LOFAR and EISCAT Status Update(with occasional science snippets)

RAF and MarioEISCATEISCAT @ VHF – the story so far...
●●●●EISCAT remote antennas (Kiruna andSodankyla) converted to VHF (224 MHz) in thelast year.
Tromso remains at 928 MHz.
1420 MHz still possible on request, but requires
one day per site and a crane...
Occasional cross-correlations seen, but big
issue with zero time-lag correlation.

EISCAT @ VHF – the story so far...

LOFAR

Array Location
 - Centred in Netherlands
 - International stations: - France (1) - Germany (5) - Sweden (1) - UK (1) - Poland (3) - Ireland (1)6Array Location
 - Dutch stations: - Core centred on Exloo - Remote stations

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Array Location
 - Core stations: - Central area of stations
with area diameter of 2km.
 - “Superterp” of six stations
in centre.

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Array Location
 - Superterp

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Data Path10

Antennas
 Each site contains two arraysof antennas.
 Low-band (LBA): 10-90MHz
 High-band (HBA):
 110-250MHz

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Low-Band Antennas- Wire dual dipoles set at 45degrees- Held in place by rubber tensioners- “Random” distribution within
station to reduce sidelobes- Usable band:10-90 MHz- Below 30MHz, ionosphericcontamination- Above 88MHz, FMwaveband

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High-Band Antennas- Metal plate “bow-tie” dual dipoles, encased inpolystyrene- Grouped into “tiles” of 4x4 antennas, with tile
spacing of 1.25m (1 wavelength at 240MHz).
- Analogue beam-former “points” tile in given
direction, ~30 degree beam width- Usable band:110-250 MHz- Three filters: 110-190MHz,170-230MHz, 210-250MHz

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Clock and Filter Selections

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Frequency and Time Resolutions

 Full bandwidth of the digitised signal (100MHz or 80MHz
depending on the clock) split into 512 “subbands”.
 Up to 488 subbands, covering 96/76MHz in total, can be
returned to Central Processing (CEP) due to data rate
limitations.
 Frequency resolution is 0.1953125/0.156250 MHz.
 Time resolution at this stage is 5.12 μs.
 In CEP, subbands can be split into channels (16 to >256)with a corresponding decrease in time resolution.

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Beam Definitions

Dipole/Tile beam:- Sensitive to whole sky>30 degrees elevation(LBA) or ~30 degrees
wide (HBA tile)

Station beam:- 2 to >8 degrees wide,
depending on station
size and frequency.

“Tied-array beam”:- Form array of stations.
- <1 degree wide,
depending on frequency
and number of stations
used.

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Observing Trade-offs

 Observing with single stations: - Each sub-band corresponds to a “beamlet” at station level.
 - Hence, bandwidth must be balanced against number ofbeams at station level.
 Tied-array beams:
 - Additional beam-forming done in the correlator.
 - Only available with core stations.
 - Allows formation of up to ~190 beams using the fullbandwidth.

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Final Data Products

 Data returned can be combined to return raw visibilities forimaging: Imaging data “pipeline” will return sky images, asspecified by the user.
 Or they can be returned as “beam-formed” data: Equivalent to single-dish measurement.
 Time/Frequency data returned for combined array and/orindividual stations.
 Or both simultaneously...

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IPS with LOFAR: the Science wedidn't show...

Mode “357” - Combining LBA andHBA modes●

A third of the LBAs in mode 3 (10-90 MHz)●A third of HBAs in mode 5 (110-190 MHz)●A third of HBAs in mode 7 (210-270 MHz)●●Specify beams such that all available subbands are spreadover the three modes.

Caveats:– Sensitivity is seriously reduced.
– Spread of subbands results in gaps between them:●

In 8-bit mode, can cover every third subband

Ooh... Pretty...

4-bit mode – 192 MHz of bandwidth

Issue with peak sensitivity of LBA saturating. Need to use
maximum attenuation, but some saturation still visible.

IPS Cross-Correlations – the curseof zero-lag correlations...

Weaker source

Strong source