Upload
justus
View
24
Download
0
Tags:
Embed Size (px)
DESCRIPTION
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 - PowerPoint PPT Presentation
Citation preview
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