We are now offering 5Å Ca II H line visual series filters for photographic, visual and academic purposes. As the K line was selected for purely academic purposes, the benefits of using the H line for amateur observers are significant.
Traditionally, university research has employed the Calcium K line for studies of Calcium on the Sun. These studies have been instrumental in determining depth of the solar atmosphere. Calcium offers TWO strong absorption lines at 393.3nm and at 396.9nm, known as the Calcium K and H lines. Researchers have previously avoided the H line for academic research, as it is very close to the Hydrogen Epsilon line. For purposes of clarity in isolating Calcium, and because imaging sensors offer similar sensitivity to the K as the H lines of Calcium, the K has to date been the target of choice in Calcium filters. However, as the visual spectrum ends at approximately 400nm, the further we venture below this wavelength, the more difficult it is for observers to visually see the image. The H line for visual observations, however, is much closer to the visual spectrum and much higher contrast in high contrast violet color. Some observatories have been imaging in Ca II H line, such as the Dutch Open Telescope with outstanding results.
K-line filters do not require F/30 configurations. They may be used at F/15 or F/20. They also do not use the same RED or Yellow glass Energy Rejection Prefilter. Rather, we recommend dielectric "hot mirror" UV/IR cut filters introduced prior to focus. Additionally, we suggest an additional neutral density solar film on apertures above 150mm or for long-term dedicated applications.