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Radio `source’ - PowerPoint PPT Presentation


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Radio `source’. Goals of telescope: maximize collection of energy (sensitivity or gain) isolate source emission from other sources… (directional gain… dynamic range). Collecting area. LBA: Long Baseline Array in AU. EVN: European VLBI Network (more and bigger dishes than VLBA).

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Presentation Transcript
slide1

Radio `source’

  • Goals of telescope:
  • maximize collection of energy (sensitivity or gain)
  • isolate source emission from other sources… (directional gain… dynamic range)

Collecting area

slide2

LBA: Long Baseline Array in AU

EVN: European VLBI Network(more and bigger dishes than VLBA)

slide4

Example 3: Array

High redshift quasar with

continuum flux density Sn = 1 mJy

(Ta = SnAeff /2k)

Ka = Ta / Sn = Aeff /2k [K/Jy]

= 0.7 K/JyParkes

= 6 x 0.1 = 0.6 K/JyACTA

rms = DS = (fac)(Tsys /Ka)/(B tint)1/2

ATCA (B=128 MHz): 1 mJy = 5 rmsmeans DS= 0.2 mJy

rms = DS = (fac)(Tsys /Ka)/(B tint)1/2

= (1.4)(30/0.6)/(B tint)1/2

tint= (70/0.0002)2/(128x106)

~16 min

slide5

ARRAYS:

Sensivity depends

on collecting area

Angular resolution

~ l/D

D

slide6

Example 3: Array

High redshift quasar with

continuum flux density Sn = 1 mJy

(Ta = SnAeff /2k)

Ka = Ta / Sn = Aeff /2k [K/Jy]

= 0.7 K/JyParkes

= 6 x 0.1 = 0.6 K/JyACTA

rms = DS = (fac)(Tsys /Ka)/(B tint)1/2

ATCA (B=128 MHz): 1 mJy = 5 rmsmeans DS= 0.2 mJy

rms = DS = (fac)(Tsys /Ka)/(B tint)1/2

= (1.4)(30/0.6)/(B tint)1/2

tint= (70/0.0002)2/(128x106)

~16 min

slide7

Sensivity depends

on collecting area

Angular resolution

~ l/D

D

slide8

Maps from Arrays (or Aperture Synthesis Telescopes):

  • intensities indicated in ‘units’ of `milli-Jansky per beam’ [why?]
  • can compute noise level sJy using radiometer equation
  • can compute beam size from Q ~l/D so W ~ pQ2/4 sterad
  • best to think of ‘mJy/beam’ as Intensity, In = 2kTB/l2
  • then, uncertainty is DTB ~ sJy /W
  • IMPORTANT: lose surface brightness sensitivity when dilute the
  • aperture by separating the array telescopes !!!
  • Hurts ability to see diffuse emission.
slide9

Source

Strength

Angle

Fourier Transform

Effect of observing complex source with a ‘beam’

Zoom of FT

slide10

view convolution of source with beam as filtering in the Spatial Frequency Domain

Fourier Transform

Zoom of FT

Filter

slide11

The `microwave sky’ (all sky picture from

WMAP map.gfsc.nasa.gov)

Example of importance of

Spatial Frequency Content

slide16

L = 50

(spatial frequency)

slide18

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.

slide19

Simple cross correlation

radio interferometer:

on-axis source

slide21

M

Radio `source’

L

Interferometer

Response

Angle, Q

  • Consider:
  • ‘point source’ response … full amplitude, but fringe ambiguity
  • ‘resolved source’ response … source fills + and – fringes => signal
  • cancels and response -> 0.
slide24

U-V sampling comes from forming interferometers among all pairs of telescopes in the array:

Locations on Earth

Instantaneous UV Coverage

Earth rotation

slide26

See:www.narrabri.atnf.csiro.au/astronomy/vri.html

to access the Virtual Radio Interferometer simulator.