![Page 1: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/1.jpg)
ObservingTechniquesandCalibration
David Frayer (Green Bank Observatory)
![Page 2: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/2.jpg)
TheGBTprovidesalotofobservingchoices
• Pickreceiverbasedonfrequency• Pickbackendbasedonobservingtype(line,continuum,pulsar,….)
• Pickobservingtechniquesbasedonsciencegoals(pointsource,largefield,narrowlinesvsbroadlines….)
• Calibrationstategiesdependonreceiverandscienceneeds
![Page 3: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/3.jpg)
AvailableGBT
receivers
2
What frequency do you need?
![Page 4: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/4.jpg)
AvailableGBTBackends
(Ka-band)
(Ka-band)
![Page 5: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/5.jpg)
Whatareyoudoing?:
Continuum Continuumfull-stokes
Line Pulsar VLB Radar
DCR Mode-1VEGAS
VEGAS GUPPI Mark5VLBArecorder
Radarbackend
CCB(Ka) Mueller matrixcalibration(functionofparallacticangle)
{29modes}
VEGAS-Pulsar
Mustang(3mm)
{Searchmode,timingmode}
Reductionusesspecializedscripts
Observing Mode vs Backend Capabilities
![Page 6: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/6.jpg)
VEGASModes:
16 separate spectrometer channels (8 dual polarization channels) that can be divided between beams and different frequencies as needed and can support up to 8 spectral sub-windows per spectrometer.Maximum data rate ~160GB/s, but most projects at <1MB/s
![Page 7: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/7.jpg)
Pickingyourobservingmode
![Page 8: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/8.jpg)
Thetelescopemeasures:Ta=“antennatemperature”
• Ta(total)=Tsource +{Trx +Tbg +Tatm +Tspill}• Where{….}=othercontributions• WantTsource,socarryoutON– OFF• Ta(ON)=Tsource + {….}• Ta(OFF)={….}• SoTa(ON)-Ta(OFF)=Tsourceè NeedtocarryoutON-OFFobservationsandtherearedifferentobservingtechniquesformeasuringON-OFF
![Page 9: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/9.jpg)
Different Observing Modes to derive the reference data (OFF)
Types of reference observationsØFrequency Switching – In or Out-of-band
ØPosition Switching– Reference-Off– Mapping-Off
ØDual-Beam Position Switching• Nod -- Move telescope• SubBeamNod -- Move Subreflector
![Page 10: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/10.jpg)
In-Band Frequency Switching
![Page 11: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/11.jpg)
Out-Of-Band Frequency Switching
![Page 12: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/12.jpg)
Position Switching
ON source OFF sourceTsource + Teverything else Teverything else
Arbi
trary
Uni
ts
![Page 13: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/13.jpg)
Position Switching: ON-OFF on Sky
ON - OFF(Tsource + Teverything else) - (Teverything else)
Arbi
trary
Cou
nts
![Page 14: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/14.jpg)
Beam Switching – Subreflector or tertiary mirror
• Removes any ‘fast’ gain/bandpass changes• Low overhead. ½ time spent off source
![Page 15: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/15.jpg)
Subreflector Nodding with multi-beam receivers (SubBeamNod)
• Removes any ‘fast’ gain/bandpass changes• Low overhead. ~All the time is spent on source
![Page 16: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/16.jpg)
Nodding with dual-beam receivers -Telescope motion (NOD)
• Removes any ‘fast’ gain/bandpass changes • Overhead from moving the telescope. All the time is spent on source
![Page 17: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/17.jpg)
Mapping Techniques• Point map
•Sit, Move, Sit, Move, etc.• On-The-Fly Mapping
•Slew a column or row while collecting data•Move to next column row•Basket weave•Should oversample ~3x Nyquist along direction of slewReference/OFF from a “source-free” map position or separate “OFF” spectrum taken.
![Page 18: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/18.jpg)
ExampleDaisyScanMap
![Page 19: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/19.jpg)
FrequencyvsPositionSwitching
• Narrowlineinnon-crowdedspectrumèFrequencySwitching(FS)
• NarrowlineincrowdedspectralregionorsignificantRFIè PositionSwitching(PS)
• Broadlineè PSØNarrowline<10km/sØBroadline>100km/s
![Page 20: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/20.jpg)
ObservingMode– SmallSourceIfsourcesize<beam,LineObs,andPS:• Nod{twobeams}– ifnotlimitedbybaselines• SubBeamNod {twobeams}– ifbaselinelimited
• OnOff {onebeam}• Track(withandw/ooffset)Ifsourcesize<beam,LineObs andFS:• TrackIfsourcesize<beam,ContinuumObs:• Daisymap(efficientwaytodealwith1/fnoise)
![Page 21: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/21.jpg)
ObservingMode– LargeSource
ØMap>FOVofinstrument• RaLongMap and/orDecLatMapØMap<~FOVofinstrument• RaLong/DecLat mapping• Daisy• Boxscans• PointMap (Grid)ifneedingadeepspectrum
![Page 22: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/22.jpg)
PerformanceoftheGBT
![Page 23: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/23.jpg)
NoiseLevels(Tsys)forTypicalWeather
![Page 24: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/24.jpg)
GBTSurfaceImprovedin2009
![Page 25: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/25.jpg)
GBTApertureEfficiencyandGain(K/Jy)
Very good efficiency at lower-end of 3mm band
![Page 26: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/26.jpg)
GBTPointingandSurfacePerformance
• ~5-10arcscec blindpointing• ~2arcsec offsetpointing• ~1arcsec trackingaccuracy• Rms (surface)~0.35mm– nocorrectionsduringday• Rms (surface)~0.3mm– nocorrectionsduringnight• Rms(surface)~0.23mmwithcorrectionsatnight• Long-termGoal:Rms(surface)~0.20mm
![Page 27: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/27.jpg)
Observing:AntennaOptimization• Shouldpoint+focus every30min-1hrdependingonfrequencyandtimeofday(point+focus takes~5min)
• AutoOOF (whichtakes~30min)isusedtocorrectthesurfaceforthermaleffectsforQ-bandandW-bandatnight.
• Daytimesurfacechanges<1hrtimescalesandtheAutoOOF solutionscancausemoreharmthangoodafter~1hrfromtheAutoOOF (soitistypicallynotusefultousethe“thermal”correctionsduringtheday).
![Page 28: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/28.jpg)
Calibration
![Page 29: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/29.jpg)
Calibration of Data(ON – OFF)/OFF
[(Tsource + Teverything else) - (Teverything else)]/ Teverything else
=(Source temperature)/(”System” temperature)
% T
sys
![Page 30: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/30.jpg)
GBT Definition of Ta
Ta = (ON – OFF)
TsystemOFF
From diodes, Hot/Cold loads, etc.
Blank Sky or other
![Page 31: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/31.jpg)
Determining Tsys
Noise Diodes
All GBT receivers besides 4mm, Argus, and Mustang use noise diodes.
![Page 32: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/32.jpg)
Determining Tsys
Tsys = Tcal * OFF/(ON – OFF)
GBT: Flicker diode on/offTsys = Tcal * OFF/(ON – OFF) + Tcal/2
Noise Diodes
Typically choose low Tcalvalue to minimize Tsys and high Tcal value for very bright sources (for Rx that have two options)
![Page 33: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/33.jpg)
Determining Tsys
Hot & Cold Loads
Hot LoadThot
Cooling SystemTcold
Gain: g =(Thot – Tcold)/(Vhot –Vcold) [K/Volts]
Tsys = g Voff
Example GBT 4mm Rx
![Page 34: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/34.jpg)
Installationof4mmcalibrationwheelandexternalcover
Look for this during GBT tour:
![Page 35: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/35.jpg)
4mmCalibrationWheelWheelPosition(defined wrtBeam1)
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
![Page 36: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/36.jpg)
CalSeq-autoScan(GFMdisplay)
Beam1
Beam2Sky
Cold1 Cold2
Warm2Warm1
When you click on a calseq scan, GFM reports the gains and Tsys in the console window
![Page 37: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/37.jpg)
AbsoluteCalibrationonknownastronomicalsources(pointsources)è Correctsforanyerrorsin theadoptedTdiode/gainsmeasuredinthelabandcorrectsforthetelescoperesponse
Observe and process source and known calibrator (3cX) source data in the same way, then the flux density of the source S(source) is simply:
S(source)/S(3cX) = T(source)/T(3cX),
where S(3cX) is known.
Absolute calibration typically known to 5-15%
![Page 38: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/38.jpg)
AbsoluteCalibration tiedtoMarsviaWMAP
Mars WMAPobservations with model in red
VLA calibration (1-50 GHz):
Ø <20 GHz ~1% accurate
Ø 20-50 GHz: ~3% accurate
Perley & Butler 2013
![Page 39: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/39.jpg)
VLAStableCalibrators
GBT Calibration “Plan”:
Eventually tie GBT to VLA calibration scale for 1-50 GHz, and we will use ALMA for 3mm absolute calibration
![Page 40: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/40.jpg)
ComparisonofCalibrationScales(Ott 1994vsVLA2013)
![Page 41: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/41.jpg)
GBTCalibrationMeasurements2015-2016
![Page 42: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/42.jpg)
DonotblindlyaccepttheGBTNoiseDiodeCalibration
• Noisediodesarerecommendedtobesentbackforre-calibrationevery6monthstomeetlaboratoryspecs– weneverdodothis– wecouldexpectdriftsontimescaleof1-2years.
• TheKFPAhasvariablenoisediodes.• ThenoisediodeswerelastcalibratedempiricallyfortheGBT10+yearsago…..
• Therearesignificantvariationsinthenoisediodesasafunctionoffrequency.
• Youshouldcalibrateyourdata.
![Page 43: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/43.jpg)
EstimateofErrorinGBTCalibrationBand GBT/VLACalibration
340 0.50
800 1.17
L 0.92
S 0.88
C 1.32
X 1.00
Ku 0.87
KFPA 0.90
Ka 1.01
Q 0.83
Based on 15A486 calibration program and ongoing observing programs over 2015-2016. Results based on averaging both polarizations.
![Page 44: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/44.jpg)
The atmosphere is important at high frequency (>10 GHz)
• Opacity– Tsys = Trcvr + Tspill +Tbg * exp(-tau*A) + Tatm * [exp(-tau*A) – 1]– Air Mass A~ 1/sin(Elev) (for Elev > 15°)
• Stability– Tsys can vary quickly with time– Worse when Tau is high
GBT site has many days with low water vapor per year (<10mm H2O are ok for 3mm, 50% of time)
![Page 45: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/45.jpg)
BackgroundInformationonCalibration
![Page 46: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/46.jpg)
TemperatureScales
ØTa=Tsys (ON-OFF)/OFF(uncorrected antenna temperature)
ØTa’=Taexp(τoA)ØTmb =Ta’/ηmb (ηmb~1.3ηa)
ØTa*=Ta’/ηl (mm-telescopes typicallyreturnTa*)
ØTr*=Ta’/(ηl ηfss)Ø Ta’/Sν =2.84ηa (fortheGBT)
![Page 47: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/47.jpg)
CalibrationwithTwoLoads
![Page 48: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/48.jpg)
CalibrationwithOneLoad,TA*
With a chopper wheel/ vane and a simple temperature sensor, one can calibrate to the approximate Ta* scale without any knowledge of the sky.
![Page 49: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/49.jpg)
Tsys forTA* scaledifferentthanTsys forTA
![Page 50: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/50.jpg)
Point-SourceCalibration:FluxDensityvsAntennaTemp
Prec =½Ae SνΔν =kTa’ΔνAe=ηa (π/4)D2
èSν =3520Ta’/(ηa [D/m]2)i.e.,Ta’/Sν =2.84ηa fortheGBT(ηa=0.71atlowν)Usedforpoint-sourcecalibration:Ø MeasureTaØ Correctforatmosphereà Ta’
Ø KnowSνØ Deriveηa
![Page 51: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/51.jpg)
ExtendedSources:Tmb vs Tsource
![Page 52: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/52.jpg)
![Page 53: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/53.jpg)
GaussianSource
![Page 54: Observing Techniques and Calibration · 0 Observing Sky Sky 1 Cold1 Cold Warm 2 Position2 1/4wave circ Sky 3 Position3 Sky Sky 4 Cold2 Warm Cold 5 Position5 Sky 1/4wave Circ Beam1](https://reader034.vdocuments.us/reader034/viewer/2022052016/602e580af308d474556f43b4/html5/thumbnails/54.jpg)
ConcludingRemarksØ Toobserveweaksignals,oneneedstomeasureON-OFFØ SeveraldifferentobservingtechniquescanbeusedtogiveON-OFF(freq-switched,positionswitched)
Ø Atcmwavelengths,weusenoisediodestocalibratethedata,whileatmmwavelengthsambient/coldloadsareused
Ø Usersshouldcorrectforatmosphereatallfrequencies,butitiscrucialathighfreq.
Ø Usersshouldobserveaknowncalibratoroncepersemesterpertargetfrequencytocalibratethenoisediodesandinstrumentaleffectsempirically.
Ø Absolutecalibrationshouldbedoneingoodweather.