Formerly known as the Penn array, MUSTANG is a focal plane array designed for the 100 m diameter Green Bank telescope. In September 2006 it became the first instrument to use the GBT at 90 GHz and it is also the GBT's first focal plane array. Large focal plane arrays represent a significant step in single dish receiver technology increasing the mapping speed of the telescope by the number of detectors. Systematic effects that are normally removed by fast chopping of the telescope beam on the sky can be removed by comparing neighboring pixels while carrying out slow scans. Noise from the atmosphere or electronics will be common to many pixels whereas the sky signal will not and the two can be separated with no loss in observing efficiency.
MUSTANG has an 8 by 8 planar array of Transition Edge Sensor (TES) Bolometers for detectors. The array is used without feedhorns and it is read out with time based SQUID multiplexing electronics. High density polyethylene lenses and capacitive mesh filters are used to focus light onto the detectors, define our 81 to 99GHz bandpass, and control the illumination of the telescope. Each of these components is described below.
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The MUSTANG arrayEach detector is a 2.9mm square membrane of silicon suspended by micron thick legs to a silicon frame. The optical power being absorbed by the membrane is measured by a TES thermometer on each membrane. Wiring to the TES runs along the frames that support the detectors. The detector array was designed and built by Harvey Moseley's group at Goddard as is the multiplexed readout electronics, the Mark III, which is based on the NIST time based multiplexing system. Bolometers detect light by measuring the temperature of an absorber as it heats up or cools down depending on the intensity of the radiation landing on it. TES bolometers are slightly different. The superconducting TES on the membrane is voltage biased and heats up to its transition temperature (~0.480K) without any light landing on it. As more optical power lands on the detector the amount of current required for this heating drops. This current is read out using sensitive SQUID amplifiers. TES bolometers have several advantages over conventional bolometers including speed and very low noise. |
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The MUSTANG opticsInstead of horn feeds and waveguide MUSTANG uses High density polyethylene lenses, a Lyot stop, and filters to define our bandpass and control our illumination of the telescope. Bolometers detect all light that lands on them. So that the detectors are not overloaded by each optical element is cooled and cold baffles stop any out of band light from objects hotter than 0.3K from reaching the detectors. Other black baffles stop reflections from creating false 'ghost' images. Also built into the optics is magnetic shielding to protect the sensitive SQUID amplifiers. Lens based optics enable MUSTANGS detectors to be spaced so that they are slightly more than half a beam width apart on the sky (0.7fλ). This means the sky is close to fully sampled in a single pointing and the scanning requirements on the GBT are relaxed - moving a structure as large as the GBT at high speeds without vibrations is hard so this is a large advantage. |
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The CryogenicsOur bolometers need cooling to under 0.3K (-459.13F). As expendable cryogens would be impractical to use on the GBT we developed a close cycle system based on a 3K pulse tube cooler. The final stage of cooling is achieved by a Helium-3 sorption refrigerator, which is run off a helium-4 sorption refrigerator which in turn runs off the pulse tube. |