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Two-band spectral filtering in instruments for measuring solar magnetic fields
Astron. Astrophys. Suppl. Ser.
ltering in instruments for measuring
V.M. Grigoryev 0
N.I. Kobanov 0
0 Institute of Solar-Terrestrial Physics , P.O. Box 4026, Irkutsk , Russia
In this paper we formulate the basic concept of a bichromatic image technique. Speci c examples of its application are analyzed in the context of research into solar plasma characteristics by pinpointing its merits and demerits. Specialized requirements to the spectral devices are set forth, which may be summarized as: a) the presence of two nearby identical spectral passbands; b) the mutual orthogonality for the polarization of the light that has passed through neighboring bands; and c) the possibility of controlling the relative position of the bands. Several alternative plausible implementations of the method are considered with the use of Fabry-Perot interferometers (FPI) and magnetooptical lters (MOF) for measuring the longitudinal magnetic eld strength and the intensity eld. A new design of a lter magnetograph is proposed, based on MOF, with the combination of two optical resonance cells into one.
lter magnetographic methods
1. Introduction
This paper is primarily concerned with a technique for
obtaining longitudinal magnetic eld magnetograms, based
on two-passband spectral ltering. Longitudinal magnetic
eld measurements usually use intensity fluctuations
measured in the blue and red wings of a spectral line at the
frequency of the modulator that consecutively transmits
the left and right-handedly circularly polarized radiation.
In this case, measurements in the two line wings are made
either simultaneously with two photodetectors or
sequentially with a single detector
(Babcock & Babcock 1952;
Beckers 1968; Cacciani 1981)
, i.e. measurements in each
of the spectral bands are separated either spatially or
temporally. Note that the identity problem of dynamic
characteristics of several photodetectors is a long-standing
complicated challenge in experimental astrophysics. A single
photodetector and the modulated signal are increasingly
favored by the investigator because these two factors act
to improve dramatically the sensitivity of measurement.
With the advent of CCD and high-speed computers, the
essence of the problem did not alter. As before,
achieving a higher sensitivity involves the possibility of using a
single photodetector for measuring a particular
parameter in each pixel, as well as using the rapid
modulationdemodulation. It is by no means accidental that this
problem was further addressed in publications devoted to the
LEST Project
(Povel 1990; Keller et al. 1992; Povel et al.
1994)
. The above considerations are characteristics both
for most one-channel magnetographs and for
multichannel instruments based on using lter systems and
CCDreceivers; the latter are gaining increasing acceptance. Of
course, with the advent of CCD, stokes-polarimeters have
been and are being developed
(Elmore et al. 1992; Mein
1991; Bendlin et al. 1992; Mickey et al. 1996)
, which
measure the distribution of all Stokes parameters in the line
pro le at each image point. When using methods for
solving inverse problems of radiation transfer in a line, they
provide a wealth of information about the magnetic eld
structure and the velocity through the layer thickness
where the line forms. However, such observations do not
enjoy reasonably high time resolution, but from 8 min
to 70 min (Mickey et al. 1996). Some problems in solar
physics that involve the study of shortlived and highly
dynamic phenomena (flares, ejections, eruptive prominences)
require a time resolution over 1 min. Conventional
magnetographs with lter systems and CCD
(Hagyard et al.
1982; West 1985; Sakurai et al. 1991)
will remain e
ective for much time to come in such observations to make
measurements in xed spectral bands of the blue and red
line wings. It is important and useful to further upgrade
such systems with the aim to maximize their time
resolution and sensitivity. Moreover, the method under
consideration does not rule out the possibility of scanning a
line pro le. And these issues are covered by the present
paper. The central idea is that intensity fluctuations are
measured simultaneously in two spectral bands 1
and 2 (in the blue and red wings) with a single
photodetector, without any spatial separation into two
images. As applied to lter magnetographs, this technique
may be called the method of bichromatic image. The idea
of the bichromatic image for measuring Hk was suggested
in 1971 by Ramsey for birefringent lters and
independently by a group including these authors at SibIZMIR
(Lebedev et al. 1972)
for the di raction spectrograph.
Subsequently, the Ramsey's idea of obtaning two bands
in birefringent lters for measuring all Stokes
parameters was implemented by Chinese investigators
(Guoxiang
1990)
. This same concept was used in di erential
line-ofsight velocity measurements and for measuring @I=@ the
spectral line pro le
(Kobanov 1983, 1993)
with (...truncated)