# NavList:

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

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Re: Celestial training planetarium 1950
From: Peter Monta
Date: 2016 Feb 14, 17:51 -0800

The drawing does not immediately convey the size of the thing until you realize that those are stairs spiralling up into the interior of the dome.

It looks like something out of "Duck Dodgers in the 24 1/2th Century".

... The solution for this problem applied in this Link Simulator is described in the article, if I understand it correctly. It says that "23 of the important navigational stars" are "collimated for navigational purposes". But it's not quite clear to me how this would be set up, and how would it appear when displayed? My guess is that those stars would be projected from something like slide projectors poking through the dome. Would they look right visually? This collimated projection would ensure that the stars would have the proper relative positions in some region around the center of the dome? How big would that observing area be?

That would be my guess too, that each of the 23 stars has its own collimator poking through the dome.  A slide projector would work, but you'd have to place the entrance pupil of the sextant within the collimated beam, and for a slide projector that would be only 50 mm.  Given that price seems to be no object, I'd suggest a large fast paraboloidal mirror for each collimator, maybe 250 mm or so in diameter and something like f/4 (need not be diffraction limited).  Maybe that would be enough for two or three students at a time to get their sextants within the beams; the sweet spot would be a sphere at the center of the platform 250 mm in diameter.

The appearance would be normal, starlike.  At worst, if the collimator has an obstruction in the beam as part of the light-source mount, the star would look funny when it's out of focus, but in focus it would look just like a star.  As you move your head outside of the sweet spot, the star will disappear.

I need one of these domes.  With all the light pollution, it's a rare night that 23 stars are visible.  (Ok, I exaggerate a bit.)  Along the same lines, a VR headset will make a good planetarium.  If it's AR (augmented reality), the headset could also tell you "here's the location of the object you cannot see because of light pollution" with a little blinking cursor.  Am I bitter?  No...

Cheers,
Peter

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