NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2014 Apr 3, 20:59 -0400
I've had a private conversation with someone who owns a Friebrrger surveying sextant with the pentaprism attachment. This would be the one furnished with the instrument. A couple of comments from that person. His name shall be Anon.
Anon has tried to use this as proscribed for large horizontal angles but found it very difficult to use. He suggests that a theodolite would be a much better choice for this
Anon has tried to use this for lunars beyond 120° and found it impossible to get the sun and moon into alignment, let alone into contact. Anon thought that maybe with a tripod this might be feasible but did not make the attempt.
Anon suggested perhaps the only thing this pentaprism arrangement would be good for is the sun's altitude with an AH. Precisely the measurement Greg wants for meridian crossing.
Anon, like me, can see no use for the up arrangement of the pentaprism. I still don't understand why Nicolas would do that. He's a bright fellow, I'm sure he had a reason.
I'm sure if I got this wrong, Anon will update me.
When I suggested that he just join NavList so all could discuss directly, he explicitly declined. His perception ( I've cleaned up and summarized the language here) is that some folks here lack respect for other opinions and other folks. Anon did not want to subject himself to "us". I certainly am not going to finger anybody although Anon was quite explicit as to persons and in language. Maybe, just maybe, if you see yourself here, you could be more collegiate. This goes for me too!
Brad
Thanks Bill, figure 12 does show the pentaprism in two orientations; up and down. I missed that.
Greg will want to use the down orientation, such that his range of measured angles is extended. This is what Greg was after and the pentaprism in this orientation does that. Nicolas agrees with the that orientation and purpose, explicitly. In this orientation, the maximum angle is extend to 215°… with a minimum measured angle being 90°.
Nicolas points out that he uses the up orientation to verify his index error at 90° (??). He sets the index to 90° and with the pentaprism in the up orientation, he can see the same thing as a normal index check. He is at a different position along the arc. Was he attempting to determine arc error ala the Heath patent using prisms? That's not stated nor is any other purpose. I'm not sure what to make of this but I never claimed to know everything! Perhaps someone knows a purpose for this orientation and is willing to explain it. In the meantime, I continue to see little or no utility in this orientation. Reducing the maximum angle measured from 125° to 35° doesn't seem very useful. Nor is an index correction at 90° as arc error will not be linear from 0° to 90° and what to extrapolate when over 90°? {My actual thought is that he did it simply because he could. There is no real purpose!}
I do wish Nicolas would rejoin us here on NavList. It would be better if he would speak for himself.
Brad
Brad
Nicolas has arranged the penta prism so that it can add or subtract 100 grads (90 degrees) from the reading. See Figures 11 and 12 on his web site, which shows the light path going up and down, respectively.
Bill Morris
Pukenui
New Zealand
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