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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


Experience of nrh observations which benefit for lofar ksp

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



Experience of nrh observations which benefit for lofar ksp

Negative bowl removed (hardly) by CLEAN. Accuracy of low T ?B 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


Experience of nrh observations which benefit for lofar ksp

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