Adaptive Dynamic Radio Open-source Intelligent Team (ADROIT): C
Radio `source’
description
Transcript of Radio `source’
Radio `source’
Goals of telescope:
• maximize collection of energy (sensitivity or gain)
• isolate source emission from other sources… (directional gain… dynamic range) Collecting area
EVN: European VLBI Network (more and bigger dishes than VLBA)
LBA: Long Baseline Array in AU
Nonthermal
Example 3: ArrayHigh redshift quasar with continuum flux density S = 1 mJy
rms = S = (fac)(Tsys /Ka)/(B tint)1/2
= (1.4)(30/0.6)/(B tint)1/2
tint = (70/0.0002)2/(128x106)
~ 16 min
Ka = Ta / S = Aeff /2k [K/Jy]
= 0.7 K/Jy Parkes = 6 x 0.1 = 0.6 K/Jy ACTA
(Ta = S Aeff /2k)
ATCA (B=128 MHz): 1 mJy = 5 rms means S = 0.2 mJy
rms = S = (fac)(Tsys /Ka)/(B tint)1/2
D
Sensivity depends on collecting area
Angular resolution
~ /D
ARRAYS:
Example 3: ArrayHigh redshift quasar with continuum flux density S = 1 mJy
rms = S = (fac)(Tsys /Ka)/(B tint)1/2
= (1.4)(30/0.6)/(B tint)1/2
tint = (70/0.0002)2/(128x106)
~ 16 min
Ka = Ta / S = Aeff /2k [K/Jy]
= 0.7 K/Jy Parkes = 6 x 0.1 = 0.6 K/Jy ACTA
(Ta = S Aeff /2k)
ATCA (B=128 MHz): 1 mJy = 5 rms means S = 0.2 mJy
rms = S = (fac)(Tsys /Ka)/(B tint)1/2
D
Sensivity depends on collecting area
Angular resolution
~ /D
Maps from Arrays (or Aperture Synthesis Telescopes):
• intensities indicated in ‘units’ of `milli-Jansky per beam’ [why?]
• can compute noise level Jy using radiometer equation
• can compute beam size from /D so ~ 2/4 sterad
• best to think of ‘mJy/beam’ as Intensity, I = 2kTB/2
• then, uncertainty is TB ~ Jy /
• IMPORTANT: lose surface brightness sensitivity when dilute the aperture by separating the array telescopes !!! Hurts ability to see diffuse emission.
Fourier Transform
Zoom of FT
SourceStrength
Angle
Effect of observing complex source with a ‘beam’
Fourier Transform
Zoom of FT
view convolution of source with beam as filtering in the Spatial Frequency Domain
Filter
The `microwave sky’ (all sky picture from WMAP map.gfsc.nasa.gov)
Example of importance ofSpatial Frequency Content
L = 1
L = 2
L = 10
L = 50
(spatial frequency)
L = 210
Interference Fringes and “Visibility” …. (Visibilities)
The term “visibility” has its origin in optical interferometry, where fringes of unresolved sources has high “fringe visibility.” The term “visibilities” in radio astronomy generally refer to a set of measurements of the visibility function of a celestial source.
Simple cross correlationradio interferometer: on-axis source
Radio `source’
InterferometerResponse
Consider:• ‘point source’ response … full amplitude, but fringe ambiguity• ‘resolved source’ response … source fills + and – fringes => signal cancels and response -> 0.
L
M
Angle,
The fringe spacing and orientation corresponding to a single ‘u-v’ point:
U-V sampling comes from forming interferometers among all pairs of telescopes in the array:
Locations on Earth Instantaneous UV Coverage Earth rotation
See: www.narrabri.atnf.csiro.au/astronomy/vri.html to access the Virtual Radio Interferometer simulator.
“Dipoles”