NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2014 Apr 5, 14:01 -0400
Yes, this topic is resurrected again .
I have had a private conversation with Nicolas about the survey instrument and the purposes of the various orientations.
For the UP orientation, Nicolas used this to calibrate the pentaprism itself. It seems that there is an offset to the orthagonality of the folded beam. It is not necessarily 90° true. So by putting the pentaprism in the up orientation it can be calibrated against the sextant, when the sextant is set to 90°…under the assumption that you know the arc error at 90. He states further that he has found no other use for this orientation. There is no expectation that the offset of the pentaprism will change over time.
The down orientation extends the maximum angle. No further discussion is required.
When attempting angles smaller than 90°, Nicolas states that the prism does not occupy the entire FOV of the horizon path, and as such, he can use the sextant in a normal fashion. That is, for small angles, the use of the pentaprism is avoided entirely
Nicolas actually owns and apparently has used this instrument, based upon the details provided to me
Brad
Hello Luc
Perhaps the light bulb is starting to glow dimly for me.
It appears from the catalog that the prism housing is on a pin and that through the use of this, the pentaprism can be in two orientations, up and down. My understanding of German is poor, so I don't know if one rotates the pentaprism in place on the sextant or the pentaprism is lifted out of the sextant from one orientation and replaced in sextant in the other.
Assuming the pentaprism is rotated in place on the sextant to the up orientation, then what you indicate makes sense! Instead of referencing to 0°, the angles are referenced to -90°. And as you state, this gives a range of -95° to +35 (the same 130° of divided arc). Since however, we care for the magnitude of the angle, we ignore the sign (that's the epiphany) yielding two ranges, to wit 0 to 35 and 0 to 95 degrees. Combining the up and down pentaprism ranges yields a total range of measured angles of 0° to 215°.
Under the assumption that the pentaprism is lifted out and then replaced, then the obvious question is why the observer wouldn't just leave it off for the lower range. This leads me to believe that this is not the arrangement and that rotate in place is the arrangement.
Thanks Luc & Bill, for helping me to understand this.
Brad
On Apr 4, 2014 4:32 AM, "Luc Van den Borre" <luc{at}nuclide.com> wrote:On 4/04/2014 3:11, Brad Morris wrote: > 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. It's a surveying sextant, meant to be used horizontally. If a sextant's normal range is -5° to 125°, the range with the pentaprism 'pointing down' will be 85° to 215°, so if the prism were fixed you wouldn't be able to measure angles smaller than 85° without unscrewing the prism housing from the frame. With the pentaprism in the other position the range will be -95° to 35°, returning the ability to measure angles smaller than 85°. Both the C&P and Freiberger pentaprism attachments can be rotated 90°, of course: http://www.cassens-plath.de/katalog/index.html?startpage=90 http://www.fpm.de/index.php?c=1&s=pentaprisma Luc
