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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