Arecibo Spectral line observing demo - basic concepts -- [Slides have been borrowed from talks by Frank Ghogo and Karen O’Neil of Green Bank, NRAO]

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Presentation transcript:

Arecibo Spectral line observing demo - basic concepts -- [Slides have been borrowed from talks by Frank Ghogo and Karen O’Neil of Green Bank, NRAO]

Intrinsic Power P (Watts) Distance R (meters) Aperture A (sq.m.) Flux = Power/Area Flux Density (S) = Power/Area/bandwidth Bandwidth (β) A “Jansky” is a unit of flux density

Power WA detected in a radio telescope Detected power (W, watts) from a resistor R at temperature T (kelvin) over bandwidth β(Hz) -[Black Body radiation] Power WA detected in a radio telescope Due to a source of flux density S Factor of one-half because detector is only sensitive to one polarization. power as equivalent temperature. Antenna Temperature TA Effective Aperture Ae

Antenna Beam Pattern (power pattern) Gain(K/Jy) Gain(305-m) ~10 K/Jy Note Pn is normalized to maximum = 1; Like optical PSF Kraus, 1966. Fig.6-1, p. 153.

System Temperature Thermal noise ΔT = total noise power detected, a result of many contributions Thermal noise ΔT = minimum detectable signal

Determining Tsource Tmeas (α,δ,az,za) = Tsrc(α,δ,az,za)

Determining Tsource Tmeas (α,δ,az,za) = Tsrc(α,δ,az,za) + TRX, other hardware + Tspillover (za,az) + Tcelestial(α,δ,t) + TCMB + Tatm(za) Tmeas = Tsource + Teverything else

Determining Tsource Tmeas (α,δ,az,za) = Tsrc(α,δ,az,za) + TRX, other hardware + Tspillover (za,az) + Tcelestial(α,δ,t) + TCMB + Tatm(za) Tmeas = Tsource + Teverything else

Off Source Observations Position Switching ON Source OFF Source

Determining Tsource ON source OFF source Tsource + Teverything else Teverything else Arbitrary Units

(Tsource + Teverything else) - (Teverything else) Determining Tsource ON - OFF (Tsource + Teverything else) - (Teverything else) Arbitrary Counts

Determining Tsource (ON – OFF)/OFF [(Tsource + Teverything else) - (Teverything else)]/ Teverything else =(Source temperature)/(”System” temperature) % Tsys

Determining Tsource (ON – OFF)/OFF ??? (ON – OFF)/OFF [(Tsource + Teverything else) - (Teverything else)]/ Teverything else =(Source temperature)/(”System” temperature) % Tsys

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

Determining Tsource Tsource Tsystem Result = (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 Tsys Noise Diodes

Determining Tsys Noise Diodes Tsrc/Tsys = (ON – OFF)/OFF Tdiode/ Tsys = (ON – OFF)/OFF Tsys = Tdiode * OFF/(ON – OFF)

OFF Determining Tsource Tsource = (ON – OFF) Tsystem 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 Ideal Telescope:

Tsource = (ON – OFF) Tsystem 1 OFF GAIN Determining Tsource Tsource = (ON – OFF) Tsystem 1 OFF GAIN Theoretical, or Observational Blank Sky or other From diodes, Hot/Cold loads, etc.