NavList:

Magnus,

Excellent tips. Thanks. The astigmatizer is perhaps an under utilized lens on a sextant and as you say has late twilight and nighttime star observation usefulness. I have one on my Navy mark 5 aircraft bubble sextant which does a good job of splitting the bubble as well as providing a sense of vertical. Use of the astigmatizer seems to be out of favor and not included in the filter sets of most sextants.

Greg Rudzinski

[NavList] Sv: Re: Sv: Re: Sv: Re: Sv: Re: Use of AH for professional navigation
From: sjoquist.magnus---com
Date: 10 Oct 2011 12:05

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.
4. If you constantly measure the objects X minutes too high (low) you may have a ?personal error? which can be corrected for in the calculations or directly when you shoot the star. It is quite common that beginners have a couple of minutes of personal error even when taking twilight sights of stars with good horizon (no offense, I hope). Lower limb observations of sun and moon are easier in that respect.

Best,
/Magnus

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