The Robert C Byrd Green Bank Telescope Frank Ghigo, National Radio Astronomy Observatory US VLBI Meeting, Socorro, Nov 2011 VLBI usage: About 15% of GBT time goes to VLBI projects. Including 2 Large projects: Megamaser cosmology Planet Search Also HSA, globals, EVN

Subreflector and receiver room

On the receiver turret

Inside the receiver room

GBT active surface system Surface has 2004 panels –average panel rms: 68  2209 precision actuators

GBT Receivers

BB240GD.GB.py import os isAstrid = 0 if 1: try: if os.getenv('ASTRIDVLBA') == '1': isAstrid = 1 except: pass if not isAstrid: from edu.nrao.evla.observe import Mark5C from edu.nrao.evla.observe import MatrixSwitch from edu.nrao.evla.observe import RDBE from edu.nrao.evla.observe import VLBALoIfSetup from edu.nrao.evla.observe import Parameters from edu.nrao.evla.observe import bbc second = 1.0/ deltat2 = 1 obsCode = 'BB240GD' stnCode = 'GB' mjdStart = *second recorder0 = Mark5C('-1') recorder0.setMode('Mark5B') recorder0.setPSNMode(0) recorder0.setPacket(0, 0, 40, 5008) subarray.setRecorder(recorder0) : dbe0 = RDBE(0, 'ddc') dbe0.setALC(1) dbe0.setFormat('Mark5B') dbe0.setPSNMode(0) dbe0.setPacket(0, 0, 40, 5008) subarray.setDBE(dbe0) loif0 = VLBALoIfSetup() loif0.setIf('B', '4cm', 'R', 9100, 'L') loif0.setIf('D', '4cm', 'L', 9100, 'L') loif0.setPhaseCal(1) loif0.setDBEParams(0, -1, -1, 10, 0) loif0.setDBEParams(1, -1, -1, 10, 0) loif0.setDBERemember(0, 1) loif0.setDBERemember(1, 1) channelSet0 = [ \ bbc(0, , 16, 'L', 2, 0), \ bbc(1, , 16, 'L', 2, 0), \ bbc(0, , 16, 'L', 2, 0), \ bbc(1, , 16, 'L', 2, 0), \ bbc(0, , 16, 'L', 2, 0), \ bbc(1, , 16, 'L', 2, 0), \ bbc(0, , 16, 'L', 2, 0), \ bbc(1, , 16, 'L', 2, 0) \ ] source0 = Source( , ) source0.setName('J ') source1 = Source( , ) source1.setName('GJ1116A') source2 = Source( , ) source2.setName('J ') source3 = Source( , ) source3.setName('GJ278C')

More BB240GD.GB.py # Scan 1 = No0002 # pointing scan for the GBT subarray.setSource(source4) recorder0.setPacket(0, 0, 40, 5008) if isAstrid: source4.setPeak(True) subarray.setRecord(mjdStart + 661*second, mjdStart+1141*second, 'No0002', obsCode, stnCo de ) if array.time() < mjdStart *second: subarray.execute(mjdStart + 475*second) else: print "Skipping scan which ended at time " + str(mjdStart+1141*second) + " since array.time is " + str(array.time()) source4.setPeak(False) # Scan 2 = No0003 subarray.setSource(source8) recorder0.setPacket(0, 0, 40, 5008) subarray.setRecord(mjdStart *second, mjdStart+1340*second, 'No0003', obsCode, stnCode ) if array.time() < mjdStart *second: subarray.execute(mjdStart *second) else: print "Skipping scan which ended at time " + str(mjdStart+1340*second) + " since array.time is " + str(array.time())

MHz filter

MHz filter, zoom

90 GHz observing in Green Bank?? ~500 hours per year with  < 0.1 and winds < 3 m/s Surface rms ~ 240  aperture efficiency ~30% at 90 GHz Tracking, 9” beam –Half-power tracking tests: about 1” rms over 15 min –Offset pointing : about 2-3” rms Pointing, servo Improvements Pointing model, including thermal effects Servo upgrades – digital servo system for Az and El motions Eliminate servo resonances Minimize structural resonances, reduce settling time

Tracking at half-power

4mm receiver GHz

4mm receiver – inside the dewar

Calibration turntable

¼ wave plate

4mm receiver in the turret

Tsys calibration Tsys is be calculated from power measurements synchronized with the hot/cold load settings. Measurements will be done at intervals of a few to 60 minutes. For VLBI, ideally the RDBE will measure power in each channel, from which Tsys can be calculated if the state of the calibration wheel is known. In general, the DCR (digital continuum receiver), running in parallel with other back ends will detect total power in the IF and its data can be used to calculate Tsys; the values will be made available in a table. –But DCR inputs follow IF bandpass filters of 80, 240, or 1280 MHz.