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VLBI part 2 - techniques and practices (PI proposal file (Deposited on…
VLBI part 2 - techniques and practices
What is long
Not: 10e2 - 10e4
Very long: 10e5-10e7
Space >10e7
pros and cons
Pros
1 mas resolution, 100 times sharper than HST
Cons
Image fidelity
Sensitivity
Planning and organsation
Radio Astron ceased because of satellite failures
two ways to consider VLBI
1) Wavefront from the same source with an angular sky frequency arrive at two antennas sinusoidal osciallations wit a small time offset where psitau varies with source direction on sky (ie. Fringe pattern) measures source structure if geometric delay corrected for
2) Multiply and average signals at the same time -> we near zero result (averaging two random noise time series) but if adjust one to correct tau then get a signal (fringe) -> precisely determine geometric delay (to 0.1nsec level) -> Measure antenna positions & continental drifts o few cm/year
Geodetic VLBI - defines/measures:
Global terrestrial reference frame (ITRF) - where things are on earth
International celestial reference frame (ICRF) - where things are in the sky
Measure Earth Orientation/rotation parameters (UT1 etc)
VLBI can measure changes in baseline length over continental scales ... distance measuring continental drift , global sea level rises, tidal effects (solid earth and sea),atmospheric loading etc.
Rel. Antennas is all measuring geometric delay
VLBI vs smaller arrays
May see objects on the sky for only a few hours / day
See through different atmosphere
Significant antenna movement (Continental drift)
Cannot use same clock signal to timestamp data - implies that arrays are not phase coherent before calibration
Challenge to bring data from all antennas together for correlation
Different calibration steps (Flux calibration noise diode vs on sky, fringe fitting)
Process of VLBI for astronomers
Wait for deadline to submit proposal
Wait for decision
Wait for time allocation
Plan observations together with correlator
Wait for oberservation
Wait for correlation
Post processing
Proposal stage
Almost all VLBI arrays operate on open science proposal allocation.
Open to anyone
Because of logistics an organisation, allocated a set day for observing
Block schedule
PI proposal file
Deposited on central server
Defines all observing scans/frequencies
Uses VLBI_SCHED programme or now with python interface 'psyched'
excellent experiment planning tool
Indiviual VLBI observations -
Antenna control computer 'VLBI field system'
Interprets VEX file for telescope control
Goes to VLBI antenna
Calibrator choices
Fringe finders/bandpass/phase delay rate etc.
Be careful, must have flux on appropriate scales and ideally close to target (phase cal) - eg. VLBI cal list, fringe-finder list and so forth
VLBI Tuning and Sampling
Frequency down conversion and amplification
To a Sampler, to Data stream to formatter/recorder
Usually 2-bit is adequate
Recording and shipping
Past (Sometimes still present)
Mark 5 recorder/formatter with "Disk packs"
Physical shipping of disk packs in boxes to correlator (weeks to months)
Present
Fila10G VDIF formatter
e-shipping viainternet (Days to weeks
Flexbuff storage computer
Future
eVLBI: direct fast internet transfer to correlator (realtime)
Note: smaller interferometers - VLA eMerlin steam data in real time always
VLBI Correlator
Large Correlator centre - remote from telecopes e.g. JIVE, Dingeloo, nL or Bonn, DE or Socorro
Incoming data via internet, e-shipping, or disk packs
high typical data rates (tens Gbps)
Multiplies and averages al pairs of time streams
Correlated visibilities - one complex number per baseline pre integration time per frequency per polarisation "few GBps"