Download tg_on-off2

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 ()
A “Jansky” is a unit of flux density
Detected power (W, watts) from a resistor R
at temperature T (kelvin) over bandwidth (Hz) [Black Body radiation]
Power WA detected in a radio telescope
Due to a source of flux density S
power as equivalent temperature.
Antenna Temperature TA
Effective Aperture Ae
Gain(K/Jy)
Gain(305-m)
Antenna Beam Pattern (power pattern)
~10 K/Jy
Kraus, 1966. Fig.6-1, p. 153.
System Temperature
= total noise power detected, a result of many contributions
Thermal noise T
= minimum detectable signal
Determining Tsource
• Tmeas ( ,az,za) =
Tsrc( ,az,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
Off Source Observations
Position Switching
ON Source
OFF Source
Determining Tsource
OFF source
Teverything else
Arbitrary Units
ON source
Tsource + Teverything else
Determining Tsource
Arbitrary Counts
ON - OFF
(Tsource + Teverything else) - (Teverything else)
Determining Tsource
(ON – OFF)/OFF
[(Tsource + Teverything else) - (Teverything else)]/ Teverything else
% Tsys
=(Source temperature)/(”System” temperature)
Determining Tsource
???
(ON – OFF)/OFF
[(Tsource + Teverything else) - (Teverything else)]/ Teverything else
% Tsys
=(Source temperature)/(”System” temperature)
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
(ON – OFF)/OFF
[(Tsource + Teverything else) - (Teverything else)]/ Teverything else
Result
=
Tsource
Tsystem
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)
Determining
T
source
(ON – OFF)
Tsource =
Tsystem
OFF
Blank Sky or other
From diodes, Hot/Cold
loads, etc.
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
Determining
(ON
– OFF) Tsource
=
Tsystem
OFF
1
GAIN
Theoretical, or
Observational
Blank Sky or other
From diodes, Hot/Cold
loads, etc.