2nd LOFAR KSP meeting Potsdam 2009 July 24-25

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

• Interferometry– Baselines, field of view– Fringe stopping, Sun motion– Calibration

• Time & frequency sampling, polarization

• Perturbations: Ionosphere and RFI

• Data formats and software

Nançay Radioheliograph

• General characteristics

– Frequency range: 150 - 450 MHz– 648 baselines from 50 to 3200m (25 to 4,800 – 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

Ext2

Nançay Radioheliograph array configuration

Ext1Ext0

H1H2H7H8H16

NS1NS2

NS12

NS23

North

South

1600 m 1600 m

1248 m

NS8A0 A1 A2 A3

NS24

« Est-West » antennae 150-450 Mhz1 polarization

« North-South » antennae 5 m diameter150-450 Mhz 2 polarizations

« Est-West Extension » antenna (Ext0)« North-South Extension » antenna (NS24)

7 m diameter150-450 Mhz 2 polarizations

« Est-West Extensions » antennae (Ext1, 2) 10 m diameter150-450 Mhz 2 polarizations

« Anti Aliasing » antennae Log Periodic150-450 Mhz2 polarizations

1200 m

2nd LOFAR KSP meeting Potsdam 2009 July 24-25

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

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

2nd LOFAR KSP meeting Potsdam 2009 July 24-25

NRH -> LOFAR KSP: Interferometry

• U-V coverage– The solar corona is a broad source: u-v min < 30

• 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 Bastian et al.

(2001)

2nd LOFAR KSP meeting Potsdam 2009 July 24-25

Clean Dirty

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

UV min ~70

2nd LOFAR KSP meeting Potsdam 2009 July 24-25

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.

11 juillet 2008 :

445 432 408 361

327 299 271 228

173 151 MHz. refait 27 mai 2009 445-298

2nd LOFAR KSP meeting Potsdam 2009 July 24-25

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

• 150 - 250 MHz band Nançay (interference survey antenna)• Wide band example

2nd LOFAR KSP meeting Potsdam 2009 July 24-25

• 150 - 152 MHz band Nançay (interference survey antenna)• Narrow band examples

Special Issues at Low frequencies: Interferences

2nd LOFAR KSP meeting Potsdam 2009 July 24-25

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 : 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

• 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 : 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 : 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

• 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• 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.

NRH -> LOFAR KSP : the end

2nd LOFAR KSP meeting Potsdam 2009 July 24-25

Thank you