Okayama Observatory: Photoelectric Vector Magnetograph
The vector magnetograph at Okayama Observatory consists of polarization modulation optics and analysis electronics. Instrumental polarization produced at the two oblique mirrors is compensated for by plane parallel glass plates and Babinet plates. The beam of light then passes through a rotating waveplate and is modulated according to its polarization state. The beam is split into two by a Wollaston prism. If the rotation frequency of the waveplate is ω the circularly and linearly polarized components are modulated with frequencies of 2w and 4w, respectively. The two beams are then led to the spectrograph. The intensities of the wing of Fe I 5250 Å line and its neighboring continuum are measured with photomultipliers, and frequency-analyzed giving amplitudes of 2ω and 4ω components. The wing intensities are balanced by tilting a glass block in front of the photomultipliers. Conversely, the tilt angle of the glass block gives the Doppler shift of the spectral line.
The magnetograph started its operation in 1982 December. Although it takes 30-60 minutes of instrument adjustment, the observation itself is automatic. Usually an active region is scanned in 6 or 10 arcsecond steps. One scan of 50X45 or 70X65 data points requires 60-90 minutes to complete. Potential field lines can be computed and overlaid onto the observed magnetogram. The data were recorded onto magnetic tapes, and were published every year (1982-1995). There were about 200-300 observations per year.
Closing down of the magnetograph
The Solar Flare Telescope at Mitaka built in 1989 can observe simultaneously white light images, H-alpha images, magnetic fields, and velocity fields. Observations of magnetic and velocity fields are made with narrow band filters and CCD cameras, single map being obtained within one minute. A cross calibration of the Solar Flare Telescope with the Okayama magnetograph was carried out from 1992, and we confirmed the reliability of the data from the Solar Flare Telescope. Therefore, we stopped the routine operation of the Okayama magnetograph at the end of 1995. The control computer for the magnetograph (Melcom 70/60B) which had been used for 16 years was abolished in 1998 May. The optics and analog electronics of the magnetograph are stored for exhibition in the basement of the Solar Tower Telescope at Mitaka.
