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4, What are Hydrogen Alpha and Calcium Filters?

This was a highly specialised area several years ago but with the introduction of small portable solar scopes this type of solar observing has now become very popular. With these types of filters we are viewing the Sun using a specific light by using a "interference" filter. This type of filter blocks all other wavelengths of light passing only the a tiny part of the solar spectrum. We can see the Sun in Hydrogen-A (red light) or Calcium-K (blue light). Filters that work in this way are often referred to as "narrowband" filters.

Remember that to get a complete picture of solar activity it is worth watching the Sun in white light using either projection of a full aperture solar filter so that you can see the sunspots clearly.

Hydrogen alpha (H-a or H-alpha) is in the red end of the visible solar spectrum (at 656.3nm). By using a specially-made combination of small telescope and interference filter we are able to see (and image) solar prominences, filaments, plages and occasionally flares on the Sun that otherwise would remain invisible. 

Calcium-K (or CaK) telescopes allow you to image the Sun in the blue light of Calcium (393.4nm) also by using a specially-made combination of small telescope and interference filter. Because the image is so near the UV region of the solar spectrum some people cannot see the image clearly but it can be imaged with a camera.


A Coronado PST (Personal Solar Telescope) for viewing the Sun in Hydrogen-alpha light, taken by John Chapman-Smith A Solarscope for viewing the Sun in Hydrogen-alpha light, taken by John Chapman-Smith

The Hydrogen-alpha filter (and scope) like those shown above will show you:

Prominences: These are clouds of  luminous hot hydrogen gas seen projecting off of the edge (or limb) of the Sun. Prominences are bright because they are seen in emission against a dark sky background. As we are looking at the Sun through an interference filter that allowing us to see features that are emitting nearly all their light at the wavelength of 6563 Angstroms the prominences appear red.

Prominences come in two main types: quiescent (quiet) or eruptive. Prominences can last days or appear and disappear in hours. You will often see a number of descriptions such as: "hedgerow-type" prominence, or "smoke-stack" prominence, "mound" or "spike" prominence. These are widely-used descriptive terms used by observers to convey the general shape of a prominence with reference to terrestrial objects. 

Image of "Witches Broom" prominence in H-alpha, taken by Mick Jenkins

Filaments: These are ribbon-like features see against the solar disk. They are the same as prominences but are seen against the bright solar disk so they appear dark by contrast. Sometimes at the solar limb we can observe a prominence against the sky and a filament on the disk if that feature is large enough to stretch from the limb and onto the Sun's disk.

Plages: Also seen in the image to the right are plages. These are the bright areas visible around sunspots while observing in H-alpha light. 

Image of solar filaments in H-alpha taken by Mark Beveridge

Flares: These are bright, occasionally very bright, points of light or ribbons of bright light usually seen near sunspots on the solar disk.

Flares usually last for about 10-20 minutes depending on the flare strength. The strength of solar flares are usually reported as: A-B-class, C-class, M-class and X-class. A-B-class are not reported as they are very common and the weakest type of solar flare. C-class are slightly more powerful, M-class are stronger and X-class are the strongest. Often these classes are sub-divided by using a numbering system from 1 to 9 (so we might see the term: "M7-class solar flare" for example. The exception is X-class where the numbering can go beyond 9. 

Image of a solar flare (and a small ejecting filament) in H-alpha, taken by Richard Bailey


We also now have Calcium light filters (often referred to as "CaK") but they can only really be used with an imaging camera as our eyes are not good at seeing light at the deep blue-end of the solar spectrum.

Image of the Sun in Calcium light (CaK) taken on 2014 Feb 4 by Peter Paice This image, taken in the blue light of calcium shows the region immediately above the solar Photosphere (the lower Chrosmosphere). The very bright areas seen here in the image are closely associated with the sunspots (just visible in the picture).

Should you need advice on choosing and using these filters please email me.