Alpha Viewing

In recent years there has been an upsurge in amateur astronomers who are imaging the Sun at H-Alpha wavelengths, i.e., at 656.28 nanometers. At this wavelength, the limb prominences can be seen, even while the blindingly brilliant Sun is in the field. Normally it takes a total solar eclipse to reveal these features. It should perhaps be explained here that the problem is not simply one of standard filtering.

The bandwidth of a good H-Alpha filter is, typically, less than one Angstrom (0.1 nanometers) and the filter production is highly complex and expensive. Decades ago Edwin Hirsch of the Daystar Company was the sole supplier of such narrow-band filters, but recently the Tucson-based company Coronado has been at the forefront of this technology and have developed a number of exciting products using advanced laser techniques. On the U.K.'s Isle of Man, Solarscope also offer quality H-Alpha filters of 50-mm aperture.

Both of these modern companies produce precisely tuned, ultranarrow line-width classical Fabry-Perot air-spaced "etalons" for their H-Alpha filters. An etalon consists of a matched pair of ultrafine pitch polished and accurately figured fused silica plates. These have partially reflective and low absorption coatings for the desired transmission wavelength. To guarantee the essential fixed air space, the two etalon plates are skillfully assembled with the use of optically contacted spacers. Such filters have a very high throughput at peak resonance and a very narrow spectral transmission.

As one narrows the filter bandwidth centred on the 656.28-nanometer H-Alpha line the prominences become more and more sharp, and fine H-Alpha features on the disc emerge too. In the 1980s, the Baader Company advertised prominence telescopes in which a metal disc could be used with a telescope of a specific focal length to exactly occult the blinding solar disc. Using this method, even a 10 Angstrom H-Alpha filter would show the prominences, while the dazzling solar surface was hidden behind the metal disc. However, by moving to expensive, narrower bandwidth filters the prominences and subtle surface chromosphere features can be viewed simultaneously. Coronado makes filters and small quality refractors optimized for use with such filters. The 2005 Coronado range consists of H-Alpha telescopes from 40-mm to 90-mm aperture (ranging from $1,700 to $12,000 dollars in price) as well as individual filters priced from $900. These units typically have bandpasses less than 0.7 Angstroms. By stacking two matched H-Alpha filters together, a bandpass finer than 0.5 Angstroms can result! Recently, Coronado's wider, 1 Angstrom bandpass, 40-mm aperture f/10 PST (Personal Solar Telescope) has made H-Alpha imaging affordable to many and, coupled with a webcam, spectacular pictures of prominences can now be obtained for an outlay of under $500! The PST is mainly just a prominence telescope, and will show few fine details on the solar disc, but it is a remarkable price breakthrough. It may be thought that 40 mm is a very small aperture, but it is sufficient to resolve prominences only a few arc-seconds in width and perfectly compatible with typical daytime seeing. Like nearly all H-Alpha systems a filter tuning collar is provided to optimize the view and, in use it gives a sort of "3D" effect as tweaking it can enhance major disc detail or limb prominences.

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