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Limitations and interferometers (Assumptions (Monochromatic light (zero…
Limitations and interferometers
Limitations
Finite range of wavelengths
Fringe pattern ok at field centre but different colours are out of phase at higher relative decay
bigger range, smaller fov (FT)
FOV maths
Bandwidth smearing
Large delays away from any chosen point on the sky and nothing can be measured sensibly
nulls of sinc
radial, source spread out away from phase centre
Cure- split into lots of small bandwidth channels though this has cost, correlator limits and data analysis
Too big integration time per data point
v changes with time, so the phase will change
decorreclates
maths
decrease integration time, but this makes dataset bigger
non flat sky over large FOV
Only problem for wide-field imaging
the bigger the phase difference the less true it becomes
equation changes
maths
Primary beam
Maximum field set by the telescope aperture
interference pattern has width 1/s
not usually important unless very narrow BW and short integration times
Signal to noise
Maths
use weighting scheme as appropriate
Assumptions
Monochromatic light (zero bandwidth
Zero integration time
b||sigma
zero-noise recievers
infinitely small telescopes
Infinitely many telescopes
no atmosphere to interduce errors in incoming wavefront
infinitely good correlator
Interferometers
VLA
Giant Metrewave Radio Telescope, India
Merlin
VLBI
limited by earth size
EVLa and e-merlin upgrades
optical fibres give higher sensitivity
because uv plane coverage in wavelengths, images higher fidelity
Low Frequency array lofar
cheap low-freq hardware
hige information processing problem
res of few arcseconds
lots of antennas, high sensitivity up to 240MHz
Atatcama Large Millimetre Array
30-950 GHz
20km max baseline
molecules in galaxies at cosmological z
gas in Galactic star-forming regions
SKA
1 sq km collecting area
2024 maybe
HI at cosmological z
sub arcsec resolution