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21st Century VLBI
Mike Garrett (JIVE)
Astro-lunch, 16 April 2003.
www.evlbi.org www.jive.nl
Current Developments in Continuum VLBI • Replacement of Tape by Disks:
– More reliability – Higher Data rates can be employed (> 1 Gbps)– Naturally leads to…
• Real-time VLBI: – Psuedo real-time “ftp-VLBI”– Real-time fibre-optic e-VLBI (meeting tomorrow!)– Data rates of tens of Gbps per antenna
• Replacement of analogue back-end electronics with digital systems
• Broadband receivers 1-4 GHz; 4-8 GHz etc. – Wide-band MFS Imaging
Will lead to better sensitivity (a few microJy/beam)
Not just deeper but wider…!
• Typical VLBI field-of-view few pixels on this screen!
6 arcmin [FWHP] Effelsberg beam
VLBI FoV x 100
• Compute resources required to maintain a given FoV scale as
Baseline2…
• Implications for EVN Correlator data output-rate: PC Int Project at JIVE
Offline data processing
(Local PC Cluster/GRID Access)
Some lessons learned from the HDF-N…
• EVN did not reach the thermal-noise level (Gross Calibration errors)
• Within 1 arcmin FoV the HDF-N delivered ~ 1 mJy in (summed) flux-density…
> 100 mJy of summed flux over 10 arcmin FoV !
Promise of VLBI as a phase-stable VLBI instrument at cm wavelenghts…
VLBA+GBT DWFS of Bootes Field- Garrett, Wrobel & Morganti
• In-beam phase referencing employed
• 3 x 5.5 (8) hours @ 256 Mbps
• VLBA correlator running flat out:– 0.5 second integrations,– 1024 x 62.5 kHz channels
2 x 60 Gbytes of data
• Calibration performed on averaged data.
• Full sensitivity limited to a region 3 arcmin across (due to smearing).
• Image processing is very slooooow!– Co-addition of Dirty
Images/Beams from each 8 hr run
• Work in progress… astro-ph-xxx (Jan 2003).
2 sub-mJy sources detected in inner 3’
Imaging out the VLBA P-beam
• Can we detect sources well beyond the GBT primary beam (FWHM) ?
• Data tapered successively as a function of distance from field-centre
• Sources in the field brighter than a few mJy were targeted…
Garrett, Wrobel & Morganti
VLBA + GBT24 hr @ 256Mb/s
9 microJyRms noise
Full-beam Calibration
• At frequencies 1-5 GHz, phase-stable VLBI observations can be made in ANY area of the sky !
• Standard self-calibration techniques can use the Summed Response of all detectable sources in the beam.
• Requires good input model conventional phase-referencing or in-beam referencing required initially.
• Compute requirements are considerable – requires PC cluster, parallel code/data streams and possibly access to GRID resources.
Future Challenges• In the next few years VLBI will be transformed into a true survey instrument!
– Huge areas of sky can be imaged-out at with milliarcsec resolution and full sensitivity,
- Dozens of radio sources will be detected simultaneously in a single observation,
- Deep VLBI Surveys will be conducted of high quality fields (e.g. FLSv Strip)
– VLBI will emerge as a very efficient AGN detector (Starburst-AGN descriminator)
- What fraction of faint, obscured SCUBA and microJy source populations harbour AGN ? Will faint AGN also be (on average) more distant ?
- Milliarcsecond astrometry for every source detected, spatial comparison between VLBI structures and other high-resolution instruments of the day e.g. VLTI
- Large surveys will be conducted e.g. extending the search for gravitational lenses to image separations < 0.3”
- Surveys will be able to “piggy-back” on any observation (AVO)