NavList:

Bill,

There are visual differences when determining index error between the Sun, stars, planets, horizon, and scopes. I don't expect to get the same index error result across the board. The horizon index error has the largest spread. Stars twinkle making it difficult to consistently superimpose. Planets are larger than stars and sometimes oval. The Sun can be hard to shade for good contrast at times. Another factor to be aware of is that the more a sextant is handled the more likely the horizon mirror will be tapped or knocked which will change the index error. Just taking the sextant in and out of the box can change the index error if the horizon mirror grazes any part of the box.

Index error is not a fixed value. The mean value is constantly in flux.

7x35mm scopes are generally used on ship sextants from heights of eye around 100 feet. The higher power scope brings in a distant horizon better. I have noticed index error determination and lunar observations seem to be more consistent with the more powerful scope also.

Greg Rudzinski

[NavList] Fwd: Re: Re: Standard Deviation Question
From: Bill B
Date: 06 Jan 2013 04:37

-On 1/5/2013 12:00 AM, Greg Rudzinski wrote:
> Here are 20 index error observations of Procyon using a Tamaya Jupiter
> with 7 x 35mm scope.
......Snip
> x = 0.575'
> Sx = 0.1585'
>
> All observations are off the arc.
Greg

Out of context I am not sure what the above indicates statistically,
other than you are far better at determining index error from a star,
especially a bright star, than I am :-(

Alex suggested I use the dimmest star I could see. Frank posted an
article that taught me what some submariners and others believed--pop
out, take your shots and be done with it. (As your eye becomes dark
adapted a point source becomes a scatter of point's or star bursts.) I
gave up star IE checks except as a court of last resort. If I can come
within 1' of the values I obtain with a natural horizon or sun IE check
I'm lucky.

I thought the excellent Celestaire 7X scope would improve my results.
Nope. The Astra has performed almost flawlessly in other observations.
Seems to prove the old adage, "Don't blame poor craftsmanship on poor
tools. If you doubt it, put excellent tools in the hands of a poor
craftsman."

What is of interest to me is using the figures in your 2 January post as
an IE check vs your star IE check. Half of the difference between your
32!64 off the arc and 32!56 on the arc is a highly respectable IE of
0!04 or 2.4" off the arc vs 0.574' off the arc for Procyon.

I'm very accurate and precise with star-to-star measurements as I can
swing one through the other, but one-star IE checks are not for my old
eyes--with or without eyeglasses.

Bill B

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