3. Which telescope is best for solar?

There are many types of telescope but which is best for solar? Before we look at these telescopes it’s worth mentioning a bit about telescope mountings. A telescope must be firmly mounted if you are going to do any real observing. The image must be steady and otherwise you will not see much detail. Astronomical telescope mountings have two basic designs:

  • The altazimuth mount has a simple up-down, left-right movement, and is useful if the instrument is used for terrestrial observation.  You will have to move the telescope up or down and left to right to follow the Sun during observation.
  • The equatorial mount has one axis which is tilted toward the north celestrial pole, and will allow the telescope to follow any celestrial object by moving one axis only. The mounts normally have slow motion drives, which allow you to turn the telescope very slowly to keep the object in view. The slow motion can be operated by hand or better still by a motor, which, if the mounting is correctly set up, will keep the telescope pointed at the object.

REFRACTOR:

The refractor telscope uses a lens at the front to focus the light from a celestial object. A small refractor of about 60mm is the ideal size of telescope for solar observing. If you have access to something a bit larger, such as a 80 to 90mm refractor so much the better as that will on most days show a bit more detail but avoid using anything larger than 100mm. Also remember the larger the scope, the heavier it will be. The best telescope is often the one that you use the most often because it is easy to set up and use.

The telescope shown here is a Vixen 80mm refractor and it is mounted on a Super Polaris equatorial mounting. Both are quite old now but they work so well that I would not even think of changing them. The telescope in the image has a full aperture solar filter fitted.

Eyepieces: If you use the projection method then stick to using the relatively cheap eyepieces such as Plössls or Kellners (they are still available most from second-hand suppliers). Be aware that over time the heat and UV from the Sun can damage eyepieces by making the cement between the lenses turn cloudy. If you use a full-aperture solar filter then you can use any eyepiece you like as the filter will protect the eyepiece as well as your eyes! Eyepieces such as Plössls, Kellners, Orthoscopic give sharp contrasty views.

telescope with a full aperture solar filter
A telescope with a full aperture solar filter fitted over the front of a telescope making it safe to view the Sun. Photo: Geoff Elston

REFLECTOR:

A reflector is a telescope that uses a specially curved mirror to focus the light entering the telescope. The light is reflected off the mirror, usually at the rear of the telescope body, sending the light back up the tube to be deflected into an eyepiece.

Do not use reflector type telescope for solar projection as the telescope can overheat. Instead, consider buying a full aperture solar filter to fit over the front of the telescope. Alternatively, you do not always need a solar filter to cover all of the telescope aperture but instead purchase an off-axis mask into which a smaller diameter solar filter can be screwed into one of the portholes in the mask. This reduces the telescope aperture and increases the telescope focal ratio.

a reflecting telescope
A reflecting telescope. Image from Wikipedia.

CATADIOPTRIC:

These telescopes use a combination of a lens (often referred to as the corrector plate or meniscus lens) and a specially shaped mirror to focus the incoming light to form an image.

These telescope can only be used for solar observing with a full aperture solar filter with an off-axis mask (as shown here). The solar filter aperture in the image is off-centre because the secondary reflector is in the centre of the meniscus lens at the front of the scope. If the solar filter aperture was also central it would be blocked by the secondary effectively stopping the scope from working.

Never use these scopes for solar projection as they will overheat causing damage to the telescope.

solar filter
An off-axis mask with a small solar filter over the telescope aperture making it safe to use a compound telescope for solar viewing, taken by Geoff Elston
Coronado PST
A Coronado PST (Personal Solar Telescope) for viewing the Sun in hydrogen-alpha light, taken by John Chapman-Smith

HYDROGEN ALPHA LIGHT AND CALCIUM LIGHT SCOPES:

These telescopes are specifically made to view the Sun in the red spectral line of hydrogen. By looking (and imaging) in this wavelength of light we can see the solar chromosphere. Here we find the solar prominences, filaments and solar flares. Observing the Sun in H-alpha is not cheap but it is highly rewarding especially during periods of high solar activity.

There are a number of manufacturers of H-alpha telescopes. DayStar, Lunt, and Coronado are well-known brands. There is a similar scope for imaging in the blue light of calcium (CaK) also available from Coronado. These are harder to use as you have to image the Sun rather than look at it visually, but they are rewarding if challenging to use. More here.

Next: What are hydrogen alpha and calcium filters?