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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).


DayStar Combo Quark - 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.  DayStar have developed a filter, the DayStar Quark, which can be attached to any telescope, with a few 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 an night observations.  The ideal instrument would be a refractor.  A reflector would require special energy rejection filter attached to the front of the telescope which could be quite expensive.  A refractor up to 150mm aperture only requires a normal sized energy rejection filter, just prior to the Quark attachment, costing a few tens of pounds.
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 former gives a fixed amplification which can produce quite a restricted view. 
The Combo Quark requires an optical system of focal ratio F15 (focal ratio = focal length/aperture) or greater to work effectively.  If 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. 
For the cost of the Quark and the appropriate energy rejection filter you can have a modestly-priced 90mm aperture solar telescope. 
DayStar produce two types of Quark, which work at different bandwidths.  There are two versions, a chromosphere version at a narrow bandwidth and a prominence version at 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.
For imaging the solar features a monochrome digital camera will be required along with, possibly, a tilting attachment.  Working at this specific H-alpha frequency can produce annoying interference fringes (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 disc 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 accurate focusing can take quite a while.  Take time over this process.  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 frames when stacking to form the final 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 limb.  The rest of the solar disc 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.   The stacking graphs will help with the number of images suitable for this final part of the process.  These can then be suitably colour enhanced using most imaging software packages.
The image here gives an indication of what can be achieved.