well as the skin ), likewise IR-A radiation can cause thermal injury to the eye . Normal visible light , if bright enough , can cause partial loss of sensitivity and temporary blindness . Damage to the eye is more likely to occur due to exposure to UV-A , and bright visual light , rather than IR . There is a human “ self-defense ” reaction which generally makes involuntary eye movement when the eye is exposed to extremely bright light ( eye movement , squinting , closing the eye ) which reduces the effect of the energy , and gives some protection ( Figure 1 ).
Solar filters The safest way to observe the sun is the projection method . A refractor or ( Newton ) reflector is adequate for solar projection . Do not use compound ( catadioptric ) telescopes ( e . g . Schmidt-Cassegrain , Maksutov ) . The Sun can only be observed visually when specially designed filters are used . The majority of these filters use a thin layer of chromium alloy or aluminium deposited in their surfaces . A solar filter should transmit less than 0.003 % of visible light and no more than 0.5 % of nearinfrared radiation ( Figure 2 ). Special solar glasses ( Eclipse glasses ) can be used when a large sunspot appear on the solar disk . Welder ’ s glass (# 14 ) is also suitable for “ naked-eye ” observation of sunspots . Unsafe filters include exposed and developed colour film , exposed and developed black & white film that contain no silver , film negatives , smoked glass , sunglasses ( single or multiple pairs ), photographic density filters and polarizing filters , CDs and aluminized food wrappers .
Solar eyepiece filters are also unsafe .
Solar telescopes
White Light : Most telescopes can be adapted for white light solar observing and imaging . Unlike a night-time scope , an instrument for solar observing is not expected to gather a lot of light . When observing the Sun , most of the effort is spent in reducing the amount of light using objective filters ( Figure 2 ) or Solar Herschel Wedges ( Figure 3 ). Solar telescopes are usually 150 mm or less in aperture . A 125 mm aperture telescope has a theoretical resolution of 1 arc second . Smaller telescopes ( 50 to 100 mm aperture ) are suitable for full disk observation and imaging while
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Figure 3- Solar Herschel Wedges ( APM 1 1 / 4 ”, 2 ” LUNT , 2 ” Baader ceramic ).
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Figure 2 – Solar objective filters : 1- Mylar ( Solar screen ); 2- Thousand Oaks ( glass ); 3 & 4- Baader Astrosolar .
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Figure 4- End-loading and front-loading narrow band solar filter telescope configurations .
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