RadioTelescopes a field guide M. Kesteven 24/September/2001 Conventional
RadioTelescopes a field guide M. Kesteven 24/September/2001 Conventional telescopes
Main reflector; possibly a secondary Focal plane single-pixel camera Fully-steerable
Parkes 64m Effelsberg 100m Green Bank 100m
Fixed Spherical Reflector Large focal plane Steering : probe the focal plane Simplified mechanical structure
Arecibo 300m Nancay 200m x 40m
Conventional Imaging Arrays Multiplicity of antennas multiplicity of baselines ATCA
6 x 22m 6 km baseline VLA (NRAO) 27 x 25m antennas
Up to 36 km baseline VLBA (NRAO) 10 x 25m antennas
64 x 12m high precision antennas (one possible) SKA N antennas; up to 3000 km baselines
Unconventional arrays Molonglo (U. Syd) 1 mile x 40 ft.
(843 MHz) Mauritius 2km and 1km arms
(151 MHz) Evolution : Mills Cross -> VLA Linear array -> fan beam Correlation between orthogonal fan beams ->
high resolution pencil beam = central pixel of the arrays synthesised image. N
V( ) e n N j
2 ( nx ) sin( )
~ sin(L / ) (L / )
Procedure: - Correlate the EW arm against the NS (multiply and average) - Since each source on the sky has its own noise signal, only sources common to the two beams give a non-zero signal
Relate to Synthesis Array : P(u ) S ( )e j 2 (u ) d S ( ) P (u )e
Mills Cross : j 2 ( u ) du
VxV y* v x (n) v*y (m) n m
S ( , )e j 2 ( n m ) n m
P (u ) n m
S (0,0) The (u,v) plane :
pointing offset : sin( ) c x
Mills Cross with multiple E-W beams Fully implemented Mills Cross : Given enough hardware, we could map a square patch of sky, each pointing defined by xEW and yNS we are doing
the FT in hardware. VxV y* vx (n)v y (m) e P(u )e S ( x , y ) ( x c x / )
j ( n x m y ) j ( n x m y )
Single Dish equivalent to superimposed arrays. An incoming wavefront excites currents on the surface these re-radiate, and combine in a phased way at the receiver. A correctly focussed antenna has constant path length for all
rays from wavefront to the focus - Rx output V( x )V* ( x ) Westerbork
14 x 25m antennas 2.7 km E-W baseline Parkes, 64m
Effelsberg, 100m Green Bank, 100m Arecibo, 300m
Nancay, 200x40m Parkes 64m Green Bank 100m
* common to both beams VEW VNS
Molonglo Synthesis Telescope : 2 East-West arms, each mile long. Continuous tracking (part mechanical, part electrical, adjusting the delays). Equivalent to an East-West array (eg, ATCA).
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