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3rd Spanish solar and heliospheric physics meeting, Granada, June, 20113rd Spanish solar and heliospheric physics meeting, Granada, June, 2011
3rd Spanish solar and heliospheric meeting, Granada, 9 June, 2011
assessing modern magnetographs
jose carlos del toro iniesta (SPG, IAA-CSIC)valentín martínez pillet (IAC)
3rd Spanish solar and heliospheric meeting, Granada, 9 June, 2011
what this talk is all about• assessment of the capabilities of modern spectropolarimeters
and magnetographs
• useful during design (tolerances) and exploitation phases (uncertainties)
• pair of nematic LCVR-based instruments (IMaX & SO/PHI)
• demonstrate that they can reach optimum εi regardless of the optics between the modulator and the analyzer
• obtain values for optimum retardances
• derive formulae for
• detection thresholds for B and vLOS depending on the S/N and εi
• inaccuracies and instrument instabilities
3rd Spanish solar and heliospheric meeting, Granada, 9 June, 2011
an optimum polarimeter
• a modulator made up of two nematic LCVRs can be optimum (martínez pillet et al., 2004)
• if axes are at 0º and 45º with S2 > 0 direction
• ideally maximum efficiencies can be reached for both vector and longitudinal analyses
• instrumental polarization may corrupt efficiencies
• we show that ideal efficiencies can be reached by simply tuning voltages
• we first demonstrate that the ideal result can be reached for the single polarimeter
3rd Spanish solar and heliospheric meeting, Granada, 9 June, 2011
polarimetric efficiencies (i)
• (ε1,ε2,ε3,ε4) ≤ (1,1/√3,1/√3,1/√3) (del toro iniesta & collados, 2000)
• 2 nematic LCVRs especially good (martínez pillet et al., 1999)
• optimum theoretical modulation with four measurements: M1 = R(0,ρ); M2 = R(π/4,τ); M4 = L(0); M ≡ M4 M2 M1 ⇒
Oij = (1, cos τi, sin ρi sin τi, - cos ρi sin τi)
• optimum longitudinal modulation (I -/+ V): M1 = R(0,0); M2 = R(π/4,±π/2); M4 = L(0)
M1 M2 M4
3rd Spanish solar and heliospheric meeting, Granada, 9 June, 2011
polarimetric efficiencies (i)
• (ε1,ε2,ε3,ε4) ≤ (1,1/√3,1/√3,1/√3) (del toro iniesta & collados, 2000)
• 2 nematic LCVRs especially good (martínez pillet et al., 1999)
• optimum theoretical modulation with four measurements: M1 = R(0,ρ); M2 = R(π/4,τ); M4 = L(0); M ≡ M4 M2 M1 ⇒
Oij = (1, cos τi, sin ρi sin τi, - cos ρi sin τi)
• optimum longitudinal modulation (I -/+ V): M1 = R(0,0); M2 = R(π/4,±π/2); M4 = L(0)
M1 M2 M4
| || || |
3rd Spanish solar and heliospheric meeting, Granada, 9 June, 2011
polarimetric efficiencies (ii)• the etalon, a retarder: M3 = R(ϑ,δ)
• M = M4 M3 M2 M1 ⇒ Oij = M1j (τi,ρi) ➔ M1j (τi,ρi)
= ±1/√3, j = 2,3,4, are transcendental equations with solution and M11 = 1 ➔ optimum polarimetric efficiencies can be achieved
• trivial cases: ϑ = 0,π/2; δ = 0
M1 M2 M4
3rd Spanish solar and heliospheric meeting, Granada, 9 June, 2011
polarimetric efficiencies (ii)• the etalon, a retarder: M3 = R(ϑ,δ)
• M = M4 M3 M2 M1 ⇒ Oij = M1j (τi,ρi) ➔ M1j (τi,ρi)
= ±1/√3, j = 2,3,4, are transcendental equations with solution and M11 = 1 ➔ optimum polarimetric efficiencies can be achieved
• trivial cases: ϑ = 0,π/2; δ = 0
M1 M2 M4M3
3rd Spanish solar and heliospheric meeting, Granada, 9 June, 2011
polarimetric efficiencies (iii)
• a train of mirrors (no matter the number and the relative angles) has a Mueller matrix like (Collet)
• all modulation matrix elements turn out to be multiplied by (a+b) ➔ no effect on the result!
• since mirrors are retarders plus partial polarizers, any differential absorption effect is included
• calibration necessary for non-ideal instruments
M1 M2 M4
E =
!
""#
a b 0 0b a 0 00 0 c d0 0 !d f
$
%%&
3rd Spanish solar and heliospheric meeting, Granada, 9 June, 2011
polarimetric efficiencies (iii)
• a train of mirrors (no matter the number and the relative angles) has a Mueller matrix like (Collet)
• all modulation matrix elements turn out to be multiplied by (a+b) ➔ no effect on the result!
• since mirrors are retarders plus partial polarizers, any differential absorption effect is included
• calibration necessary for non-ideal instruments
M1 M2 M4M3 E
E =
!
""#
a b 0 0b a 0 00 0 c d0 0 !d f
$
%%&
3rd Spanish solar and heliospheric meeting, Granada, 9 June, 2011
noise in action (i)
• we only measure photons ➔ everything depends on photometric accuracy (syst. errors ideally absent)
• noise: limiting factor ⇒
• signal-to-noise ratio ⇒
• detectability is smaller in polarimetry than in pure phototmetry
(S1, S2, S3, S4) ! (I, Q, U, V ) !Siv ,B,!," ! "i
(S/N)i =!
S1
!i
"
c(S/N)i =
!i
!1(S/N)1
martínez pillet et al. (1999) and del toro iniesta & collados (2000)
!i !"
3!1
3rd Spanish solar and heliospheric meeting, Granada, 9 June, 2011
magnetographic inaccuracies (i)
• magnetographic formulae
• error propagation yields
• if S/N = (S/N)1 = 1700 (1000 for S2, S3, and S4) then δ(Blon) = 5 G and δ(Btran) = 80 G for IMaX
• T, V, and other instabilities and defects of LCVRs ⇒ changes in the retardances ⇒ demodulation changes ⇒ cross-talk between the Stokes parameters ⇒ covariances (asensio ramos &
collados, 2008)
Blon = klonVs
S1,cBtran = ktran
!Ls
S1,candLs !
1n!
n!!
i=1
"S2
2,i + S23,iVs !
1n!
n!!
i=1
ai |S4,i |,
!(Blon) =klon
S/N"1
"4!(Btran) = ktran
!"1/"2
S/N= ktran
!"1/"3
S/Nand
3rd Spanish solar and heliospheric meeting, Granada, 9 June, 2011
magnetographic inaccuracies (ii)
• efficiency variances can be seen as functions of retardance variances (del Toro Iniesta & Collados 2000)
• retardance variances can be written as functions of birefringence, thickness, and wavelength variances
• birefringence variance is a sum of thermal and voltage variances
!2max,i =
!4j=1 O2
j i
Np! "!2
max,i= f ("2
"j,"2
#j)
!2!L
"2L
=!2"
#2 +!2
t
t2 +!2#0
$20
!2! = q2
T!2T + q2
V!2V
IMaX
0.3 K or 1.2 mV instabilities induce a 5 % repeatability error in Blon and a 2.5 % repeatability error in Btran
3rd Spanish solar and heliospheric meeting, Granada, 9 June, 2011
velocity inaccuracies (i)
• velocities
• key instrumental ingredient: etalon
• the technique: e.g., Fourier tachogram
• scientific requirements on v directly translate onto roughness, temperature and voltage stabilities, and noise
v =2c!"
#"0arctan
I!9 + I!3 ! I+3 ! I+9
I!9 ! I!3 ! I+3 + I+9
!2v = f (v , "#)!2
!" + g(v , #0, "#, Ii , si )(k2T !2
T + k2V !2
V ) + h(#0, "#, Ii )!2I
3rd Spanish solar and heliospheric meeting, Granada, 9 June, 2011
velocity inaccuracies (ii)
• assume λ0 = 6173 Å and δλ = 100 mÅ (SO/PHI)
• a roughness instability inducing σδλ = 1 mÅ produces σv = 1 ms-1 for speeds of 100 ms-1! (and is linear in v)
• imagine temperature and noise contribute equally. then, σT/σI = 5.7 and S/N =1700 ⇔ σT = 10 mK
• pure photon noise of σI = 10-3 Ic induces σv = 7 ms-1
• uncertainties larger than 45 mK or 3.4 V produce σv > 100 ms-1 (and this can be an issue for global helioseismology)
!2v = f (v , "#)!2
!" + g(v , #0, "#, Ii , si )(k2T !2
T + k2V !2
V ) + h(#0, "#, Ii )!2I