A. Cacciani and P.F. Moretti
Physics Department, University ``LA SAPIENZA,'' Piazzale Aldo Moro 2,
00185 ROMA-ITALIA.
The MOF developed first by A. Cacciani (1966,1967) is discussed in several papers by different authors (see, e.g., Cacciani 1994, for general references). It displays four important characteristics: 1) high transmission, 2) high spectral resolution, 3) large field of view, 4) absolute spectral reference and stability. The last characteristic is a consequence of the fact that it is intrinsically based on the properties of atomic absorbing vapours that, on the other end, limit the working wavelengths to few atomic resonance lines. However, a limited tunability is allowed around the working wavelengths and two narrow transmission bands (25 mÅ each) are switchable on different parts of a spectral line.
Let us describe the working principle avoiding mathematical formulæ using instead simple physical considerations. The interested reader can find a complete theoretical analysis, e.g., in Cacciani et al. (1994).
Let us consider two crossed polarizers. They transmit no light until
a third polarizer is interposed at arbitrary angle between them. If
the axis of this third polarizer is parallel to
the others, we get transmission because the initial polarization is
changed during its way. Similarly we get transmission any time we
insert a polarizing agent (e.g., birefringent optics) between two
crossed polarizers. In order to obtain a double band monochromatic
transmission, the MOF takes advantage of the Zeeman effect and
related phenomena (Righi and Macaluso-Corbino effects). The Righi
effect is associated with the inverse Zeeman effect, i.e., the
residual light after absorption by a gas in a longitudinal magnetic
field is left circularly polarized so that it can go through the
crossed linear polarizer. The Macaluso-Corbino effect is analogous to
the Faraday effect but is limited to the neighborhood of the spectral
line since it is associated with the Zeeman splitting of the
refraction index variation across the spectral line itself. The final
result is a double band spectral transmission. The possibility of
selecting one or the other of the two bands is obtained using a double
system in series as illustrated in Figure 1. Finally, it is an important
characteristic of the MOF: its ability to modify the separation between
the two transmission bands (from 50 to 200 mA) without changing the
magnetic field needed to get the Zeeman effect. The overall system
results in a compact device (10
10
40 cm and 5 kg) very attractive for
space applications (see Fig.1). Figure 2 and Figure 3 illustrate its
applications.
Figure: The system is composed of two parts: the MOF itself and the
Wing Selector WS. Both are made of absorption cells containing
metallic vapors (e.g., sodium) in a magnetic field. With a suitable
choice of magnetic fields and temperatures we get the spectral
behavior shown in the bottom half of the figure: the MOF acts as an
optical filter providing two very narrow passbands selecting the two
wings of the incoming solar sodium profile, while the WS absorbs one
wing or the other depending on the retardance of the Fast Modulator
interposed between the two cells. The acquisition system is a CCD
camera and a Computer able to perform the difference between the two
filtered images. The result is the Doppler image shown in Figure
2.
Figure: MOF APPLICATION TO HELIOSEISMOLOGY: here a solar doppler
image and a portion of disk integrated (summation of all the pixel
values) doppler signal versus time is shown. The time scale is such
that 5 min=100 in the abscissa so that the solar Five Minute
Oscillations are well visible. Note that the integrated signal is
about 1m/s rms and that the noise level is far below this
value.
Figure: The Jovian spectrum is obtained summing together the solar
shifted spectra from all the reflecting points of the surface of the
rotating planet. Any additional velocity field, other than rotation,
perturbs the Jovian line profiles as shown in this figure. The MOF
double band transmission detects this variation as a differential
signal on the image between the Blue and the Red band. Using this
technique a Doppler signal was detected during the comet
Shoemaker-Levy 9 impact on Jupiter (Cacciani, Moretti et
al. 1995).
The MOF is being used extensively in physics; use has been done also in and astro-seismology; finally, the MOF is being considered for in free space and diffusing media.
Solar Seismology has become a very important field of research with several ongoing projects, both from the space and from the ground (see, e.g., the review paper of J. Harvey 1995). In Figure 2, we just give a sample of data quality achievable with the MOF.
After the highly satisfactory results obtained on the study of the interior of the sun, the scientific community is trying to use the same technique to provide information about the interior of other stars and gaseous planets (like Jupiter and Saturn). One of the first approaches was to use an MOF thanks to its performances as a very compact, stable and luminous device. References are given in Cacciani & Moretti (1994). So far, however, for the sake of simplicity, only a simple MOF has been used. In order to overcome the difficulty associated with the Keplerian motion doppler shifts, we present in Figure 3 the use of a double MOF for Jupiter seismology.
The narrowness of the transmission band provided by the MOF is a strong advantage when trying to link remote sites through luminous signals, as for example between two space satellites. This technology is being developed by a few companies in the world. The one that looks the most promising is also able to build inexpensive hardware for the MOF. The name of the company is EDDY Co., 13590 Niabi Road, Apple Valley, California 92308 fax (619)961-8458, voice (619)961-8457.
This work is supported by SPAZIALE ITALIANA, MINISTERO della RICERCA SCIENTIFICA, PROGETTO NAZIONALE ANTARTIDE
Cacciani, A. 1967, ``Strumentazione dell'Osservatorio Astronomico di Mt.Mario per la misura dei campi deboli del sole'' Atti del Convegno della Soc. Astr. Italiana, Padova/Asiago Sett. '97, p. 106
Cacciani, A., Rosati, P., Ricci, D., Egidi, A., Marquedant, R., & Smith, E.J. 1994, ``Theoretical and Experimental Study of the Magneto-Optical Filter'' Jet Propulsion Lab Document D. No 11900, Pasadena, CA
Cacciani, A. & Moretti, P.F. 1994, ``Magneto-Optical Filter (MOF): Concept and Applications in Astronomy'' SPIE, 2198, 219
Cacciani, A., Moretti, P.F., Dolci, M., Brocato, E., & Smith, E.J., 1995, ``Doppler Observations of the Impact of Comet SL9, Fragment A'' European SL9/Jupiter ESO Workshop, Garching February 1995, eds. West & Bohnhardt, pp. 181--190
Cacciani, A., Moretti, P.F., Dolci, M., Brocato, E., Smith, E.J., 1995, ``Doppler Observations of the Impact of Comet P/Shoemaker-Levy/9, Fragmaent A'' Geophysical Research Letters, 22, 2437
Harvey, J. 1995 ``Helioseismology'' Physics Today, October issue, pp. 32--38