Observation of sun

Looking back I still remember a fragmental solar eclipse, I watched at the age of four.
Today I know, in 1954 at 30th of June, the moon passed the sun at 14:00 o'clock. Observing in Germany, the sun was covered by the moon up to 93%.
In spite of such a coverage, you automatically close your eyes tightly, because an enormous luminous intensity is emitted by the sun.
Using a candle, my friends coated dark glasses with soot, in order to observe the solar eclipse. Sun observation is not possible with unprotected eyes and the soot-covered glasses are naturally no suited eye protection too. We need optical aids, if we want to increase knowledge about Sun.

What kind of aids do you need to observe the sun?
You should Visit an optician to procure a special eye protection, e.g. glasses made of foils, to observe a solar eclipse safely. If you want to look at details such as sunspots or even granules (convection cells of the sun matter), you need a telescope.
There are two types of telescope: The refractor built by lenses and the reflector, using mirrors. Generally the refractor is better suitable for the sun observation.
  • Middle enlargements between 50 and 100 as well as focal lengths around 1 meter are needed for sun observation. This is reached easily by refractors.
  • The large light collecting ability of the reflector is not needed because of the high radiating power of the sun. A large lens aperture is even obstructive. Since more distorted wave fronts are caught; caused by turbulence of air. In this case the discrimination of details at the sun surface is not reached.
  • When you assume a surface defect at the optically active surface of the observation instrument, so the error distance is passed 2-times by the light beamin case of light reflection . This results in an error that is 6-times higher, using a reflector in contrast to a refractor (supposing a refractive index ofn = 1.5).
  • A telescope, built by lenses does not need a central mirror and reaches thereby a larger contrast.
  • Since the solar spectrum has its maximum at the infrared range, the absorption by optical elements may not be neglected. Colour filters in the proximity of focus can warm up strongly. You will get density variations and air turbulences in your tube. These filters usually have a shape of flat-parallel plates. The lateral misalignment of ray-path in that elements leads to ghost images, which reduce the image contrast.
  • The cement, which is used to fasten the eyepiece lenses, can become soft by heating up in case of high absorption and the lenses may misalign. Here the 'Huygens eyepieces' work better, because they consist only in 2 lenses and their positions are locked by rings.

What can you observe at the sun?
You can differentiate 3 large ranges of sun observation:
  1. The photosphere
  2. The chromosphaere
  3. The corona

I will limit here to the photosphere. For the two other ranges you need either special narrow-band filters or an observation point that has to be in between the shadow of a solar eclipse. A total solar eclipse arises occasionally, but however takes place at most different places of the earth. Often far journeys are necessary.
To observe the photosphere of sun there are in principle two possibilities: