fibre optic rate gyroscopes. This system can control azimuthal speed with < 0.02 deg/s RMS error. To control elevation, SPIDER uses stepper-motor-driven linear actuators to rotate the cryostat, which houses the optical instruments, relative to the outer frame. With the velocity in each axis controlled in this way, higher-level control loops on the onboard flight computers can implement the pointing and scanning observation modes required for the experiment. We have accomplished the non-trivial task of scanning a 5000 lb payload sinusoidally in azimuth at a peak acceleration of 0.8 deg/s$^2$, and a peak speed of 6 deg/s. We can do so while reliably achieving sub-arcminute pointing control accuracy.
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