Spectral Line Calibration Techniques with Single Dish Telescopes. K. O Neil NRAO - GB
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1 Spectral Line Calibration Techniques with Single Dish Telescopes K. O Neil NRAO - GB
2 A Quick Review
3 Review: The Rayleigh-Jeans Approximation Planck Law for Blackbody radiation: B= 2hν 3 1 If ν~ghz, often hν << kt. Taylor series gives: B= = Source flux in Rayleigh Jeans limit: c 2 2kTν 2 c 2 2k e hν/kt -1 S= Ω s T(θ,φ)dΩ λ 2 2kT If brightness temperature is constant across source: 2kT λ 2 S = Ω S λ 2 B=Brightness, n = frequency; h = x J s; k = x J K -1 ;T = temperature
4 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
5 Determining the Source Temperature
6 Determining T source T meas (α,δ,az,za) = T src (α,δ,az,za) + T system
7 Determining T source T meas (α,δ,az,za) = T src (α,δ,az,za) + T RX + T gr (za,az)
8 Determining T source T meas (α,δ,az,za) = T src (α,δ,az,za) + T RX + T gr (za,az) + T cel (α,δ,t)
9 Determining T source T meas (α,δ,az,za) = T src (α,δ,az,za) + T RX + T gr (za,az) + T cel (α,δ,t) + T CMB + T atm (za) T meas = T source + T everything else
10 Determining T source ON OFF T source + T everything else T everything else Arbitrary Units Arbitrary Units channel channel
11 Determining T source ON - OFF (T source + T everything else )-(T everything else ) Arbitrary Units channel
12 Determining T source (ON OFF)/OFF [(T source + T everything else )-(T everything else )]/ T everything else % T sys channel
13 Determining T src T source = (ON OFF) OFF???
14 Choosing the Best OFF
15 Off Source Observations Baseline Fitting Image on right courtesy of C. Conselice
16 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
17 Off Source Observations Frequency Switching
18 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 must be known System must be stable Will not work with significant (changing) standing waves
19 Off Source Observations Position Switching ON Source OFF Source
20 Off Source Observations Position Switching Little a priori information needed Typically gives very good results Disadvantages: System must be stable Requires re-pointing the telescope Sky position must be carefully chosen Source must not be too extended
21 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 Switch sources/beams periodically Sky position must be carefully chosen Source must not be too extended
22 Off Source Observations Variations Baseline fitting with an average fit Offers a very good fit May lose detailed information for individual fits System must be very stable
23 Off Source Observations Variations Position Switching on an Extended Source Alternative if frequency switching is not an option May lose detailed information for individual fits System must be very stable
24 Off Source Observations Variations Position Switching on Strong Continuum ON Source 1 OFF Source 1 ON Source 2 OFF Source 2
25 Off Source Observations Variations Position Switching on Strong Continuum Possibly only alternative if T src > few x T sys Designed to remove residual standing waves Result: R= [On(ν) Off(ν)] source1 [On(ν) Off(ν)] source2 From ATOM by Ghosh & Salter
26 Off Source Observations Variations Position Switching on Strong Continuum [(On Off)] 1 [(on Off)] 2 [(On Off)/Off] 1 [(on Off)/Off] 2 Standard (On Off)/Off From ATOM bu Ghosh & Salter
27 Off Source Observations Variations Position Switching on Strong Continuum Standard (On Off)/Off [(On Off)/Off] 1 [(on Off)/Off] 2 [(On Off)] 1 [(on Off)] 2 From ATOM by Ghosh & Salter
28 Determining T source (ON OFF)/OFF [(T source + T everything else )-(T everything else )]/ T everything else Result = T source T system Units are % * System Temperature Need to determine system temperature to calibrate data
29 Determining System Temperature
30 Determining T sys Theory Measure various components of T sys: Decreasing Confidence T RX Can be readily measured/monitored T CMB Well known (2.7 K) T cel (α,δ,t) Can be determined from other (tel.) measurements T atm (za) Can be determined from other (tel.) measurements T gr (za,az) Can be calculated
31 Determining T sys Noise Diodes
32 Determining T sys Noise Diodes T src /T sys = (ON OFF)/OFF... T diode /T sys = (On Off) / Off T sys = T diode * Off/(On Off)
33 Determining T sys 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
34 Determining T sys Noise Diodes - Considerations Frequency dependence Time stability
35 Determining T sys Noise Diodes - Considerations Frequency dependence Time stability Accuracy of measurements
36 Determining T sys Noise Diodes - Considerations 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. Have numerous losses through path from diode injection to back ends σ 2 measured value = σ 2 standard cal + σ 2 instrumental error + σ 2 loss uncertainties
37 Determining T sys Noise Diodes - Considerations Frequency dependence Time stability Accuracy of measurements σ 2 measured value = σ 2 standard cal + σ 2 instrumental error + σ 2 loss uncertainties σ 2 total = σ 2 freq. dependence + σ 2 stability + σ 2 measured value + σ 2 conversion error
38 Determining T sys Two Diodes (the Y-Factor) T Y = 1 + T off T T off = 1 - YT 2 T 2 + T off Y - 1 Can be more accurate than just one diode Ignores effects of the antenna
39 Determining T sys Hot & Cold Loads Same idea as two diodes Takes antenna into account True temperature measurement (no conversions)
40 Determining T sys Hot & Cold Loads Cooling System T cold
41 Determining T sys Hot & Cold Loads Hot Load T hot
42 Determining T sys Hot & Cold Loads Absorber (T hot/cold ) T off = T 1 - YT 2 Y - 1
43 Determining T sys Hot & Cold Loads
44 Determining T sys Hot & Cold Loads Same idea as two diodes Takes antenna into account True temperature measurement (no conversions) Requires a reliable load able to encompass the receiver, with response fast enough for on-the-fly measurements
45 Theory: Determining T sys 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
46 Determining T source T source = (ON OFF) T OFF system Blank Sky or other From diodes, Hot/Cold loads, etc. Telescope response has not been accounted for!
47 Determining Telescope Response
48 Telescope Response Main Beam Brightness: T MB = η beam T measured Flux Density: S = 2k T(θ,φ) Pn (θ,φ)dω λ 2 Units: W m -2 Hz -1 or Jy (1 Jy = W m -2 Hz -1 ) Ω = Solid angle of tel. pattern; η beam = telescope efficiency; λ = wavelength; S=Flux, k = constants, T = temperature; P = antenna power pattern
49 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)
50 Ideal Telescope: Telescope Response
51 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
52 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
53 Telescope Response Pre-determined Gain curves: Allows for accurate representation of gain at all positions Saves observing time Can be only practical solution
54 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)
55 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!
56 Great, you re done? done! Determining T source T source = (ON OFF) T system 1 OFF GAIN Blank Sky or other Theoretical, or Observational From diodes, Hot/Cold loads, etc.
57 A Few Other Issues
58 Other Issues: Pointing Results in reduction of telescope gain Typically can be corrected in software
59 Other Issues: Focus Results in reduction of telescope gain Corrected mechanically
60 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
61 Comatic Error: Other Issues: Coma & Astigmatism sub-reflector shifted perpendicular from main beam results in an offset between the beam and sky pointing Image from ATOM 99-02, Heiles
62 Other Issues: Image from ATOM 99-02, Heiles 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
63 The End List of useful references pp in book
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