NavList:

Greg ?

I have been off the air for a couple of days.

Byron Franklin (and probably others) have mentioned the importance of letting sufficient time to work on your night-vision, as well as resting your eye a moment before doing the final step in the observation.

The old look-out technique not to stare yourself blind on the object itself, but to move your eye and rather scan the area around it has also been covered and is important for a good sight.

Some more hints which may or may not have been mentioned in this discussion (now or earlier):

1.       Use of astigmatizing lens (I think this is also called ?elongation lens?) helps a bit.

2.       By moving the telescope as far out from the sextant frame as possible (possible on many marine sextants) you get more light from the horizon than what you get when it is clamped in its standard position.

3.       Use as much shading as possible if the object (star/planet) is very bright. Reduces the contrast between the horizon and the object which is good, also helps to maintain your night vision.

----Ursprungligt meddelande----
Från: gregrudzinski@yahoo.com
Datum: 2011-10-06 18:02
Till: <NavList@fer3.com>
Ärende: [NavList] Re: Sv: Re: Sv: Re: Sv: Re: Use of AH for professional navigation

Magnus,

I had posted a few days ago asking if anyone had ever been able to do night celestial during new moon using an estimated horizon. Based on your description it looks as if a pair of young eyes can do this.

I believe there is a bit of confusion regarding the artificial horizon (AH). There are two types- one which presents a bubble to the eye when looking through the sextant scope and the other which is a shielded reflective surface (liquid) that is placed on the ground or table and doubles the sextant angle when the reflected body in the AH is superimposed with the body as seen via the sextant index mirror. The AH which doubles the sextant angle won't work on a ship that is underway. The bubble aircraft sextant will work aboard a small craft that is secure to the dock but unfortunately no good for an underway small craft at sea. At the dock I normally get plus or minus 5NM intercepts just as you have observed with your experience aboard ship.

Aircraft bubble sextants are very affordable on Ebay and worth the gamble on finding a fully functional example.

Greg Rudzinski

[NavList] Sv: Re: Sv: Re: Sv: Re: Use of AH for professional navigation
From: sjoquist.magnus---com
Date: 6 Oct 2011 12:14

I know practically nothing of air-navigation, but compared to a Cessna the accelerations of a cargoship it may be so that the ship is more steady. Whatever vibrations You have on a ship can be handled by standing with slightly bended knees and a fairly loose grip (fingertip) on the sextant (sitting is more difficult but probably there is no alternative on an aircraft). Rolling and pitching of the ship is not a serious problem if your sea-legs are fairly well trained. On a smaller boat, or on a very fast-going ship when the bow continiously hits the waves there would be a challenge, but I have never used sextants (for measuring vertical angles) with AH's on such crafts.

Night-observations can also, as You surely know, also be done without any AH at all provided Your night-vision is good and if You accept something around 10 NM-precision. Being 65 I cannot any longer brag with my night vision, but younger collegues take star- (and planet-) sights with very good results (no Moon around). With "very good" under nightconditions I here mean around 5 NM.
Textbooks are OK, in general, but if the text does not match my own experiences I disregard the former.

Correct me if I am wrong, but did not submarines occasionally take astro-sights ("periscopal")? Would be interesting to know what precision they had in their LOP's. Does not sound too easy imagining a dark windy night wintertime North Atlantic (read some of that in textbooks, though).
/Magnus

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