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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. 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 ?

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  1. 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

  2. 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

  3. 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

  4. 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

  5. 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

  6. Nançay Radioheliograph: 5m antennas (north-south array) 2nd LOFAR KSP meeting Potsdam 2009 July 24-25

  7. 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

  8. 2nd LOFAR KSP meeting Potsdam 2009 July 24-25 Bastian et al. (2001)

  9. 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

  10. 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

  11. 11 juillet 2008 : 445 432 408 361 327 299 271 228 173 151 MHz.refait 27 mai 2009 445-298

  12. 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

  13. NRH -> LOFAR KSP : Interferences • 150 - 250 MHz band Nançay (interference survey antenna) • Wide band example 2nd LOFAR KSP meeting Potsdam 2009 July 24-25

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

  21. 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

  22. NRH -> LOFAR KSP : the end Thank you 2nd LOFAR KSP meeting Potsdam 2009 July 24-25

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