Transcript Obs.Techn. Course Part B, 20-apr

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: Array
High redshift quasar with
continuum flux density Sn = 1 mJy
(Ta = Sn Aeff /2k)
Ka = Ta / Sn = Aeff /2k [K/Jy]
= 0.7 K/Jy Parkes
= 6 x 0.1 = 0.6 K/Jy ACTA
rms = DS = (fac)(Tsys /Ka)/(B tint)1/2
ATCA
(B=128 MHz):
1 mJy = 5 rms means 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
ARRAYS:
Sensivity depends
on collecting area
Angular resolution
~ l/D
D
Example 3: Array
High redshift quasar with
continuum flux density Sn = 1 mJy
(Ta = Sn Aeff /2k)
Ka = Ta / Sn = Aeff /2k [K/Jy]
= 0.7 K/Jy Parkes
= 6 x 0.1 = 0.6 K/Jy ACTA
rms = DS = (fac)(Tsys /Ka)/(B tint)1/2
ATCA
(B=128 MHz):
1 mJy = 5 rms means 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
Sensivity depends
on collecting area
Angular resolution
~ l/D
D
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.
Source
Strength
Fourier Transform
Angle
Effect of observing complex source with a ‘beam’
Zoom of FT
view convolution of
source with beam as
filtering in the Spatial
Frequency Domain
Fourier Transform
Zoom of FT
Filter
The `microwave sky’
Example of importance of
Spatial Frequency Content
(all sky picture from
WMAP map.gfsc.nasa.gov)
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 correlation
radio interferometer:
on-axis source
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.
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”