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Determining Tsource
• Tmeas (,az,za) =
Tsrc(,az,za)
+ Tsystem -> Rx, other hardware
The noise that is picked up as the signal is received and passed down through the
processing system
Determining Tsource
• Tmeas (,az,za) =
Tsrc(,az,za)
+ TRX, other hardware
+ Tspillover (za,az)
+ Tcelestial(t)
• Tmeas (,az,za) =
Tsrc(,az,za)
+ TRX, other hardware
+ Tspillover (za,az)
+ Tcelestial(t)
+ TCMB
+ Tatm(za)
Determining Tsource
• Tmeas (,az,za) =
Tsrc(,az,za)
+ TRX, other hardware
+ Tspillover (za,az)
+ Tcelestial(t)
+ TCMB
+ Tatm(za)
Tmeas = Tsource + Teverything else
Determining Tsource
• Tmeas (,az,za) =
Tsrc(,az,za)
+ TRX, other hardware
+ Tspillover (za,az)
+ Tcelestial(t)
+ TCMB
+ Tatm(za)
Tmeas = Tsource + Teverything else
Determining Tsource
Determining Tsource
ON source OFF source Tsource + Teverything else Teverything else
Arb
itrar
y U
nits
Determining Tsource
(ON – OFF)/OFF[(Tsource + Teverything else) - (Teverything else)]/ Teverything else
=(Source temperature)/(”System” temperature)
% T
sys
Determining Tsource
(ON – OFF)/OFF[(Tsource + Teverything else) - (Teverything else)]/ Teverything else
=(Source temperature)/(”System” temperature)
% T
sys
???
Off Source Observations
Two basic concepts
• Go off source in sky• Go off source in frequency/channels
Like most things in science:Easy to state, complicated in practice
Off Source Observations
Position Switching• Little a priori information needed• Typically gives very good results• Can be done through:
– Moving the telescope (re-pointing onto blank sky)– Moving a secondary or tertiary dish– Beam switching (two receivers on the sky)
• Disadvantages: – System must be stable over time of pointings– Sky position must be carefully chosen– Source must not be extended beyond positions– May take significant time (not true for beam switching)
Off Source Observations
off source in frequency/channels
• Typically gives very good results
• Requires well understood line of interest
• Can be done through: – Baseline fitting– Frequency switching– Some combination of the two
Off Source Observations
Baseline Fitting
• Simplest & most efficient method• Can be from average of multiple scans or single fit
• Not feasible if:Line of interest is large compared with bandpassStanding waves in data Cannot readily fit bandpassDo not know frequency/width of line of interest
• Errors are primarily from quality of fit
Off Source Observations
Frequency Switching
• Allows for rapid switch between ON & OFF observations• Does not require motion of telescope• Can be very efficient• Disadvantages:
Frequency of line of interest should be known*System must be stable in channel spaceWill not work with changing baselines, wide lines
*But not always…
Mapping an Extended Source
Off Source ObservationsVariations
Possible alternative if frequency switching is not an option
System must be very stable
Off source must truly be off!
Off Source ObservationsVariations
There are many other variations to the main themes given:
• Position switching by moving subreflector• Using average offs from a variety of
observations• Frequency switching with many baseline scans • Etc, etc
The ideal off:• Is well understood• Has minimal systematic effects• Adds little noise to the results• Maximizes the on-source telescope time
Determining Tsource
(ON – OFF)/OFF[(Tsource + Teverything else) - (Teverything else)]/ Teverything else
Tsource
Tsystem
Result =
Units are % System Temperature
Need to determine system temperature to calibrate data
Determining Tsystem
Theory
Measure various components of Tsys:
TCMB ― Well known (2.7 K)
TRX, hardware ― Can be measured/monitored
Tcel(t) ― Can be determined from other measurements Tatm(za) ― Can be determined from other measurements
Tgr(za,az) ― Can be calculated
Decreasing
Confidence
Tsrc/Tsys = (ON – OFF)/OFF
Tdiode/ Tsys = (ON – OFF)/OFF
Tsys = Tdiode * OFF/(ON – OFF)
Noise Diodes
Determining Tsys
Noise Diodes - Considerations
• Frequency dependence
Determining Tsys
Lab measurements of the GBT L-Band calibration diode, taken from work of M. Stennes & T. Dunbrack - February 14, 2002
Noise Diodes - Considerations
• Frequency dependence
• Time stability
Determining Tsys
Lab measurements of the GBT L-Band calibration diode, taken from work of M. Stennes & T. Dunbrack - February 14, 2002
Determining Tsys
Noise Diodes - ConsiderationsNoise Diodes - Considerations• Frequency dependence
• Time stability
• Accuracy of measurements
Typically measured against another diode or other calibrator
Errors inherent in instruments used to measure both diodes
Measurements often done in lab -> numerous losses through path from diode injection to back ends
2 measured value = 2
standard cal + 2 instrumental error + 2
loss uncertainties
Determining Tsys
Noise Diodes - Considerations• Frequency dependence
2total = 2
freq. dependence
+ 2stability
+ 2measured value
+ 2conversion error
• Time stability
• Accuracy of measurements
Determining Tsys
Hot & Cold Loads
• Takes antenna into account
• True temperature measurement
Hot LoadThot
Cooling SystemTcold
Determining Tsys
Hot & Cold Loads
• Takes antenna into account• True temperature measurement
Requires:
• Reliable loads able to encompass the receiver
• Response fast enough for on-the-fly measurements
Determining Tsys
Theory:Needs detailed understanding of telescope & structureAtmosphere & ground scatter must be stable and understood
Noise Diodes:Can be fired rapidly to monitor temperatureRequires no ‘lost’ timeDepends on accurate measurements of diodes
Hot/Cold Loads:Can be very accurateObservations not possible when load onMust be in mm range for on-the-fly measurements
In reality
, all t
hree methods
could
be combined fo
r best
accuracy
Determining Tsource
Tsource = (ON – OFF)
Tsystem
OFF
From diodes, Hot/Cold loads, etc.
Blank Sky or other
Telescope response has not been accounted for!
Telescope Response
Ideal Telescope:
– Accurate gain, telescope response can be modeled
– Can be used to determine the flux density of ‘standard’ continuum sources
– Not practical in cases where telescope is non-ideal
(blocked aperture, cabling/electronics losses, ground reflection, etc)
Telescope Response
• ‘Bootstrapping’:
Observe source with pre-determined fluxesDetermine telescope gain
Tsource = (ON – OFF) Tsystem 1
OFF GAIN
GAIN = (ON – OFF) Tsystem
OFF Tsource
Telescope Response
• ‘Bootstrapping’:
Useful when gain is not readily modeledOffers ready means for determining telescope
gain
Requires calibrator flux to be well known in advance
Not practical if gain changes rapidly with position
Telescope Response
Pre-determined Gain curves:
Allows for accurate gain at all positionsSaves observing timeCan be only practical solution (complex
telescopes)
Caveat:Observers should always check the predicted
gain during observations against a number of calibrators!
Tsource = (ON – OFF)
Tsystem
1
OFF GAIN
From diodes, Hot/Cold loads, etc.
Blank Sky or other
Theoretical, or Observational
Determining Tsource
Great, you’re done!Great, you’re done?
Other Issues:Focus
Results in reduction of telescope gainCorrected mechanicallyAlways check focus!!!
Other Issues:Side Lobes*
• Allows in extraneous or unexpected radiation
• Can result in false detections, over-estimates of flux, incorrect gain determination
• Solution is to fully understand side lobes
*Covered more fully in talk by Lockman
Beam
Other Issues:Coma & Astigmatism
Comatic Error: – sub-reflector shifted perpendicular from main
beam– results in an offset between the beam and sky
pointing
Image from ATOM 99-02, Heiles
Other Issues:Coma & Astigmatism
Astigmatism: deformities in the reflectors
Can result in false detections, over-estimates of flux, incorrect gain determination
Solution is to fully understand beam shape
Image from ATOM 99-02, Heiles
Other Issues:The Atmosphere
This is a huge issue.
Can result in false detections, over-estimates of flux, incorrect gain determination
Solution is to fully understand beam shape
Image from ATOM 99-02, Heiles
Other Issues:The Atmosphere
This is a huge issue.
So huge it deserves its own talk.
Can result in false detections, over-estimates of flux, incorrect gain determinationSolution is to fully understand beam shape
Image from ATOM 99-02, Heiles
Other Issues:The Atmosphere
This is a huge issue.
So huge it deserves its own talk.
Which is next.
Can result in false detections, over-estimates of flux, incorrect gain determinationSolution is to fully understand beam shape
Image from ATOM 99-02, Heiles
Conclusion:(applies to all science research)
• Astronomy instrumentation and calibration is complicated
• Learn the system you are using well
• Talk with the instrument staff frequency, and visit often
• To produce good science you must understand the instrument and techniques you are using
You are ultimately responsible for the quality of your data!