This paper is primarily concerned with a technique for
obtaining longitudinal magnetic field magnetograms, based on
two-passband spectral filtering.
Longitudinal magnetic field 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 modulation-demodulation.
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 filter systems and CCD-receivers; 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 profile 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 field 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 filter systems and CCD (Hagyard et al.
1982; West 1985; Sakurai et al. 1991) will
remain effective for much time to come in such observations to
make measurements in fixed 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 profile.
And these issues are covered by the present paper. The
central idea is that intensity fluctuations are measured
simultaneously in two spectral bands 
and 
(in the blue and red
wings) with a single photodetector, without any spatial
separation into two images. As applied to filter
magnetographs, this technique may be called the method of bichromatic
image. The idea of the bichromatic image for measuring 
was
suggested in 1971 by Ramsey for birefringent filters and
independently by a group including these authors at SibIZMIR (Lebedev
et al. 1972) for the diffraction spectrograph. Subsequently, the
Ramsey's idea of obtaning two bands in birefringent filters for measuring
all Stokes parameters was implemented by Chinese investigators (Guoxiang
1990). This same concept was used in differential line-of-sight
velocity measurements and for measuring 
the spectral line profile (Kobanov 1983, 1993) with
diffraction spectrograph. The goal of this paper is to generalize the concept
of the method and demonstrate the possibilities of using it in
conjunction with different spectral instruments.
It should also be emphasized that using the method of bichromatic
image allows for scanning the spectral line profile
in both conventional and filter magnetographs. This is achieved
by varying the amount of the spectral interval between 
and
.