EYE SAFETY WHEN POINTING AT THE SUN
Note: Watching astronomical events in which the Sun is involved or objects close to the Sun requires special safety !
NEVER look through an optical device like a telescope or binoculars at the Sun or your eyes will be damaged resulting in PERMANENT BLINDNESS and no opthamologist can help you !!
When pointing the telescope at the Sun, e.g. for alignment reasons, use a safe objective filter, e.g. Baader foil
or a glass filter, e.g. Thousand Oaks. Only close to sunset and sunrise and a very hazy sky the Sun can safely be viewed unfiltered.
When no filter available you can align by watching that the bright spot is centered in the eyepiece without looking through the eyepiece !!!
See the picture above left. In this case, to prevent overheating eyepieces or star diagonals when pointing to an unprotected Sun during alignment, mask off the objective lens with a cardboard shield with a hole of at most 5cm in it. The picture above center shows a cardboard blind of 4cm on the Televue Genesis. The black strip is a piece of duct tape for removing the filter easily. A full aperture of a 10cm pointed at the Sun will dissipate 8 Watts of heat in the eyepiece and a 20cm 30 watts ! It is the same as pointing a burning glass of the same diameter to the eyepiece which you would never do !
With Newtonians, SCTs, Maksutovs or Dobsonians
, make the hole eccentrical, otherwise the hole is in front of the secondary mirror and no light at all passes into the telescope, see image above right. When putting the cardboard shield on front of a Newtonian tube, be careful no dust or small pieces fall into the tube and do not exert force to the spider legs of the secondary mirror. Take care that the shield does not fall off when you point the scope at the Sun.
When you put (the objective / primary mirror of) the telescope in the shade of a well-foliated tree, the sunlight is even more attenuated (but looking to the eyepiece is still unsafe!).