[time-nuts] Information on the Danjon Astrolab

[Geoff Powell]

In article <44D7E964.4000604 at pacific.net>, Brooke Clarke
<brooke at pacific.net> writes
>Hi:
>
>After seeing a reference to "Splitting the Second" by Tony Jones on 
>Tom's web page I read it and found that the Photographic Zenith 
>Telescope was replaced by the "Danjon Astrolabe".   But Google provides 
>almost no information about it.  A patent search has turned up a number 
>of optical systems, mostly for spacecraft, that use star separation for 
>navigation, and if used at a know location on the earth could be used 
>for timing.  For these see my  Stellar Time Keeping web page: 
>http://www.pacificsites.com/~brooke/StellarTime.shtml
>
>Does anyone know how a "Danjon Astrolabe" works?

IIRC, the Danjon astrolabe uses a mercury pool, and a semi-reflecting
prism.

The optical telescope is aligned horizontally, and the prism provides 2
light paths, one aimed upward at some angle (normally 45 or 60 degrees)
and the other downwards. The mercury pool reflects the downward beam
upwards.

As a result, when you look through the telescope, you see double images.
As the skies go through their apparent diurnal rotation, objects will
pass through the nominal viewing angle, and the image doubling will go
away. The observer presses a button at the magic moment to record the
time.

Advantages - 

can use more stars, not just those at the zenith.
can observe more stars in a night, leading to improved accuracy in time
determination.

Disadvantage - 

requires manual operation, with resultant errors due to "personal
equation" - the eye-hand delay.

Accuracy of the Danjon astrolabe and the PZT were considered similar,
but the automated capabilities of the PZT won out - I think.

Source for this - the book "Geodesy" by Bomford. Sorry, no ISBN or
publisher (I'm typing from memory)
>
>Have Fun,
>
>Brooke Clarke
>-- 
>
>w/Java http://www.PRC68.com
>w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml
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>
>
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-- 
Geoff Powell