Let us briefly summarize the results of our treatment. First we must mention the advisability of employing the bichromatic image technique in experimental solar research. As shown earlier in the text, the technique has a number of merits. The main of them are: doubling of the light, reduction of the number of frames from one-fourth to half, reduction of the exposure time and, consequently, decrease in the atmospheric seeing effects. The technique is applicable in conjunction with various spectral instruments, from the spectrograph to MOF. The bichromatic method allows for scanning the spectral line profile in both conventional and filter magnetographs.
When handling diffraction spectrograms of solar telescopes, the technique makes it possible to eliminate the influence of internal noise of the spectrograph at low modulation frequencies, upon magnetic field strength measurements. The solar spectroheliograph operated in the bichromatic image mode provides spectroheliograms free from the influence of spectrograph noise and line-of-sight velocities.
Magnetooptical filters satisfy best all requirements imposed upon spectral instruments when using the bichromatic image technique. Conceptually, such filters are two-banded, with a different polarization of the bands and with a possibility for their relative displacement. The angular aperture of MOF is virtually unlimited. There is possibility simultaneously to measure the general magnetic field of the Sun with utilization the scattered light. Unfortunately, spectral lines, with which modern MOF can operate, are far short of optimum for measuring the magnetic field strength. This factor sets serious limits on the fields of application.
Lyot birefringent filters currently in use to obtain the bichromatic image, have a small angular field, a bulky design, large light losses, and a complicated control of the band position. However, because of the wide use at solar observatories and the ease to switch over to the two-band mode of operation (by eliminating the input polarizer of one of the stages), they can gain widespread acceptance in this mode. It is particularly simple and proficient to obtain a bichromatic filtergram of intensities in the spectral line wings.
Of well-known filter techniques, the method of choice seems to be FPI based on a solid-state etalon made, where possible, of a material with a controlled refractive index. These instruments are compact, durable and readily adjustable, and have high spectral resolution. In some cases, even the demerit of such a filter, namely the high sensitivity to the angle of incidence of the incoming radiation, may be turned into a merit. For instance, for producing two spectral bands or controlling their position, as has been suggested above, or for compensating the band displacement as a consequence of solar rotation on full disk filtergrams (Rust 1984). This same disadvantage dictates the need to look for ways of reconciling with the feed optical system. It is best to mount such a filter in systems with telecentric ray path. However, in this case the influence of the FPI's spatial inhomogeneouses is increased. Possibly, recent holographic filters (Rakuljic & Leyva 1993) that feature a large angular field and a narrower spectral band, will supersede FPI. In any event it would be well to look for ways of using such filters in the bichromatic image mode.
Furthermore, as may well be noted above, 
-measurements
using the bichromatic image technique are made with a push-pull
modulation. Recent reports on the use of CCD as a push-pull
detector of optical signals (Povel et al. 1990, 1994) will
improve significantly the signal/noise ratio. In conjunction with other
advantages of the technique described here, this may well lead to
a significant improvement of the sensitivity of filter magnetographs.
Acknowledgements
We are grateful to Mr. V.G. Mikhalkovsky for his assistance in preparing the English version of the manuscript.