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I've owned both a Davis Mark 3 and a Mark 20.

The Mark 3 is a relatively unsophisticated instrument, at best suitable
for the crudest of celestial sights.

On the other hand, the Mark 15/20/25 series seem to be pretty decent
sextants.  Admittedly not the equal of a Tayama, Freiberger, Plath, or
even an Astra 3B, but I've seen a lot of people pass celestial
navigation courses (eg, the USPS courses) using them.   I've used my
Mark 20 on my boat to practice celestial.  When a small boat is rolling
and pitching, arguments about whether there's 0.1 min of backlash in the
vernier drum become pretty irrelevant.

Lu Abel

Fred Hebard wrote:
> I've got a Mark III.  It's much harder to adjust than a sextant with a
> drum.  It doesn't have a magnifier, which makes it harder to judge
> contact, and is difficult to set.  It's horizon mirror is also split,
> so it's tough to bring images into contact: there's no overlap of the
> reflected and straight images as with a half-silvered horizon mirror.
> You need to use the vernier to read to the nearest arcminute (or
> possibly to the nearest 5 or 10 arcminutes).  So you can't just read
> the minutes off the dial.  You can't read below an arcminute (or
> perhaps 5 or 10 arcminutes).
>
> The index arm is so sticky that it's tough to nudge it except in one
> direction, which is what I meant in the previous paragraph when I said
> it is difficult to set.  A difference in reading depending upon the
> direction of adjustment may occur due to slop in the pivot bearing.
> Never tried to measure that.
>
> On Sep 16, 2005, at 10:00 PM, Herbert Prinz wrote:
>
>> The Davis Mark 3 has a Vernier scale. I don't understand why it should
>> make a difference wich way you move the index arm before reading. This
>> is only a concern with micrometer drums.
>>
>> Herbert Prinz
>>
>>
>>
>> Espen S. Ore wrote:
>>
>>> Given the way the Davis Mk. III works I believe it is
>>> difficult to come any closer than 1-2", and that yes, one should move
>>> the arm in the same direction for all the shots.
>>
>>
>
>

   

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