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Frank,

Welcome back. While you were absent several questions to you accumulated.

1. About Nav List archive. All messages for 2012 are listed under January.
(I already addressed this 2 times, but you were away, and probably missed
these messages).

2. Question about your Lunar sight and Almanac.
How far back it is reliable?
I know it is OK for XVIII century. Is it OK for BC dates?
Actually I don't even know how can I enter a BC date:-)

3. You have "predicted lunars" for many more stars than your
"clear a lunar online" allows. Many years ago you said that you
consider adding stars to the calculator. Why you decided not to do this?

Alex

On Tue, 27 Mar 2012, Frank Reed wrote:

>
> I haven't posted about this in a while. After some years of experimentation, 
trying various different techniques, I have found that the best way to get an 
index correction is to take off the sextant's standard telescope and place a 
higher power spotting scope in line with the instrument on a table. Then you 
look at some suitably distant target, some miles away, and do the usual 
alignment. With a 30x scope, you should find with a metal sextant that you 
get the same IC over and over again to the limiting precision of reading of 
the micrometer (in other words, you'll get +0.4 ten times in a row without 
fail). A plastic sextant will continue to show a lot of variability, but by 
using a spotting scope, you take the observer error due to the limited 
resolution of the human eye out of the equation. Then you can assess the real 
limitations of the plastic sextant and possibly find circumstances where the 
IC is stable.
>
> For those interested in sextant calibration beyond IC (for arc error at 
angles other than zero), you can extend this by placing two sextants mirror 
to mirror on a table using one as a standard for the other. As with the index 
error check, you remove the telescopes and place a spotting scope in line 
with the usual position of the telescope on the instrument being tested. Then 
if you set both sextants to some angle, e.g. thirty degrees, the arc 
calibration is reduced to a simple index correction check. The "calibration 
standard" sextant delivers light to the sextant being tested at exactly 
thirty degrees angular deviation (assuming the calibration standard is good, 
of course) and then the sextant being tested straightens it back out so the 
observer should see two identical images superimposed. Any difference is the 
arc error for that angle.
>
> -FER
>
>
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