NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Star-star distances for arc error
From: Douglas Denny
Date: 2009 Jun 23, 09:57 -0000
And you wrote:
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From: Douglas Denny
Date: 2009 Jun 23, 09:57 -0000
----- Original Message -----
From: <frankreed@HistoricalAtlas.com>
To: <NavList@fer3.com>
Sent: Tuesday, June 23, 2009 2:12 AM
Subject: [NavList 8761] Re: Star-star distances for
arc error
And you wrote:
"Attempting to use star separations to try to
determine scale accuracy for example would not be possible due to the variables
in the measurements themselves - including the refraction component even if
calculated."
A sure indication that you have never tried it!
A sure indication that you have never tried it!
========
I have tried it and I don't rate it as
practical.
If accuracy is required, the measurements are
difficult to achieve in the first place unless the sextant is clamped. Clamping
requires special arrangements for tilting the sextant.
Also, combining of two stars overlapping, with the
inevitable abberrations seen in the telscope image itself preclude this to being
within one minute at best.
Also, the measurement can only be done to an
accuracy of the divisions of the sextant which in most cases is one minute of
arc.
Sextants vary a lot. My Hughes sextant - beautifuly built, brass frame, with
platinum divided scale, with drum/worm drive has a vernier to measure
to 10 seconds of arc but the certificate indicates accuracy of max
error of 1 minute 30 seconds at points on the scale.
My Carl Zeiss Frieberger sextant however,
does not have a vernier so only measures to one minute of arc (half a minute
estimated as best accuracy), but the certificate gives max error of the
scale as 24 seconds of arc.
To attempt to calibrate the scale error using star
distances is simply not accurate enough in my opinion.
Any measurement of that nature needs the accuracy
confidence to be at least an order greater in accuracy than the measurement
errors likely - and you are suggesting it is OK to measure two stars where
the refraction error alone can be greater than the measurement.
Even when corrections are applied, if the
altitude is less than 40 degress the refraction component is one minute of arc,
(i.e. of an order comparable with the measurement); if less than twenty
degrees it is 2.6 minutes of arc; and less than ten degrees it is
over 5 minutes of arc and the unknown effects of barometric pressure and
humidity affecting density make low altitudes very suspect indeed.
No. I am sorry, but as a method for checking
sextant scales it is just not good enough in my opinion.
The only proper way is to use a dividing table and
collimator where the dividing head is at least accurate to one tenth of a minute
of arc at all readings.
It is easy to get carried away with the theory
without considering the practical difficulties or potential errors of the
measurement itself compared to what one is measuring.
Douglas Denny.
Chichester. England.
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