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

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

Radio Astron ceased because of satellite failures

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)

Present

Mark 5 recorder/formatter with "Disk packs"

Physical shipping of disk packs in boxes to correlator (weeks to months)

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"