NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Henry Halboth
Date: 2013 Dec 17, 12:07 -0500
Randy,
1) Let us agree that the "star telescope" is a Galilean of low power, having a positive objective lens and a negative eye lens (two lenses).
2) To Lecky, the "inverting telescope" was a Keplerian, having a positive objective lens and a compound positive eyepiece (three lenses).
3) Some sextant kits of his time were also supplied with an eyepiece with an extra pair of inverting lenses (five lenses)to give an erect image. The magnifying power of the Keplerians was usually somewhere between 6 and 14 times.
In his chapter on the sextant, Lecky seems to prefer the inverting Keplerian over the "star telescope" because of its greater magnification, which allows easier judgement of when a body contacts the horizon. He mentions one of 14 to 15 powers as being especially useful when using an artificial horizon. I don't think he is referring to the advantage of an inverting Keplerian over an erecting one, in which there is more light loss and scattering at each lens surface.
For twilight observations it is light-gathering power that is needed so a large objective lens diameter is desirable. Even indiscernible increases in brightness can lead to an improvement in contrast. It is fairly easy to make a Galilean of 3 or 4 power in a reasonable length, as the lens separation is equal to the differences in their focal lengths (ignoring the signs). One of greater power would be shorter, but at the cost of an increasingly constricted field of view for a given objective lens diameter, and there is of course a practical limit to increasing this diameter, set by the need to accommodate it on the sextant.
The modern compromise would perhaps be a prismatic monocular of 6 x 30 or 7 x 35 power, though I have two Japanese military sextants mounted with 7 x 50 s.
The bright, erect, wide field view both makes acquisition of the body easier and makes it easier to judge horizon contact. Coating of lenses means that we don't have to worry as much as Lecky might have done about light loss at glass-air interfaces.The Soviet SNO-T was provided with a 6 x 30 Keplerian telescope with coated lenses, giving the minimum of glass (and weight) plus a sensible magnifying power combined with a bright image. It needs practice to get used to inverted images it gives, but in difficult twilight conditions it may be worth it. The small-boat sailor may find the image is too unstable for comfort at these magnifications.
Bill Morris
Pukenui
New Zealand
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