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VLBI - picking an interferometer (Examples (eMerlin (3 main bands, 7…
VLBI - picking an interferometer
Pick based on:
Observing frequency
Resolution
Sensitivity
No interferometer can do all this
Varying receiver technologies for different frequencies
Variable baseline lengths to acheive different resolutions
Sky coverage - on interferometer cannot see all the sky
Receiver technologies
Aperture arrays
Low cost
Variable collecting area
Large FOV
Used at low frequencies
Non-uniform directional response
Beam poorly understood
Dishes
High cost
Fixed collecting area
Used at high frequencies
Small FOV
Uniform directional responce
Beam well understood
Single Pixel : Parabolic reflector (mechanical)
Phased Array Feed (PAF): Reflector and receiver array (mechanical + electronic)
The delay added will coherently add the different elements of an aperture array in one direction, and suppress emission from other directions
Examples
LOFAR
Low band and high band antenna
High res and similar to VLBI
Murchison Wide-field array
Low frquency
Wide field of view
Australia
Giant Metrewave Radio Telescope
30 dishes, 45 m
Fully steerable
India
JVLA
New mexico
27 antennas
4 configurations
Flexible and powerful
Australia Compact Telescope Array: ATCA
6 telescopes
4 moveable configurations
Good overlap with alma
Meerkat - South africa SKA pathfinder
64 telescopes
Karoo desert of norther SA
Soon upgraded
eMerlin
3 main bands
7 telescopes
new more sensitive receivers
Wide-band deep high res imagiing
unique position
links VLA and VLBI
Automated delivery of wide-field images from large datasets
Provides short spacings to EVN
VLBI
EVN 22 radio telescopes worldwide
3 sessions per year of 3 weeks
USA 10 25m telescopes
High sensitivity array
VLBA + Phased VLA + Arecibo + Green Bank + Effelsberg
SKA
1 million m2 over 3000km
Fibre network, computing power and raw power to put everything together
Constructed in 2 phases (SKA1 and SKA2)
Low frequency science
Universe was re-ionised arond reshift 6 to 15 (from quasar spectra and CMB), by the first objects stars, mini black-holes)
Can detect the signal of the EoR from observations of redshifted HI in LOFAR, MWA, PAPER
Wide area low frequency servays
The Centimetre sky
Between 1GHz and 50Gz (due to atmospheric cut off by 02)
Calibration limite by the ionosphere at low-freq and the troposphere at high freq
Atmosphere attenuates signal and also adds noise for high opacity
Calibration and systematic are well understood
Science drivers:
Sycrtron continuum
Free-free continuum
spectral line (HI, OH, CH3OH, H2O, CO, high-z)