In experimental research into solar phenomena filtergrams and spectroheliograms have received wide recognition; they represent a solar surface image in a narrow spectral band usually corresponding to a chosen part of the spectral line profile. In some instances it is necessary to have accurate information about the brightness distribution, but when obtaining a filtergram in a single wing of the line, the brightness field will be distorted by the line-of-sight velocity field. In order that one parameter be separated from the other, one more brightness filtergram needs to be obtained in the other wing; after that, it is necessary to perform addition to obtain a brightness filtergram and subtraction to obtain a dopplerogram. This procedure involves unavoidable errors introduced by technical factors when performing these operations. In addition, the atmospheric seeing effect is enhanced in this case, and areas of the image with fast changes in line-of-sight velocity will also make their contribution to the error of measurement. If, however, the bichromatic image
Figure 1: Principle of signal measurement in two-band
filtering mode: a) for intensity filtergramm, b) for 
magnetogramm
mode is employed, then it is possible immediately to obtain in a single frame a brightness filtergram without any distortions caused by Doppler velocity. Also, due regard must be had to the fact that the intensity will be doubled; hence the signal/noise ratio increases, and the exposure time can be further reduced. Errors introduced by the operation of adding up the two frames disappear, and the atmospheric seeing influence is minimized. Unfortunately, information about line-of-sight velocity is lost in this case.
It is, however, possible to combine the merits of both variants.
To accomplish this, it is necessary to obtain one frame of the bichromatic
image 
and one frame of the monochromatic image
or 
. In this case the subsequent subtraction of twice the
second from the first and division by the first gives 
, i.e., the line-of-sight velocity. As a result, two
frames plus the subtraction/division operation provide a usual
dopplerogram and high-quality brightness filtergram with twice
the light. It is pertinent to note that the procedure of
obtaining a dopplerogram is quite as usual.
Next, we consider the performance of the bichromatic image
technique when measuring the Zeeman line splitting.
Specifically, it follows from Fig. 1b that in this case the
instrument must operate so that one phase of modulation receives
the light of intensity 
and the other receives 
.
Here 
and 
are the oppositely polarized
Zeeman components. In the Ramsey filter magnetograph the
modulator is placed ahead of the filter, and the input
polarizer is removed from the thickest (most narrow-band) cell
of the filter. In one band (
) the filter transmits the
horizontally polarized incoming light, and in the other (
)
it transmits the vertically polarized light. The polarization of
both beams at the cell output is the same. In this manner the
required operating conditions are achieved.
The principle of operation of a spectrographic analog of such an instrument
is very much alike, but its practical implementation is quite
different from Ramsey's (Lebedev et al. 1972). Immediately
behind the spectrograph entrance slit are
the electrooptical modulator 
and the polarization deflector
D. The deflector and modulator are mutually adjusted so that one
of the component deviates along the dispersion to the right and
the other to the left. The amount of deviation of polarized
beams is set approximately equal to the half-width of the
spectral line profile. The spectrograph exit slit lies
halfway between the splitting spectral components. The intensity
of the light that has passed through the entrance slit, varies
with the modulation from 
to 
. The polarization
deflector is made of one or two calcite plates. If
the polarization of one of the beams at the deflector output is
normal to the direction of the diffraction grating groovs, then
such a deflector should be complemented with a 
-plate in order
for the polarization to be transformed to a circular one and to
avoid the dissimilar effect of the grating on the intensity of
the beams. There exists also a deflector design, in which the
polarization of beams makes 
with the dispersion
direction.
Thus, we have considered two essentially different examples illustrating the capabilities of the bichromatic image technique. In connection with the trend today toward the progression of filter magnetographs based on Fabry-Perot interferometers and resonance cells with metal vapours, it would be of interest to explore the prospects for the application of the bichromatic image technique in such instruments.
First we formulate the main requirements that are placed
by this problem upon filters used. In this case we will start
from the examples considered above by identifying their common
properties. These requirements may be summarized as three basic
ones.
- The filter is to ensure the formation of a single image
simultaneously in two spectral bands.
- Each of spectral bands
must transmit the light of a certain polarization only, because
it is in this case only that a modulation of the signal becomes
possible.
- And finally, there should be a possibility of
changing the position of the bands by making them coming closer
together or moving apart as required.