Coronal Magnetism, May 21-23, 2012, Boulder, Co, USA. Moscow M.V. Lomonosov University. I.S. Kim, I.V. Alexeeva, and O.I. Bugaenko

Transcription

1 Moscow M.V. Lomonosov University I.S. Kim, I.V. Alexeeva, and O.I. Bugaenko 1

2 Weak magnetic fields diagnostics in the upper solar atmosphere δλb /Δλ = 2

3 Key items of weak magnetic fields diagnostics in the upper solar atmosphere Low-scattered light feeding optics: Analyzing equipment: Recording equipment Coronagraphs Advanced polarimeters Elmore et al. 1992, Paletou et al. 2001, Kuhn et al. 2003, Lin et al. 2004, Tomczyk et al

4 Weak magnetic fields in the upper solar atmosphere: non-object signatures in the final focal plane: Istray, Isky, Icont 4

5 Acceptable level of the stray light for Zeeman diagnostics Kim et al., 2012, Proceedings of the 2nd ATST EAST workshop Chromosphere Hα, W 0.1, B = 100 G, Istray 6 10^{-4}; He I, 1083 nm, W = 0.06, B = 20 G, Istray 4 10^{-4}. Prominences W = 0.1, B = 100 G, Istray 6 10^{-4}. W = 0.01, B = 20 G, I stray 10^{-5}. Corona Fe XIII, nm. All non-object signatures are comparable. Performance of the polarimeter is very important! 5

6 Parasitic background in the final focal plane caused by non-object signatures Newkirk&Bohlin Multiple reflections. 2. Random inhomogeneities of the refractive primary lens. 3. Departures of the surface of the primary optics from a uniform shape. 4. Diffraction of the bright light of the solar disk at the entrance aperture: 5. Scattering at microroughness of the primary optics: 6. Sky brightness. 7. Continuum corona. Lyot s method (Lyot 1930) Nagaoka 1920 Sazanov

7 Reducing the stray light 1. Diffraction at the edge of entrance aperture 1.1. Coronagraphic technique/the Lyot method: Lyot 1930, 1.2. Multi-cascade coronagraphic technique: Terrille 1988; Kim et al Apodizing in the plane of entrance aperture: Kim et al. 1995,..? 2. Scattering at micro-roughness of the primary optics 2,1. Super-smooth primary optics: RMS = 3 5 A The moderately smooth primary optics with a given profile of the microrelief: Romanov et al K = K = 10 K = ? 7

8 Diffraction of the round source at the round aperture Sazanov 1968 Reducing the stray light: 1.1. the Lyot method Calculated stray light caused by diffraction of the solar disk light at the entrance aperture The correct use of the Lyot method results in Bdif reducing by 1-2 orders of magnitude (the coronagraphic factor, K) depending on the size of masking in the primary focal plane and in the plane of Lyot s stop. Chromosphere K 10. Prominences ARF: K = 50. QP: K = 50. 8

9 Reducing the stray light: 1.2. Multi-cascade coronagraphic technique Terrille 1988, Kim et al Fabry-Perot magnetograph (Nikolsky et al. 1984). 53 cm domeless coronagraph, a piezo scanning FPI, the LiNbO3 modulator. - Measurements at optical axis - Compensation of linear polarization by rotation of a modulator assembly. - Electrical compensation of circular polarization. Kim et al., 2012, Proceedings of the 2 nd ATST EAST workshop, Fig. 2. Top: Hα prominence of July 19, 1984, P=268º, 04:46:15 UT. The dark spot is 8arc sec pinhole of the magnetograph, the black triangle at the artificial moon indicates the location of the optical axis in the picture plane. Note the fine prominence structure of < 1 arc sec. Bottom: simultaneous V and I Stokes profile and Doppler mark (the lower record corresponding to the blue shift of 7 km/s. 9

10 Reducing the stray light: 1.3. apodizing in the plane of entrance aperture Diffraction pattern in the focal plane is a result of violated continuity of the transmission function (or its derivatives) of an entrance aperture: G(ρ) = 1 for ρ < 1; G(ρ) = 0 for ρ > 1. (Bessel functions and elliptical integrals) 10

11 Reducing the stray light: super-smooth prinary optics: RMS = 3-10 A Statistical nature of RMS: the scattered energy ~RMS². Requirements for RMS for reflecting and refracting primary optics: n = 1.5, n* = -1, [(n*- 1)/(n+1)]² = 16. Super-smooth primary optics: RMS = 3 10 A. Simulation by a superposition of sinusoidal phase gratings with the depth of the relief phase, "a, and the grating constant "d" (the correlation length of inhomogeneity 11

12 Reducing the stray light: 2.2. The moderately smooth primary optics having a given profile of the microrelief Romanov et al. 1991, Soviet Journal of Opt. Tech., Fig. 3. Numerical modeling - Microroughness was simulated by a superposition of sinusoidal phase gratings with the depth of the relief phase, "a" (RMS = 25 A) and the grating constant "d" (the correlation length of inhomogeneity, d = µ. - Calculation of the PSF for the given function of the wave front. The red line: diffraction at the entrance aperture. Thus, creating a microrelief with a period of less than 0.78 µ one can localize the scattered light outside the zone of observation. 12

13 Development of new approach for data reduction Poster: 2D linear polarimetry in emission lines, Hα prominences - Reducing accidental errors: statistics. - Reducing systematic errors by the solution of 24 over-determined equations by the least square method. 13

14 Summary - Near-limb reliable Zeeman diagnostics is out of the power of the most advanced Stokes polarimeter in the presence of high level of the stray light. - Reducing the scattered light by combination of different methods depending on the object under study - Apodizing in the plane of an entrance aperture with a special mask 1 - ρ². - A super-smooth primary optics with RMS < 10 A. - A given micro-relief of moderately smooth primary optics. - Development of new approach for data reduction. 14