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I have a different experience.
For all kinds of observations (except with the sea horizon twilight
observations, in which I have no experience),
a 7x or 8x inverting or prismatic scope performed better than
the 3x Galileo scope.

Most of my observations were Lunar and star distances.

There is another advantage of a Kepler scope: because of its larger
field of view, it is much easier to catch the star (or Sun) on a
moving boat.

Alex.

> I do not seek to, nor will I, dispute the
> experts with respect to telescope
> design, however, having taken a few star sights in my time will comment on
> their use in practical navigation. I have uses 7 x 50 monoculars, high
> powered inverting scopes, nondescript antique scopes, and sight tubes in
> taking stars and have found nothing in general use to surpass the 3x scope
> provided with my Plath.
>
> Henry[?]
>
>
> On Mon, Dec 16, 2013 at 9:53 PM, Bill Morris 
> wrote:
>
>> ------------------------------
>>
>> 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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