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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2014 Apr 2, 21:39 -0400
Attached please find a diagram of the optical path of a pentaprism. Also find attached an image of Nicolas' sextant with his home made pentaprism attachment. If you trace the optical path, you will see that he has it rotated improperly.
The path on Nicolas' sextant goes up past the index mirror. That's WRONG!
The pentaprism should direct the path down away from the index mirror.
Since the pentaprism is a known entity to me, when I saw Nicolas' arrangement, I didn't understand how it was going to work. Now I understand. It won't!
Brad
Got it!
Nicolas de Hilster shows a prism that will fold the optical path up. The beam exits the horizon path, takes the 90° turn inside the pentaprism and then goes up past the index mirror! Instead of mounting the prism as Nicolas shows it, with the path turned UP, the prism needs to be rotated such that the reference path is rotated DOWN. After the beam exits the horizon path, it should fold 90° away from the index mirror and go towards the ground. Then if the index mirror is set to 0° on the arc, your measurement is 90°… set 19° on the arc and your measurement is 109°. Now that's functional.
Shame on me for accepting Nicolas' mounting at face value. He did state that he made it himself. I'll bet that the commercial mount is correct!
Pentaprisms can be had for only a few $10s. The mount must be machined and attached to your sextant. But very feasible!
Greg- It looks like you have a real solution here.
Brad
On Apr 2, 2014 11:02 AM, "Brad Morris" <bradley.r.morris---.com> wrote:Hello Peter
I've had a look at the referenced pentaprism mounted on the sextant. That's a clever idea, as it twists the reference path.
The beauty of a pentaprism is that the light bundle out is always 90° from the light bundle in. This immediately eliminates most of the setup problems in aligning the prism to the optical path of the sextant. It still requires that the plane of the sextant and the plane of the pentaprism be parallel, but any machinist can achieve that within a few thousandths of an inch. Thus the cosine error will be negligible.
I then set out to see how the sextant would be used, with the artificial horizon and the pentaprism in the optical path. My mind has been churning over the position the sextant must be in and how I would then see the celestial object in both optical paths (index path & horizon path). I must confess I cannot see how to make it work. Which path will you direct to the AH? Which path directly towards the celestial object?
I think this is along the right track. That is, a known deflection of the horizon path will change the angle distended on the sextant. Greg needs that to extend his AH work. Can the idea be reworked a little to make it truly functional?
Brad
On Apr 2, 2014 2:12 AM, "Peter Monta" <pmonta---.com> wrote:Here are a couple of links describing pentaprism attachments for sextants---search for "penta" within the pages. Only de Hilster's site shows a photo, though; the prism is evidently in the horizon-mirror path.Cheers,
http://www.jgarraio.pt/index.php?main_page=product_info&cPath=1053&products_id=17634&subtemplate=mc&language=en
http://www.dehilster.info/index.php
Peter
