Another view of the Sun

Your white-light photographs of the Sun are in the can. Now, wouldn’t prominences be exciting? You’ll need special filters for this.

Besides observing and photographing the Sun in white light, photographing the Sun in Hα light and in calcium light is an exciting challenge for many amateur astronomers. In this way, the structures of sunspot regions on the Sun’s surface or prominences and filaments can be made visible.

In Hα light, you can see the approximately 2,000km thick layer of the chromosphere, which is located above the photosphere visible in the white light of classical solar photography. Observation is much more difficult, since its luminosity is only one millionth of the luminosity of the photosphere. In the chromosphere, phenomena such as prominences and filaments are visible. But you can also see chromospheric regions of faculae and solar flares.

A Hα filter is required for observing. It is used to allow only certain wavelengths to pass through: the light with a wavelength of 656.28nm, which is produced when an atom’s hydrogen electron falls from the third to the second energy level. The following simplified rule applies for the effectiveness of such a filter,: the smaller the full width at half maximum, the more details are visible, but also the less light is available. Filters with a full width at half maximum of more than 1.0Å (0.1nm) show only prominences at the Sun's edge, while filters between 0.8Å and 0.7Å also show filaments and regions of faculae on the surface of the Sun. Filters with a full width at half maximum below 0.6Å also show more detailed and high-contrast images of the Sun's surface.

Just as the view of the Sun in the narrow band of Hα light differs significantly from the view in white light, so too does the view in calcium light (Ca-K light at 393.4nm) differ from the that in Hα and white light. One difficulty in visually observing the Sun in calcium light is that the human eye becomes less sensitive with age in this spectrum. However, CCD chips are usually sufficiently sensitive, so that a photographic documentation of the Sun viewed in Ca-K light is definitely possible.

The imaging procedure for photographing the Sun Hα and Ca-K photography is no different to that of white light. Even in Hα and Ca-K light, it is recommended to capture several hundred (or thousand) images of the Sun as a film with a CCD camera and then use this film to create an optimised composite image using digital image processing. Even in the case of narrow-band Hα and Ca-K photography, there is still sufficient light available, so that the respective exposure time of the individual images is only a fraction of a second, and even long film sequences can be created within a few minutes.

The most striking subjects for Hα solar photography are prominences: huge explosions of hydrogen gas that are closely connected to sunspots. With less expensive instruments, these impressive structures are best observed at the edge of the solar disk. The dramatic spectacle of their change in form and intensity can be observed over just a few minutes and can be photographically documented over several hours. In addition to prominences at the Sun's edge, narrow-band Hα telescopes can also be used to photograph dark filaments, i.e. prominences beyond those visible at the Sun's edge. In addition, plages, also known as Hα faculae, appear as bright areas around sunspots.

Photographing the Sun in Ca-K light reveals structures that are clearly different to those visible in white light and Hα light (even if, as with the Hα line, it only reveals a certain area of the chromosphere). Very high contrast images of solar faculae and sunspots especially can be photographed in the calcium line.

Author: Ullrich Dittler / Licence: Oculum Verlag GmbH