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Message: <blockquote data-quote="Binastro" data-source="post: 3170782" data-attributes="member: 111403">. Hi dantheman,. Really old telescopes, such as 60 mm, 3 inch and 80 mm long focus refractors from the 1950s and 1960s were usually made of brass or aluminium. These will not be affected by the Sun. Later telescopes may have plastic parts, perhaps in the focusing unit, which could melt.Two element uncemented eyepieces are best. You can also find these with microscopes, and they cost very little. However, the standard fitting for microscopes is about 23.2 mm, rather than the 24.5 mm for older telescopes. My telescope had RAS threads, which are basically 1.25 inch fit, but threaded. The eyepieces are made of brass and will last for ages.These two element eyepieces only really work properly at F/12 or slower. They might be okay at F/10.You can easily stop down, even an F/5 telescope, say 100 mm refractor. If you stop it down to 50 mm then it will be F/10.</blockquote>