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Richard M. Pisko wrote:
>> I have a graph on my web site showing the results of the second method
>> (using that same 566m reference on Terschelling), scroll down to figures
>> 9 and 10 on:
>> http://www.dehilster.info/instrumenten/theodolite2/index.html
>>
>
> Nice graphs.  The way I remember them (can't look now, as I'm downloading
> a large file), you made decent time with the set of eight observations,
> with good results.  Raw figures were two up, two down, two up, two down, I
> think.  Is that off center bias on the Roelofs Prism constant, or does it
> seem to depend on the observer?   Does it move with the housing, or with
> the rotation of the prism tube?
>
The differences between the face left, face right readings is indeed a
centring error of the prism. When I was at school myself I was taught to
take the observations in two faces because of this and although the Wild
manual does not tell you to do so it says that when "...observing on
both faces the telescope must be transitted via the eyepiece end and the
solar prism must not be touched." (see figure 8 on that same page of my
web site). The closing error indeed varies when the solar prism has been
re-attached, so clearly either the housing or the hinged prism is not
well centred. It could also be that the wedges and double wedges
(numbers 2 and 4 in figure 26 on that image no. 8 of my web site) are
not properly aligned in my Roelofs prism.
> Mostly, I now wonder how much is induced when twiddling with the alignment
> of the cross hairs to the light green "clover leaves" in the prism.  I
> generally don't touch the prism through my usual six shots on the sun
> between the bracketing shots on the reference object.
The prism housing should indeed not be touched in between observations,
apart from flipping away the prism.
>   Then, of course,
> the prism has to be flipped out of the way.  I think I should set up a
> high intensity strobe light at the same level as my RO, but out of the
> field of view, and see how consistently I can point between them.
That would indeed be interesting to know.
>   I has
> assumed the difference of calculated Az in my readings between face left
> and face right was due to errors in my theodolite's preparation, but now I
> don't know.
Last option is a collimination error. You can check this of course
easily without the prism. With the prism you can check the centring
error by making series of observations, turning the whole prism housing
approximately 90 degrees and take another series. Repeat this another
two times and you should be able to tell.
>   Something else to do when the temperature becomes more
> moderate.
>
>
That's the best time ;-)
> I saw the SunAz Java application run Monday night at the computer club.  I
> hope I now have the tools to try it at home.  We'll see.
>
Ok, great. You might need to download it again as you might have a
version that shows you only one decimal, when in gon or degrees and
rounded values in DMS mode.
> Thanks for the program,
>
>
You're welcome.

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