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. A more recent development is the “DayStar Quark” which can be used with a normal telescope with some safety precautions. More of this below.

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.

HYDROGEN ALPHA:

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

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.

Prominence
A typical prominence

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.

Filaments: These are ribbon-like features seen 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 below are plages. These are the bright areas visible around sunspots while observing in H-alpha light.

filaments and plages
Dark filaments and bright plages

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.

Flare
The bright area is a solar flare

DayStar Combo Quark (provided by Section member, Carl Bowron)

DayStar Quark
DayStar Quark Combo with Barlow, tilting unit and imaging camera. Image by Carl Bowron

There are two ways to observe the Sun at Hydrogen-Alpha frequencies, either using a designated solar telescope, or using a special solar filter attached to a “normal” telescope.

DayStar have developed a filter which can be attached to any telescope, with a few safety precautions. A designated solar telescope can only be used for solar work but any astronomical telescope with the DayStar filter can be used for both day (solar) and night (stars and planets) observations. The ideal telescope to use with the Quark stsyem is a refractor. Why is this? A reflector could be use but it would require special energy rejection filter attached to the front of the telescope and this could be quite expensive. A refractor, on the otherhand, up to a maximum of 150mm aperture requires only a normal sized energy rejection filter just in front of the Quark saving on cost.

DayStar produce two types of Quark filters, the standard Quark which has a built in 4x Barlow lens, and the Combo Quark which has no Barlow. The first gives a fixed amplification which can produce quite a restricted field of view, while the second does not.

DayStar produce two versions of Quark which work at different bandwidths. There first, a chromosphere version with a narrow bandwidth, and a second, a prominence version with a wider bandwidth. The former is designed for greater resolution of surface features, however, it will also show prominence features at increased camera gain settings.

The Combo Quark requires an optical system of focal ratio F15 (focal ratio = focal length/aperture) or greater to work effectively. If, for example, you have an 90mm F10 refractor with an aperture stop of 60mm it will give a F15 system with quite a large field of view. A normal Barlow can be attached to the front of the Quark to boost the amplification. The 60mm/F15 example with the addition of a 1.5x Barlow will transform this to a F36 system. If you now remove the aperture stop to regain the full aperture you now have a 90mm/F24 system. A 2x Barlow with the 90mm aperture results in a F37 system. The focal ratio can be adjusted to the size and resolution required for the solar feature to be observed. The increase in F number also improves the contrast in the observed features. For a fixed cost of the Quark and the appropriate energy rejection filter you can have a modestly priced 90mm aperture solar telescope.

For imaging the solar features a monochrome digital camera is needed with, possibly, a tilting attachment. Working at this specific H-alpha frequency can produce annoying interference fringes (called “Newton Rings”) at the image plane which can be removed by slightly tilting the camera, hence the tilting attachment. Tilting adapters can be purchased for most astronomical cameras for a few pounds.

Armed with a DayStar Combo Quark chromosphere filter, an energy rejection filter, a monochrome digital camera and a tilting attachment, you are now set-up to take on solar imaging in all its glory. For large expanses of the solar disk you can start off with a F15 arrangement and then for more detailed imaging the focal ratio can be adjusted by the use of various Barlow lenses.

Solar imaging is usually conducted in relatively unstable air so precise focusing can take quite a while to achieve. Take time over this process as it is important. Unlike the eye, which can rapidly adapt to subtle changes in focal length, the camera plane is fixed so the focal plane will move back and forth in front and behind the imaging chip. Take as many frames as possible, aim for at least 60 frames per second over a one minute run and be prepared to reject up to 95% of these when stacking to form the final composite image.

For imaging prominences the same technique applies but this time the camera gain will need to be increased substantially to show these fainter features around the solar rim. The rest of the solar disk will be completely white. It is possible with the brighter prominences to show these as well as surface features but only at the larger focal ratios where the contrast is better.

Having acquired the images they will need to be processed through RegiStax or a combination of Autosakkert!2 and RegiStax to produce final monochrome images. Be prepared to discard the majority of the images at the stacking phase of these programs. Finally, the resulting image can then be suitably colour enhanced using most imaging software packages.

CALCIUM:

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.

calcium image
Image made in calcium light

This image, taken in the blue light of calcium shows the region immediately above the solar photosphere (the lower chromosphere). 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 using the contact form.

Next: Making a solar observation