# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Can You Bias Ho?
Date: 2014 Apr 4, 08:14 -0400

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.

On Apr 4, 2014 4:32 AM, "Luc Van den Borre" <luc@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
```

View and reply to this message: http://fer3.com/arc/m2.aspx?i=127443

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