![]() ![]() However, the use of a standard barlow to achieve f/30 in an off-axis stopped-down Newtonian Telescope may only allow part of the field of view to be in the passband at any one time. It still requires at least an f/30 light cone, but this can be easily achieved with auxiliary lenses, or by stopping down larger apertures. This, plus the lower quality quartz elements cuts the cost of the filter nearly in half. The tiltable stack also means no filter heater electrical power or warm-up time is needed. Not only does this variable tilt allow the user to compensate for temperature variations, but it also makes possible rapid tuning of the filter for viewing Doppler-shifted features. C), and deals with the temperature control problem by using an adjustable tilt in the filter stack to change the optical path length through the filter. It uses somewhat lower quality quartz, operates from 32 to 104 degrees F (0 to 40 deg. A novel variation on this design is DayStar’s less expensive T-SCANNER 0.7 Angstrom filter. They are also not rapidly tunable for viewing features in the off-centerline “wings” of H-alpha (unless equipped with a stack-tilt feature). They require both electrical power for the ovens, and fairly long f/ratios (f/30 or more) in order to work properly, as the “field angles” of the light hitting the etalon must be very shallow. These filters are quite expensive (over $3000), due to the high quality requirements of the quartz elements, and the critical temperature control. Without this control, the passband would wander away from H-alpha fairly quickly. These filters are quite temperature sensitive, and are often encased in special ovens to keep them within one degree of the required operating temperature (often, they run at over 100 degrees F). Interference antireflection coatings are placed on the front and back “windows” of the telescope objective for further filtration to protect the filter from excessive solar heating. The standard filter stack uses a narrow-band blocking filter, a Fabry-Perot etalon with solid spacer crystal, and a broadband trimming filter. These filters have very sharp passband edges, and sub-Angstrom versions allow detailed views of both limb and disk detail. With the proper plate spacing, the light undergoes interference, producing a series of very narrow resonant passbands one of which (at the H-alpha line) can be selected via a blocking filter arrangement. ![]() An etalon is a pair of plane-parallel optical surfaces which are partially reflecting and partially transmitting. One design, produced by DayStar Filters of California, is a multi-component interference filter using as its core a Fabry-Perot etalon. ![]() ![]() For viewing fine chromospheric disk detail, a more complex high quality sub-angstrom bandwidth filter is usually needed. For better prominence viewing, much more expensive Coronagraph-type viewers are available which use solar occulting disks coupled with a narrow bandpass filter and high-quality optics. It is suitable mainly for viewing limb prominences, sunspots, and very bright major flares, since the filter does not have the sharp-edged sub-Angstrom passband needed for revealing much chromospheric disk detail. One design, produced by Lumicon (2111 Research Drive, #5S, Livermore, California 94550), is a 1.5 Angstrom FWHM (Full Width at Half Maximum) Prominence filter, using multi-layer dielectric coatings on a glass surface similar to their nebular filter designs. The other method of H-alpha viewing involves a special narrow band filter. For those interested in building one, the basic details are described in the January 1969 issue of Sky and Telescope magazine. It is rather bulky, and thus is used mainly in a heliostat-fed horizontal solar telescope. The instrument has the advantage of rapid tunability not only around H-alpha, but in other spectral lines which show emission, such as the K-line of Calcium. One method of doing this involves using a spectrohelioscope, an image-scanning spectroscope using pairs of moving slits to allow monochomatic viewing of the sun. H-ALPHA FILTERING SYSTEMS: Unlike continuum “white light” observations of the sun, observing the chromosphere requires a very narrow bandwidth filter centered on the Hydrogen Alpha spectral line (6562.8 Angstroms), which not only reduces the intensity of the sunlight to a safe level, but eliminates much of the photosphere’s contribution to the image. ![]()
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