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I was busy with finishing procrastinated work when that very nice
post came by, and didn't comment.  Thanks to Frank for resuscitating it.

I'd just like to comment on one thing regarding these determinations
of sextant error at fixed intervals along the arc.  One would expect
that some smoothly varying curve passing through the points would
describe the errors, so that one could interpolate between points to
estimate an error.  Unfortunately, this may not necessarily be so.
This became obvious to me on EBay in inspecting some certificates
issued to late-model Hughes sextants by the British Navy that showed
a continuous curve.  I don't recall the methadology used to generate
the curves, if it was listed.  The curves were all over the place
with substantial peaks and valleys occurring between fixed points.
This, of course would be less of a problem with error determinations
at 15d intervals than 30d intervals, but is still a potential
problem.  Perhaps Mr. Morris will poke around with his calibration
machine to test this hypothesis.  I would think the high end of the
MkII scale would be a fruitful starting location.

I'd also be curious to learn Mr. Morris' opinion of by how much one
must undershoot a reading to reload the thrust bearing at the reading.

Fred Hebard

On Jan 13, 2008, at 7:21 PM, frankreed@HistoricalAtlas.net wrote:

>
> Two weeks ago, Bill Morris wrote about calibrating his sextants.
>>
>>
>> "My 1981 instrument meets Alex's specification but not mine: 15d.
>> +1"; 30d
>> +1"; 45d +4"; 60d +7"; 75d +8"; 90d +11"; 105d +9"; 120d +8"."
>>
>> That's excellent.
>>
>> "My Mk II s/n 14176, 1942 in near-new condition with a certificate
>> from Long
>> Beach Shipyard dated 7 January 1986, does well too, except above
>> 90 degrees.
>> I give the original certificate figures followed in brackets by my
>> own,
>> which are each based on the mean of three careful readings: 15d
>> +3"(+1");
>> 30d -14"(-12"); 45d -17"(-16'); 60d -25"(-22"); 75d -30"(-31"); 90d
>>  -33"(-40"); 105d -12"(-31"); 120d 0"(-30"). For most practical
>> purposes
>> except lunars, the differences are insignificant."
>>



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