the gbt 4mm rx

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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 2011Commissioned: Jan-Mar 2012Amplifier upgrade: Nov 2012

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2.3 acre collecting area

485 ft

151 ft

At 100 m, the GBT is the largest fully steerable telescope in the world. Unblocked Aperture

Active SurfaceOperates from ~100 MHz to 100 GHzFully Steerable>85% of total sky covered δ≥-46°Pointing to 1”-2” accuracySurface good for 3mm work

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Improvements to Surface Makes 3mm Possible

(From Todd Hunter, PTCS group)

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2209 actuatorsThe Active SurfaceCurrently rms < 240μm at night, the goal is ~200μm

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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”)

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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)

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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]

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Installation, calibration wheel and external cover

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4mm Calibration Wheel

Wheel Position (defined wrt Beam1)

Beam 1 Beam 2

0 Observing Sky Sky

1 Cold1 Cold Warm

2 Position2 1/4wavecirc

Sky

3 Position3 Sky Sky

4 Cold2 Warm Cold

5 Position5 Sky 1/4waveCirc

Beam1 Beam20

21

3

45

Cold

Warm

1/4

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CDL LNA’s optimized for 4mm Band (installed Nov 2012)

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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)

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Latest Performance Measurements

Tsys~60-70K over central band and under 100K from 70-90 GHz (tau_90 GHz~0.08)

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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)

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4mm Rx: Cold starless-cores molecular freeze-out D-species enhanced

WVU 2/23/12

N2D+ in L1544 at 77 GHz (S. Schnee et al.)

N2H+ in W3OH at 93 GHz

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HCN/HCO+ in M31 Molecular Clouds, A. Schruba et al.

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4mm: Dense gas and Molecular Diversity in Nearby Galaxies

M82 H2CO(formaldehyde)& HC3N (J. Mangum)

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4mm Mapping

M82 HCO+(1-0) contours on VLA 6cm image(A. Kepley et al.)

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Orion-KL Spectral Line Survey 67-93.6 GHz (Frayer et al.)

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Zoomed in spectrum on Orion-KL

81-82 GHz

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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*.

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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