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

A Quick Review

Review: The Rayleigh-Jeans Approximation Planck Law for Blackbody radiation: B= If ~GHz, often h << kT. Taylor series gives: B= = Source flux in Rayleigh Jeans limit: S= ∫ Ω s T(  )dΩ If brightness temperature is constant across source: S = Ω S e h /kT - 1c2c2 2h c2c2 2kT 2 2kT 2 2k 2 2kT B=Brightness,  = frequency; h = x J s; k = x J K -1 ;T = temperature

Review: Antenna Temperature Antenna theorem: A e  A =  r 2 Measured flux: S = Ω A = Temperature: T A = (A e / 2 ) ∫∫ T src (  )P n (  )d  = (  r /  A ) ∫∫ T src (  )P n (  )d  2 2kT A A = area;  = Solid angle of tel. pattern;  r = fractional power transmission;  = wavelength; S=Flux, k = x J K -1, T = temperature; P = antenna power pattern AeAe 2kT A

Review: Antenna Temperature Temperature: T A = (A e / 2 ) ∫∫ T src (  )P n (  )d  = (  r /  A ) ∫∫ T src (  )P n (  )d  Point Source: T A = (  r /  A ) ∫∫ T src (  ) d  =  r (  S /  A )T avg Source > Beam, T=T const T A = (  r T const /  A ) ∫∫ P n (  ) d  =  r (  const /  A )  ′ b  r = fractional power of antenna transmission;  A = antenna solid angle; P n (  ) = antenna power pattern;  b = solid angle subtended by main beam and side lobes

Review: Minimal Detectable Temperature Set by the system noise T sys = T A + (1/  )T R + T LP [1/  -1] Sensitivity is rms noise of system:  T rms = K S T sys / √ (  t n)  B rms = (2k/ 2 ) K S T sys / √ (  t n)  S rms = (2k/A e ) K S T sys / √ (  t n) T min ~ 5 X  T rms T R = receiver temperature; T LP = transmission line temperature;  = efficiency transmission; K S = telescope sensitivity constant (~1);  n = pre-detection bandwidth (Hz); t = integration time, one record; n= number of records

Review: Antenna Temperature Telescope observes a point source (flux density S) Telescope feed replaced with matched load (resistor) Load temperature adjusted until power received equals power of the source This is equal to the Antenna Temperature

Determining the Source Temperature

T meas ( ,az,za) = T src ( ,az,za) + T system Determining T source

T meas ( ,az,za) = T src ( ,az,za) + T RX, other hardware + T spillover (za,az)

Determining T source T meas ( ,az,za) = T src ( ,az,za) + T RX, other hardware + T spillover (za,az) + T celestial (  t)

T meas ( ,az,za) = T src ( ,az,za) + T RX, other hardware + T spillover (za,az) + T celestial (  t) + T CMB + T atm (za) Determining T source

T meas ( ,az,za) = T src ( ,az,za) + T RX, other hardware + T spillover (za,az) + T celestial (  t) + T CMB + T atm (za) T meas = T source + T everything else Determining T source

ON OFF T source + T everything else T everything else channel Arbitrary Units

Determining T source ON - OFF (T source + T everything else ) - (T everything else ) channel Arbitrary Units

Determining T source (ON – OFF)/OFF [(T source + T everything else ) - (T everything else )]/ T everything else % T sys channel

Determining T source T source = (ON – OFF) OFF ???

Choosing the Best OFF

Off Source Observations Baseline Fitting Image on right courtesy of C. Conselice

Off Source Observations Baseline Fitting Simplest & most efficient method Not feasible if: Line of interest is large compared with bandpass Standing waves in data Cannot readily fit bandpass Errors are primarily from quality of fit

Off Source Observations Frequency Switching

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 space Will not work with changing baselines

Off Source Observations Position Switching ON SourceOFF Source

Off Source Observations Position Switching Little a priori information needed Typically gives very good results Can also be done with tertiary throw Disadvantages: System must be stable over time of pointings Requires re-pointing the telescope Sky position must be carefully chosen Source must not be extended beyond positions

Off Source Observations Beam Switching Same idea as position switching Removes need to move telescope Source is always in one beam Disadvantages/Caveats: Requires at least two receivers, precise hardware Switch sources/beams periodically Sky position must be carefully chosen Source must not be extended beyond throw

Baseline fitting with an average fit Offers a very good fit May lose detailed information for individual fits System must be very stable with time Off Source Observations Variations

‘Position Switching’ on an Extended Source Off Source Observations Variations Alternative if frequency switching is not an option May lose detailed information for individual fits System must be very stable

Position Switching on Strong Continuum Off Source Observations Variations ON Source 1 OFF Source 1 ON Source 2OFF Source 2

Position Switching on Strong Continuum Possibly only alternative if T src > few x T sys Designed to remove residual standing waves Requires two source with similar power levels Result: [On( ) – Off( )] source1 [On( ) – Off( )] source2 R= Off Source Observations Variations From ATOM by Ghosh & Salter

Position Switching on Strong Continuum Off Source Observations Variations Standard (On – Off)/Off [(On – Off)/Off] 1 [(On – Off)/Off] 2 [(On – Off)] 1 [(On – Off)] 2 From ATOM bu Ghosh & Salter

Position Switching on Strong Continuum Off Source Observations Variations From ATOM by Ghosh & Salter Standard (On – Off)/Off [(On – Off)/Off] 1 [(On – Off)/Off] 2 [(On – Off)] 1 [(On – Off)] 2

Determining T source (ON – OFF)/OFF [(T source + T everything else ) - (T everything else )]/ T everything else T source T system Result = Units are % * System Temperature Need to determine system temperature to calibrate data

Determining System Temperature

Determining T sys Theory Measure various components of T sys: T CMB ― Well known (2.7 K) T RX, hardware ― Can be measured/monitored T cel (  t) ― Can be determined from other measurements T atm (za) ― Can be determined from other measurements T gr (za,az) ― Can be calculated Decreasing Confidence

Determining T sys Noise Diodes

Determining T sys

T src /T sys = (ON – OFF)/OFF. T diode / T sys = (On – Off) / Off T sys = T diode * Off/(On – Off) Noise Diodes Determining T sys

Noise Diodes - Considerations Frequency dependence Determining T sys Lab measurements of the GBT L-Band calibration diode, taken from work of M. Stennes & T. Dunbrack - February 14, 2002

Determining T sys Noise Diodes - Considerations Frequency dependence Time stability

Determining T sys Noise 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

Noise Diodes - Considerations Accuracy of measurements Determining T sys Typically measured against another diode or other calibrator Errors inherent in instruments used to measure both diodes Measurements often done in lab. Have numerous losses through path from diode injection to back ends  2 measured value =  2 standard cal +  2 instrumental error +  2 loss uncertainties

Determining T sys Noise Diodes - Considerations Frequency dependence  2 total =  2 freq. dependence +  2 stability +  2 measured value +  2 conversion error  2 measured value =  2 standard cal +  2 instrumental error +  2 loss uncertainties Time stability Accuracy of measurements

Determining T sys Two Diodes (the Y-Factor) Y = T off = Ignores effects of the antenna T 1 + T off T 2 + T off T 1 - YT 2 Y - 1

Determining T sys Hot & Cold Loads Takes antenna into account True temperature measurement (no conversions)

Hot & Cold Loads Determining T sys Cooling System T cold

Determining T sys Hot & Cold Loads Hot Load T hot

Determining T sys Hot & Cold Loads Absorber (T hot/cold ) T 1 - YT 2 Y - 1 T off =

Determining T sys Hot & Cold Loads

Determining T sys Hot & Cold Loads Takes antenna into account True temperature measurement (no conversions) Requires: Reliable load Able to encompass the receiver Response fast enough for on-the-fly measurements

Determining T sys 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 should be combined for best accuracy

Determining T source T source = (ON – OFF) T system OFF From diodes, Hot/Cold loads, etc. Blank Sky or other Telescope response has not been accounted for!

Determining Telescope Response

Telescope Response Main Beam Brightness: T MB =  beam T measured Flux Density: 2k ∫∫T(  ) P n (  )d   2 Units: W m -2 Hz -1 or Jy (1 Jy = W m -2 Hz -1 ) S =  = Solid angle of tel. pattern;  beam = telescope efficiency;  = wavelength; S=Flux, k = constants, T = temperature; P = antenna power pattern

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 Ideal Telescope:

Telescope Response ‘Bootstrapping’: Observe source with pre-determined fluxes Determine telescope gain T source = (ON – OFF) T system 1 OFF GAIN GAIN = OFF T system (ON – OFF) T source

Telescope Response ‘Bootstrapping’: Useful when gain is not readily modeled Offers ready means for determining telescope gain Requires flux of calibrator sources be known in advance Not practical if gain changes rapidly with position

Telescope Response Pre-determined Gain curves: Allows for accurate representation of gain at all positions Saves observing time Can be only practical solution

Telescope Response Pre-determined Gain curves: Average Gain [(polA+polB)/2]: gainavg(az,za,f=1415MHz) = *za *(za-14) *(za-14) x10- 08*cos(az) x10-07*sin(az) x10-08*cos(2*az) *sin(2*az) *cos(3*az) *sin(3*az) gainavg(az,za,f=1175MHz) = *za *(za-14) *(za-14) x10- 06*cos(az) x10-07*sin(az) x10-07*cos(2*az) x10-07*sin(2*az) *cos(3*az) *sin(3*az) gainavg(az,za,f=1300MHz) = *za *(za-14) *(za-14) x10- 07*cos(az) x10-07*sin(az) x10-07*cos(2*az) x10-07*sin(2*az) *cos(3*az) *sin(3*az) gainavg(az,za,f=1375MHz) = *za *(za-14) *(za-14) x10- 07*cos(az) x10-06*sin(az) x10-08*cos(2*az) x10-07*sin(2*az) *cos(3*az) *sin(3*az) gainavg(az,za,f=1550MHz) = *za *(za-14) (za-14) x10- 07*cos(az) x10-07*sin(az) x10-08*cos(2*az) x10-07*sin(2*az) *cos(3*az) *sin(3*az)

Telescope Response Pre-determined Gain curves: Allows for accurate representation of gain at all positions Saves observing time Can be only practical solution Caveat: Observers should always check the predicted gain during observations against a number of calibrators!

T source = (ON – OFF) T system 1 OFF GAIN From diodes, Hot/Cold loads, etc. Blank Sky or other Theoretical, or Observational Determining T source Great, you’re done!Great, you’re done?

A Few Other Issues

Other Issues: Pointing Results in reduction of telescope gain Typically can be corrected in software

Other Issues: Focus Results in reduction of telescope gain Corrected mechanically

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

The End List of useful references pp in book