As a Japanese National space mission with international collaboration, Solar-B (2005 launch) will carry a spectro- polarimeter (SP) to be operated in visible light to obtain the first high angular resolution, precision measurements of solar vector magnetic fields from space. The SP is part of the Focal Plane Package (FPP) fed by a diffraction-limited 50-cm optical telescope. The SP will be operated exclusively at the photospheric 630 nm Fe I lines. It features a rotating, low-order crystalline quartz retarder for polarization modulation and a reflecting Littrow spectrograph design that is shortened by using diffraction from the 12micrometers wide slit to fill the grating. Polarization analysis is accomplished by a modified Savart plate beam splitter. A custom CCD detector with two active areas, one for each beam from the beam splitter, allows continuous high duty-cycle sampling of polarization. The spectrograph slit will sample a 0.16 x 164 arcsec2 rectangle of the solar image, which may be scanned across the slit by up to +/- 160 arcsec in order to build up vector magnetic field maps of the solar photosphere. Along with simultaneous, co-spatial imaging and polarimetry with the filter imagers of the FPP, the SP will provide a precise view of active and quiet solar magnetic fields that control the structure, dynamics, and energetics of the upper solar atmosphere.

SOHO offers an unprecedented opportunity to probe the interior of a star with the techniques of helioseismology. Continuous distortion-free observations of the Sun will provide unique measurements of its internal structure and dynamics. The MDI instrument provides the capability to obtain observations with very high spatial resolution and high precision at all times, with statistical errors well below the expected solar noise background at all frequencies. The real limitation will be gaining a sufficient understanding of the systematic error sources with characteristic periods longer than several hours.

The Michelson Doppler Imager is an instrument for doing helioseismology from Lagrangian point between the Earth and Sun. It is a full sun imager that includes 12 cm refractor, a high speed image stabilization system, a narrowband filter system (Ni 6768 angstroms), and a 1024 × 1024 CCD detector. One hundred milliangstrom spectral analysis is provided by a combination of dielectric filters, a fixed Lyot filter, and a pair of tunable solid Michelson interferometers.

Narrowband birefringent filters have traditionally been used by solar physicists to simultaneously isolate narrow spectral bands over wide spatial regions of the sun. Recent development of optical communication and detection systems, however, has stimulated broader interest in birefringent filters because of their unique filtering abilities. Tenability and large fields of view make these filters attractive for imaging and non-imaging applications. The purpose of this paper is to illustrate the capabilities of birefringent filter concepts by presenting examples of actual working units which have been built at the Lockheed Solar Observatory. An adequate theoretical background is available in the literaturel'2'3 but practical considerations are generally neglected. As a result, there is little appreciation for the capabilities and limitations of current birefringent filter technology. The following discussion will concentrate on the practical aspects of the filters and will also consider the restrictions imposed by material limitations. Although this presentation is mainly concerned with the practical capabilities of the filters, brief discussions of the theoretical foundations are included for convenience. Specifically, short descriptions of the basic polarization interference principle, the wide field element construction and the tuning technique are included.

A waveplate is any optical device which retards one polarization component of an incident wave relative to the orthogonal component. This subject is much too general for our purposes so we will restrict the discussion to retardations of one quarter or one half of a wavelength. In particular, we will be concerned with phase retarders which are constant or nearly so over a significant spectral region.

This article reviews the types and capabilities of birefringent filters. The general operating principles of Lyot (perfect polarizers), partial polarizing, and Solc (no internal polarizers) filters are introduced. Appropriate techniques for tuning each filter type are presented. Field of view of birefringent filters is discussed and is compared to Fabry-Perot and interference filters. The trans-mission and throughput advantages of birefringent filters are shown. Finally, the current state of the art in practical filters is reviewed.

This paper describes one phase of an ongoing development program in tunable optical filter techniques. Like previous efforts in this program, the goal of this phase is to find practical implementations of sophisticated concepts in birefringent filters. In particular, in this phase we are concerned with a practical implementation of Solc filters. The implementation which is described below incorporates theoretical developments reported previously in a mechanical design suited for laboratory development. In Section 1, a brief introduction to Solc lossless filters is presented. A particular type of lossless filter, the generalized split element, is introduced in Section 2. Its advantages and limitations are noted. The application of waveplates and their engineering implication is described in Section 3. Finally, in Section 4, the actual mechanical and optical components are shown. Attention is given to the relationship between the spectral objectives and the engineering implementation.