NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Inverting telescope for sextant
From: Alexandre Eremenko
Date: 2013 Dec 17, 10:47 -0500
From: Alexandre Eremenko
Date: 2013 Dec 17, 10:47 -0500
Don, > I am a bit confused. I thought that a simple two lens telescope inverted > the image, and a three lens telescope produced an erect image. There are two types of two-lens telescope: a) Galilean (straight) and b) Kepler (inverting). The difference is in the smaller lens (eyepiece). It is positive in Kepler and negative in Galileo. > In nautical literature of the 18th and 19th century, I have come across > references to naval officers using a 'night glass'. "Night glass" refers to a telescope with large objective lens. It gathers more light than a naked eye. Most of them are Galilean, because of the reason I mentioned: with the same diameter of the objective lens, the Galilean is shorter. Of course, it can be used day and night. Alex. > The ordinary day time > telescope was a terrestrial one, with an erect image. After sunset, they > might use a night glass with one less lens, which gave an inverted image > but more light transmission. > > Don Seltzer > ---------------------------------------------------------------- > NavList message boards and member settings: www.fer3.com/NavList > Members may optionally receive posts by email. > To cancel email delivery, send a message to NoMail[at]fer3.com > ---------------------------------------------------------------- > > : http://fer3.com/arc/m2.aspx?i=125788 > > > >