A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Francis Upchurch
Date: 2016 May 7, 00:40 -0700
Thanks gentlemen for the reassurance on this.
I suspect if you are just using it for horizon filter, no problem anyway. Chronic UV light exposure is mainly associated with long term risks like macular degeneration and cataracts.
However, acute damage from looking directly at the sun through a telescope (i.e the index mirror on a sextant) is more likely related to the infrared end of the spectrum. You can literally burn a hole in the retina like starting a fire with a magnifying glass!
So, I would want any index filter to stop both UV and infrared. Also , hence the advice from Astronomy web sites for observing eclipses, transits etc., to put the solar filter over the main primary lens of the telescope, not the eye objective. You can easily burn a hole through the eye objective filter and then the eye! I've now stopped using the simple and for the eye, the safe method of direct projection of the sun image onto a white screen.This is safe for the eye, but actually, not for the telescope which can get pretty hot and if the focus goes wrong, you could easily start a fire with what is essentially a very powerful magnifying lens!
Sadly, it looks like mother nature is going to protect my eyes on Monday by placing thick cloud filters in front of the sun. So I hope someone else out there in sunier climes will do some Mercury transit observations?