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The GBT 4mm Rx . David T . Frayer (NRAO). Instrument’s purpose: 67-93.3 GHz spectral-line science {Called “4mm” since middle of band is 80GHz (3.75mm) and plans for future higher frequency Rx at 3.5-2.6mm}. First Light Spectrum: May 2011 Commissioned: Jan-Mar 2012
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The GBT 4mm Rx David T. Frayer (NRAO) Instrument’s purpose: 67-93.3 GHz spectral-line science {Called “4mm” since middle of band is 80GHz (3.75mm) and plans for future higher frequency Rx at 3.5-2.6mm} First Light Spectrum: May 2011 Commissioned: Jan-Mar 2012 Amplifier upgrade: Nov 2012
485 ft 151 ft At 100 m, the GBT is the largest fully steerable telescope in the world. Unblocked Aperture Active Surface Operates from ~100 MHz to 100 GHz Fully Steerable >85% of total sky covered δ≥-46° Pointing to 1”-2” accuracy Surface good for 3mm work 2.3 acre collecting area
Improvements to Surface Makes 3mm Possible (From Todd Hunter, PTCS group)
The Active Surface 2209 actuators Currently rms < 240μm at night, the goal is ~200μm University of Georgia, Sept 2011 4
GBT Effective Collecting Area (ηa * Area) Assumes current ~240um rms surface errors 35% at 90 GHz most sensitive facility at Q and 4mm (ALMA ”band2”)
Weather at Green Bank good for 3mm observations for 1000-2000hrs per year Hours available per year with acceptable winds and tau: (from Ron Maddalena’s weather studies)
Rx Block Diagram • Specs: • Dual beam (4.7’) separation (cold load between beams) • Dual linear polarization for both beams, 1/4plate allows circular polarization for 1 beam for VLBI observations • Frequency Range: 67-93 GHz • IF system 4 separate bands: [FL1 67-74 GHz], [FL2 73-80 GHz], [FL3 79-86 GHz], [FL4 85-93 GHz]
4mm Calibration Wheel Cold 1/4 1 2 3 0 Beam1 Beam2 5 4 Warm
4mm Performance and Tsys Improvement Boxes are original system, while diamonds show Tsys after new amplifiers. Aperture and main-beam efficiency of the dish given to the right as dotted and solid lines respectively. (tau_90GHz~0.18 for both sets of measurements)
Latest Performance Measurements Tsys~60-70K over central band and under 100K from 70-90 GHz (tau_90 GHz~0.08)
Science with 4 mm Rx, Lines: • Dense gas tracers in star-forming regions and nearby galaxies (HCN, HNC, HCO+all at ~90GHz) • D-species in cold cloud cores (~70-80GHz) • Astro/bio-chemistry (throughout the band)
4mm Rx: Cold starless-cores molecular freeze-out D-species enhanced N2D+ in L1544 at 77 GHz (S. Schnee et al.) N2H+ in W3OH at 93 GHz
4mm: Dense gas and Molecular Diversity in Nearby Galaxies M82 H2CO (formaldehyde) & HC3N (J. Mangum)
4mm Mapping M82 HCO+(1-0) contours on VLA 6cm image (A. Kepley et al.)
Zoomed in spectrum on Orion-KL 81-82 GHz
Example VLBA+GBT-4mm fringes GBT VLBI will probe the physics near the base of black hole jets in nearby galaxies and directly measure the size of the galaxy via parallax of SagA*.
Web Links… 4mm Web Page: http://www.gb.nrao.edu/4mm/ 4mm Wiki: https://safe.nrao.edu/wiki/bin/view/GB/Gbt4mmRx 4mm Commissioning Wiki (latest info on performance and notes for users): https://safe.nrao.edu/wiki/bin/view/GB/Gbt4mmRxCommissioning