By Birger Andresen, Trondheim Astronomiske Forening.
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| Solar filters from Orion. Photo: Celesta Optikk |
Using a solar filter is necessary.
It’s only during the total phase of a total solar eclipse you can look at the Sun directly without filter.
The solar filter must have been made for observation of the Sun so that not only visible light, but also a sufficient amount of heat radiation and ultraviolet light will be blocked. Do not use tinted glass, overexposed film, several layers of neutral filters or two polarization filters in a cross.
There are two kinds of solar filters available; those which are mounted in front of the main lens (eyepiece filter), and those which are mounted onto the ocular.
NEVER use solar filters which are mounted onto the ocular.
Whatever the producer says, the heat will become strong enough to cause cracks in filters mounted onto the ocular. This may lead to injuries to the eye, causing blindness or other permanent damage. These filters should be thrown away!!
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Use of solar filters to safely observe the Sun. Click on the photo for full resolution Illustrator
EPS (CMYK)
Illustration: Trond Erik Hillestad
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Amongst the filters which are mounted in front of the lens/telescope there are two kinds; those which transmit light throughout the whole cross-section of the main lens (full-aperture filter) and those which only transmit through a part of the cross-section (off-axis filter). For photographing partial eclipses which of the filters you use is unimportant. Solar filters can be made out of metal-coated glass or aluminized Mylar or Baader.
It’s extremely important that the solar filter is safely mounted onto the lens/telescope to avoid it falling off due to strong wind or similar disturbances.
You also need a solar filter on finder scopes if in use, both to protect your eyes and to avoid certain kinds of finder cross-hairs to burn. Make sure the lens cover is on the finder scope if you do not have a solar filter for it. If so, you can use the solar shadow of the finder scope to adjust the telescope.
