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Paper: New Results from GISMO, a 2 mm Camera Using a Backshort-Under-Grid TES Bolometer Array
Volume: 417, Submillimeter Astrophysics and Technology: A Symposium Honoring Thomas G. Phillips
Page: 451
Authors: Staguhn, J.; Benford, D.; Allen, C.; Arendt, R.; George, J.; Fixsen, D.; Maher, S.; Sharp, E.; Ames, T.; Chuss, D.; Dwek, E.; Marx, C.; Miller, T.; Moseley, S. H.; Navarro, S.; Schinnerer, E.; Sievers, A.; Walter, F.; Wollack, E.
Abstract: In October 2008, we demonstrated for the second time our 2 mm bolometer camera GISMO (the Goddard IRAM Superconducting 2 Millimeter Observer) for astronomical observations at the IRAM 30 m telescope in Spain. GISMO uses a monolithic 8×16 Backshort Under Grid (BUG) array with superconducting Transition Edge Sensors (TES). The instrument’s fast beam yields 0.9 λ/D pixel sampling, which optimizes GISMO for detecting sources serendipitously in large sky surveys, while the capability for diffraction limited imaging is preserved. In order to ensure GISMO’s operation under all weather conditions, we used a 40% neutral density filter at 4 K. In less than ideal weather conditions we obtained map sensitivities for the working pixels of between 40 mJy s-1/2 and 50 mJy s-1/2. These numbers are consistent with our atmospheric model calculations. The predicted map NEFDs range from 28 to 56 mJy s-1/2 for GISMO’s optical configuration (including the 40% neutral density filter) and observing conditions ranging between 10% and 40% line of sight opacities. With an optimized observing strategy for point sources of known position, this sensitivity can further be increased by a factor of 1.5, which we demonstrate by means of analyzing of our signal to noise ratio achieved for point source crossings in the time stream data. The noise in our co-added maps (with few thousand seconds of integration time) scales very well with the square root of time down to sub-mJy levels. We have now designed a cryogenic mechanism that will allow us to move neutral density filters in or out of the beam during future observations. Our simulations indicate that we will gain a factor of 2.4 in observing efficiency if we take out the neutral density filter under good weather conditions. Under typical conditions (20% line of sight sky opacity) we expect to achieve a good pixel map sensitivity of 22 mJy s-1/2 and a map NEFD of 1 mJy in an observation lasting about 8 minutes.
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