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George wrote [NavList 9430]

>All he needs to do, once it's been placed firmly on a well-set tripod, is to
>set up his instrument so that its vertical axis is truly vertical, for which
>the built-in levels should be quite adequate. Then to set it so that its
>telescope swings in a near-meridian plane. One way to do this is to
>establish North (in the Northern hemisphere), by bisecting the most-Eastern
>and most-Western azimuth swings of Polaris. However, that's only possible at
>certain times of year; at others, two different stars may have to be used,
>but this presents no problem now that stars have been catalogued so
>precisely. Then swing by 180� to look Southwards.

A couple of practical points are worth noting 
(again, as Hanno is new to the list) in the use 
of transit theodolites for measuring azimuth.

First is that for altitudes greater than about 50 
degrees, it is usually not possible to get your 
eyeball behind the telescope using a standard 
transit theodolite. I think Hanno says he lives 
in San Diego, whose latitude is low enough that 
he will be able to see Polaris without difficulty 
using a standard transit theodolite. Here in 
Northern Europe, it would not be possible to do so.

There are so called 'broken' theodolites, where 
the viewing is done along the axis about which 
the telescope and vertical scales rotate. There 
is a 45 degree mirror which redirects the view up 
the telescope. There is no limit to the altitude 
one can view using such a theodolite, the most 
famous example of which is the Wild T4 - but you 
won't find one of those on ebay! What you do find 
quite commonly on ebay are theodolites for 
tracking weather balloons. These are 'broken' 
theodolites and would be excellent for CN alt/az 
measurements - except that their scales are quite 
coarse, usually in degrees only I think, judging 
by what I can see from the ebay pictures.

Second, is that the error in azimuth goes as the 
tangent of the error in vertical alignment of the 
theodolite. So, if the theodolite is 1 minute off 
vertical and you are looking at an object at an 
altitude of 45 degrees, the error in azimuth 
could be up to 1 minute. Luckily, standard 
transit theodolites are self limiting in this 
regard - see my first point above.

Third and final point. As George mentioned, when 
doing measurements on the sun or moon, it is 
usual to take a measurement as the edge of the 
body (top of bottom for altitudes, sides for 
azimuth) touches the appropriate cross hair and 
then make a correction to get the centre of the 
body. For altitudes, the correction (to first 
order) is simply the semi-diameter of the body. 
For azimuths, the correction is the semi-diameter 
divided by the cosine of the altitude.

With both these last two points, it can be seen 
that it is advantageous to observe bodies of low 
altitude to avoid significant errors in azimuth.

Geoffrey Kolbe


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