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”