Spectral line calibration techniques with single dish telescopes k o neil nrao gb
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Spectral Line Calibration Techniques with Single Dish Telescopes K. O’Neil NRAO - GB. Determining the Source Temperature. Determining T source. T meas ( a,d ,az,za) = T src ( a,d ,az,za). Determining T source. T meas ( a,d ,az,za) = T src ( a,d ,az,za).

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Spectral Line Calibration Techniques with Single Dish Telescopes K. O’Neil NRAO - GB

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Spectral line calibration techniques with single dish telescopes k o neil nrao gb

Spectral Line Calibration Techniques with Single Dish TelescopesK. O’NeilNRAO - GB


Determining the source temperature

Determiningthe SourceTemperature


Determining t source

Determining Tsource

  • Tmeas (a,d,az,za) =

    • Tsrc(a,d,az,za)


Determining t source1

Determining Tsource

  • Tmeas (a,d,az,za) =

    • Tsrc(a,d,az,za)

Or at least, that is what we want…


Determining t source2

Determining Tsource

  • Tmeas (a,d,az,za) =

    • Tsrc(a,d,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 t source3

  • Tmeas (a,d,az,za) =

  • Tsrc(a,d,az,za)

  • + TRX, other hardware

  • + Tspillover (za,az)

Determining Tsource


Determining t source4

  • Tmeas (a,d,az,za) =

  • Tsrc(a,d,az,za)

  • + TRX, other hardware

  • + Tspillover (za,az)

  • + Tcelestial(a,d,t)

Determining Tsource


Determining t source5

  • Tmeas (a,d,az,za) =

  • Tsrc(a,d,az,za)

  • + TRX, other hardware

  • + Tspillover (za,az)

  • + Tcelestial(a,d,t)

  • + TCMB

  • + Tatm(za)

Determining Tsource


Determining t source6

  • Tmeas(a,d,az,za) =

  • Tsrc(a,d,az,za)

  • + TRX, other hardware

  • + Tspillover(za,az)

  • + Tcelestial(a,d,t)

  • + TCMB

  • + Tatm(za)

  • Tmeas =Tsource+Teverything else

Determining Tsource


Determining t source7

  • Tmeas(a,d,az,za) =

  • Tsrc(a,d,az,za)

  • + TRX, other hardware

  • + Tspillover(za,az)

  • + Tcelestial(a,d,t)

  • + TCMB

  • + Tatm(za)

  • Tmeas =Tsource+Teverything else

Determining Tsource


Determining t source8

Determining Tsource

ON sourceOFF source

Tsource+ Teverything else Teverything else

Arbitrary Units


Determining t source9

Determining Tsource

ON-OFF

(Tsource+ Teverything else)- (Teverything else)

Arbitrary Counts


Determining t source10

Determining Tsource

(ON–OFF)/OFF

[(Tsource + Teverything else)- (Teverything else)]/ Teverything else

=(Source temperature)/(”System” temperature)

% Tsys


Determining t source11

Determining Tsource

???

(ON–OFF)/OFF

[(Tsource + Teverything else)- (Teverything else)]/ Teverything else

=(Source temperature)/(”System” temperature)

% Tsys


Choosing the best off

Choosing the Best OFF


Off source observations

OffSource 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 observations1

OffSource Observations

Position Switching

ON Source

OFF Source


Off source observations2

OffSource 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 observations3

OffSource 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 observations4

OffSource 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 bandpass

    Standing waves in data

    Cannot readily fit bandpass

    Do not know frequency/width of line of interest

  • Errors are primarily from quality of fit


Off source observations5

OffSource Observations

Frequency

Switching


Off source observations6

OffSource 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 space

    Will not work with changing baselines, wide lines

    *But not always…


Spectral line calibration techniques with single dish telescopes k o neil nrao gb

OffSource ObservationsVariations

Mapping an Extended Source

Possible alternative if frequency switching is not an option

System must be very stable

Off source must truly be off!


Spectral line calibration techniques with single dish telescopes k o neil nrao gb

OffSource 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 t source12

Tsource

Tsystem

Result =

Determining Tsource

(ON–OFF)/OFF

[(Tsource + Teverything else)- (Teverything else)]/ Teverything else

Units are % System Temperature

Need to determine system temperature to

calibrate data


Determining system temperature

DeterminingSystemTemperature


Determining t system

Determining Tsystem

Theory

Measurevarious components of Tsys:

TCMB ― Well known (2.7 K)

TRX, hardware ― Can be measured/monitored

Tcel(a,d,t) ― Can be determined from other measurements

Tatm(za) ― Can be determined from other measurements

Tgr(za,az) ― Can be calculated

Decreasing

Confidence


Determining t sys

Determining Tsys

Noise Diodes


Determining t sys1

Determining Tsys

Noise Diodes

Tsrc/Tsys = (ON – OFF)/OFF

Tdiode/Tsys = (ON – OFF)/OFF

Tsys = Tdiode * OFF/(ON – OFF)


Determining t sys2

Determining Tsys

Noise Diodes - Considerations

  • Frequency dependence

Lab measurements of the GBT L-Band calibration diode, taken from work of M. Stennes & T. Dunbrack - February 14, 2002


Determining t sys3

Determining Tsys

Noise Diodes - Considerations

  • Frequency dependence

  • Time stability

Lab measurements of the GBT L-Band calibration diode, taken from work of M. Stennes & T. Dunbrack - February 14, 2002


Determining t sys4

Determining Tsys

  • Noise Diodes - Considerations

  • Frequency dependence

Noise Diodes - Considerations

  • 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

s2 measured value = s2 standard cal + s2 instrumental error + s2 loss uncertainties


Determining t sys5

Determining Tsys

Noise Diodes - Considerations

  • Frequency dependence

  • Time stability

  • Accuracy of measurements

s2total = s2freq. dependence

+ s2stability

+ s2measured value

+ s2conversion error


Determining t sys6

Determining Tsys

Hot & Cold Loads

  • Takes antenna into account

  • True temperature measurement

Cooling System

Tcold

Hot Load

Thot


Determining t sys7

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 t sys8

Determining Tsys

Theory:

Needs detailed understanding of telescope & structure

Atmosphere & ground scatter must be stable and understood

Noise Diodes:

Can be fired rapidly to monitor temperature

Requires no ‘lost’ time

Depends on accurate measurements of diodes

Hot/Cold Loads:

Can be very accurate

Observations not possible when load on

Must be in mm range for on-the-fly measurements

In reality, all three methods could

be combined for best accuracy


Determining t source13

From diodes, Hot/Cold loads, etc.

Determining Tsource

Tsource = (ON–OFF)Tsystem

OFF

Blank Sky or other

Telescope response has not been accounted for!


Determining telescope response

DeterminingTelescope Response


Telescope response

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 response1

Telescope Response

  • Ideal Telescope:


Telescope response2

Tsource = (ON –OFF)Tsystem1

OFFGAIN

GAIN = (ON – OFF)Tsystem

OFF Tsource

Telescope Response

  • ‘Bootstrapping’:

    Observe source with pre-determined fluxes

    Determine telescope gain


Telescope response3

Telescope Response

  • ‘Bootstrapping’:

    Useful when gain is not readily modeled

    Offers ready means for determining telescope gain

    Requires calibrator flux to be well known in advance

    Not practical if gain changes rapidly with position


Telescope response4

Telescope Response

Pre-determined Gain curves:

Allows for accurate gain at all positions

Saves observing time

Can be only practical solution (complex telescopes)

Caveat:

Observers should always check the predicted gain during observations against a number of calibrators!


Determining t source14

Determining Tsource

Tsource = (ON –OFF)Tsystem 1

OFF GAIN

Theoretical, or Observational

Blank Sky or other

From diodes, Hot/Cold loads, etc.

Great, you’re done!

Great, you’re done?


A few other issues

A FewOther Issues


Other issues pointing

Other Issues:Pointing

Results in reduction of telescope gain

Always check telescope pointing!


Other issues focus

Other Issues:Focus

Results in reduction of telescope gain

Corrected mechanically

Always check focus!!!


Other issues side lobes

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

Beam

*Covered more fully in talk by Lockman


Other issues coma astigmatism

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 astigmatism1

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


Other issues the atmosphere

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


Other issues the atmosphere1

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 determination

Solution is to fully understand beam shape


Other issues the atmosphere2

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 determination

Solution is to fully understand beam shape


Conclusion applies to all science research

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!


The end

The End


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