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    Re: Lunar longitudes, not by lunar distance.
    From: Nicol�s de Hilster
    Date: 2009 Aug 09, 16:17 +0200

    Geoffrey Kolbe wrote:
    > 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. 
    I measure sun azimuth's quite regularly professionally using either a 
    Wild T2 theodolite or a Leica TCRA1101 total station, both being 
    instruments with standard telescopes. In order to overcome the problem 
    mentioned Wild (and most other theodolite manufacturers as well) 
    supplied a rectangular eye piece that can be mounted to the telescope 
    (it fits the Leica as well).
    > 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!
    Oh yes, they do appear on e-bay as well, twice even quite recently, but 
    at prices fitting their use: astronomical...
    > 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.
    >   
    When I measure azimuth's I use a solar prism, also known as a Roelofs 
    prism. This prism was invented by the Dutch professor Roelofs in 1947 
    (patent requested 19 May 1947 and granted on 16 May 1949) and produces 
    four overlapping images of the sun, the centre of which is used to aim 
    at (see http://www.dehilster.info/instrumenten/theodolite4/index.html 
    where you need to scroll down to picture 8 and further). Apart from a 
    small centring error (which will be corrected for by transiting the 
    telescope) the filter will give the sun's centre.
    > 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.
    >
    >   
    The best time to observe the sun's azimuth is in indeed the early 
    morning or late evening, especially when in the tropics. The azimutal 
    motion is then quite slow, with forthcoming better results (it is all 
    about timing).
    
    Nicol�s
    
    
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