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On 8/24/2015 4:02 PM, Mark Coady wrote:
> 4. Seem to have trouble with visual resolution, verifying index
> correction to absolute reliability.  One reading on sun diameter came
> out perfect on the checksum from the NA of 15.8,  another was off by
> .3'-.4' Part of it is a slight haze, i am having trouble get sharp edges
> to touch. I see a gap...but can't quite tell if I just touched or
> am overlapping a wee bit.

Welcome Mark

Haze could be a contributor. It does sound like you have an excellent
grasp and are doing things properly.

Despite the position of some NavList members that a well-designed
temperature-acclimated sextant will not change its IE over a string of
observations or in storage, I find my Astra IIIB can. It is well
designed. If you spend 5-10 minutes doing checks it will heat up. It is
a black instrument with optics in a black housing in the direct
sunlight, and subject to the laws of physics. Something as trivial as a
good breeze that helps cool it vs. calm can be a tipping point. For
natural horizon sights with lower bodies refraction can be a factor.
Another part of the equation is experience. Statistics seem to indicated
performance improves up to 3000 observations or more.

Another factor, as Alex put it when referring to lunars, is "Learning to
see." One can make the images tangent as they would in limb-to-limb IE
checks or sun/moon/star/planet horizon observations as far as they can
see and still be a good bit off consistently in the same direction with
lunars. (Personal vs random error, can't blame the sun's heat here.)
Note an error in lunars will magnify any problems versus traditional
sights by approximately 30 due to the nature of the beast. After enough
lunar observations to establish a base line, you can train/adjust you
standard for the degree of tangency so your observations coincide with
predicted results, exhibiting only random error.

Regarding IE, for what it is worth and older study on a mountain top (to
negate refraction) did show a marked change in IE in the same instrument
and observer during a series of experiments. That being said, anything
over plus/minus 0.2' between sets when averaging 5-10 limb-to-limb
should raise a red flag IMHO.

Best advice, keep practicing. You will see improvement.

You wrote: "I would guess there are rules agoianst drinking and navigating."

Quite the contrary. Over a long series of observations, my partner and I
found a moderate intake of adult beverages helps steady the hand and
improves precision and accuracy. He prefers whiskey (I would have bet
Vodka) and has better results than I do with my Pusser's Rum. The real
trick is getting started. I hold a piece of line or a necktie in my left
hand, wrap it around the back of my neck, then grasp the other end and
shot glass in my right hand. By pulling down slowly with the left hand I
can hoist the shot glass to my mouth without spilling a drop. After
repeating 3-4 times, I'm steady enough to hit my mouth without the aid
and start observing.

   

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