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Experience of NRH observations: which benefit for LOFAR KSP ?. A. Kerdraon Observatoire de Paris - LESIA - USN. NRH -> LOFAR KSP: outline. Interferometry Baselines, field of view Fringe stopping, Sun motion Calibration Time & frequency sampling, polarization

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experience of nrh observations which benefit for lofar ksp

Experience of NRH observations: which benefit for LOFAR KSP ?

A. Kerdraon Observatoire de Paris - LESIA - USN

2nd LOFAR KSP meeting Potsdam 2009 July 24-25

nrh lofar ksp outline
NRH -> LOFAR KSP: outline
  • Interferometry
    • Baselines, field of view
    • Fringe stopping, Sun motion
    • Calibration
  • Time & frequency sampling, polarization
  • Perturbations: Ionosphere and RFI
  • Data formats and software

2nd LOFAR KSP meeting Potsdam 2009 July 24-25

nan ay radioheliograph
Nançay Radioheliograph
  • General characteristics
    • Frequency range: 150 - 450 MHz
    • 648 baselines from 50 to 3200m (25 to 4,800 l)
    • Spatial resolution: ~4 to 0.3 arcmin (depending on frequency, declination, snapshot/synthesis)
    • Field of view: from 3 to 0.5 degrees
    • Stokes I and V
    • Time resolution: 5 ms* number of frequencies
slide4

Nançay Radioheliograph array configuration

1600 m

1600 m

H16

H8

H7

H2

H1

NS1

Ext0

Ext1

Ext2

NS2

  • « Anti Aliasing » antennae
    • Log Periodic
    • 150-450 Mhz
    • 2 polarizations

A0

A1

A2

A3

NS8

  • « Est-West » antennae
    • 150-450 Mhz
    • 1 polarization

1248 m

NS12

North

  • « Est-West Extension » antenna (Ext0)
  • « North-South Extension » antenna (NS24)
    • 7 m diameter
    • 150-450 Mhz
    • 2 polarizations

South

  • « Est-West Extensions » antennae (Ext1, 2)
    • 10 m diameter
    • 150-450 Mhz
    • 2 polarizations

NS23

  • « North-South » antennae
    • 5 m diameter
    • 150-450 Mhz
    • 2 polarizations

1200 m

NS24

nan ay radioheliograph east west array flat antennas
Nançay Radioheliograph: East - west array flat antennas
  • Low gain antennas: (~wide band dipoles)
  • Severe sensitivity limitation at high frequency
  • One linear polarization

2nd LOFAR KSP meeting Potsdam 2009 July 24-25

nan ay radioheliograph 5m antennas north south array
Nançay Radioheliograph: 5m antennas (north-south array)

2nd LOFAR KSP meeting Potsdam 2009 July 24-25

nrh lofar ksp interferometry
NRH -> LOFAR KSP: Interferometry
  • U-V coverage
    • The solar corona is a broad source: u-v min < 30 l.
      • Main problem: negative bowl due to poor uv sampling around the origin. Strong impact on quiet Sun TB.
    • Diffusion of radio waves in the corona broadens sources: baselines > 10-20 km are not useful (probably)
  • Field of view: > 6 degrees (> inverse of UV min)
    • CMEs may be observed at very high altitudes
    • To a lesser extent, type III also
    • This is a primary beam problem

2nd LOFAR KSP meeting Potsdam 2009 July 24-25

slide9

Negative bowl removed (hardly) by CLEAN. Accuracy of low TB in coronal holes ?

UV min ~70 l

Clean

Dirty

2nd LOFAR KSP meeting Potsdam 2009 July 24-25

nrh lofar ksp interferometry1
NRH -> LOFAR KSP: Interferometry
  • Fringe stopping and coordinates
    • Absolute accuracy ~1 arsec (better if we use long baselines)
    • Sun motion
      • UT/ST: it is better to make the fringe stopping in UT, but that can be corrected oofline (NRH uses UT)
      • Sun hour angle/declination slow variations: up to 1 arcmin / hour.
        • Can also be done offline ( NRH uses one solar center coordinate per day, and makes the corrections offline)
  • Imaging:
    • The preferred mode is snapshot
    • Earth rotation synthesis increases the quality of quiet corona thermal emission.

2nd LOFAR KSP meeting Potsdam 2009 July 24-25

slide11

11 juillet 2008 :

445 432 408 361

327 299 271 228

173 151 MHz.refait 27 mai 2009 445-298

nrh lofar ksp interferometry2
NRH -> LOFAR KSP: Interferometry
  • Calibration
    • Ideally: gains to a few %, phases to a few degrees
      • NRH problems: no strong point source in the sky.
        • We use most often a model of Cygnus A. Problems come from the small antenna sizes, the strange arrays configuration and the simplified correlator (which dont make all the possible correlations)
      • LOFAR should be much better.
    • Polarization calibration is done by a rotation of the antennas (there is no polarized calibrator).
      • Related problem: crosstalk between the 2 polarization of the antennas should be as low as possible (or corrected ?): instrumental polarization should be <1% (Type III polarization…)
    • It is difficult (impossible) to calibrate in the presence of an active sun: the best answer is stability, at least for 24 hours.
  • Have a common frequency between LOFAR and NRH (151 MHz ?)

2nd LOFAR KSP meeting Potsdam 2009 July 24-25

nrh lofar ksp interferences
NRH -> LOFAR KSP : Interferences
  • 150 - 250 MHz band Nançay (interference survey antenna)
  • Wide band example

2nd LOFAR KSP meeting Potsdam 2009 July 24-25

special issues at low frequencies interferences
Special Issues at Low frequencies: Interferences
  • 150 - 152 MHz band Nançay (interference survey antenna)
  • Narrow band examples

2nd LOFAR KSP meeting Potsdam 2009 July 24-25

nrh lofar ksp interferences1
NRH -> LOFAR KSP : Interferences
  • NRH has no RFI mitigation capability
    • Study for FASR (experimental interferometer)
      • The classic system with banks of narrow filters can remove medium level low bandwidth telecom signals, with simple detection of low bandwidth signals.
      • It is more difficult for powerful interferences
      • Solar obervations are special:
        • Not sensitive to low level interferences
        • Detection of RFI based on the power level is not possible, due to solar bursts.
  • We try to have the best status in the (very few) band ~allocated to radioastronomy:
    • 74, 151, 327, 408, 610 MHz.

2nd LOFAR KSP meeting Potsdam 2009 July 24-25

nrh lofar ksp interferences2
NRH -> LOFAR KSP : Interferences
  • For LOFAR KSP:
    • Make a simple ~real time RFI mitigation, avoid storing lots of small bandwidth correlations.
    • The situation is getting worst in the metric band, with digital audio and video broadcast:
      • Everybody should work in his country to get the best legal protection of the astronomy bands ( is it too late ?).

2nd LOFAR KSP meeting Potsdam 2009 July 24-25

nrh lofar ksp ionosphere
NRH -> LOFAR KSP : Ionosphere
  • Ionosphere at 164 MHz
  • Very severe case (includes some distorsion)
  • In most cases: smaller motion and no distorsion.
  • Likely to occur at low site angle

2nd LOFAR KSP meeting Potsdam 2009 July 24-25

nrh lofar ksp ionosphere1
NRH -> LOFAR KSP : Ionosphere
  • Generalities
    • Density inhomogeneities due to Travelling Ionospheric Disturbances (TIDs) may affect radio observations at dam to dm wavelength.
    • TIDs most often due to gravity waves, sometimes to other phenomena (including magnetosphere).
    • Effects are proportionnal to f-2
    • Gravity waves are neutral atmosphere phenomenon , which couples through collisions to electrons and ions
      • Their effect is VERY sensitive to the height of the sun (10° is a bad value).
      • They are frequent.
    • There are TID: « Bubbles » isolated disturbances
      • NRH see phase shifts (100°) crossing the arrays in ~30 sec.

2nd LOFAR KSP meeting Potsdam 2009 July 24-25

nrh lofar ksp ionosphere2
NRH -> LOFAR KSP : Ionosphere
  • Possible corrections
    • For NRH: almost none
      • Try to follow a stable source on the sun, if any (noise storm).
      • It is difficult to measure motions on the quiet sun emission.
    • For LOFAR: ?
      • Ionosphere model based on motions of radio sources (equivalent to multi object adaptative optics). Needs one source per square degree, not convenient for solar observations.
      • At low frequencies, you have to correct not only motions, but also scintillations.

2nd LOFAR KSP meeting Potsdam 2009 July 24-25

nrh lofar ksp time frequency sampling
NRH -> LOFAR KSP : time/frequency sampling
  • 0.1 sec, 200 kHz bandwidth, 5 to 20 frequencies
    • With 20 frequencies, it is possible to have a raw spectrum of different sources.
  • With the 200 kHz bandwidth, only I and V are required
    • In a spectrograph mode (one or a few stations), 4 Stokes make sense if the bandwidth is <10 kHz.
  • Burst / monitoring (= integrated) modes
    • Both need the same number of stations, observing time, correlator resources. Is monitoring mode a convenient quicklook to the observations?

2nd LOFAR KSP meeting Potsdam 2009 July 24-25

nrh lofar ksp data formats
NRH -> LOFAR KSP : Data formats
  • Store visibilities, not images
    • Processing algorithms may be improved
    • Possibility to make images in wider fields
  • Use standard FITS (Soho headers ?, Aips compatible?)
    • Think to quick look products to facilitate data access
      • Integrated data (standard images?)
      • movies
  • For solar studies, essential capabilities are:
    • Movies
    • Merging with other solar observations
    • Sources detection and tracking
    • Integration in Solarsoft
    • Specific multiscale deconvolution
  • Storage: compression with loss
    • Integrate when time variations with time are small + manual decision for exceptionnal events.

2nd LOFAR KSP meeting Potsdam 2009 July 24-25

nrh lofar ksp the end
NRH -> LOFAR KSP : the end

Thank you

2nd LOFAR KSP meeting Potsdam 2009 July 24-25

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