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> I found that checking IE by sighting on a single bright star on a clear night
> was inevitably faster and more precise than any other target. It is at
> "infinite" distance, so parallax is not an issue, and being "up" it is further
> removed from ground effects on the atmosphere. As a pinpoint it makes a very
> tiny mark to align and compare. Although I must admit, here the C&P optics
> have a further advantage, since one light path is tinted green and the other
> red, the
> star really snaps into "white" when it is precisely aligned in both. (I'm not
> aware of any other maker using tinted optics that way but it certainly is
> possible.)

Red

Thanks for your many thoughts.  Some I've tested and posted a years or so
ago.  I do like the red/green concept.  I am quite frankly poor at testing
IE with the same star.  Poor seeing in my neighborhood, and IE checks with a
star are usually all over the map.  There is also the problem Alex et al
have noted with a star(s).  Once in the sweet spot, one or both develop
"tails."   Plus the problems associated with stars moving from point source
to a "cluster" as the pupil expands for night vision.  (Again I reference
the Sept. 2005 Star & Telescope article.)

Alex and I (with the Astra) do quite well with star-to-star distances as the
stars are not identical. Alex seems to do well with one-star IE checks. I've
gone as far as testing drum eccentricity. I feel quite comfortable that the
Astra exceeds its +/- 20" accuracy-along-the-arc specs.

Fred asked if I had others check IE.  Yes, sun and stars. But as I think
about it Alex had to refocus the scope, so he was not seeing what I saw side
alignment or IE wise.

> It is also possible that some of your changing error was was thermal effects
> on the sextant as it heated in the sun. Another variation found much less
often
> at
> night.

Possible.  When I first obtained the Astra, I had scope focus shifts from
day to night.  As that was about February 2005 in Indiana, I had a good
chance to extensively test any number of combinations and concluded that any
significant focus shifts were indeed day to night driven.  Sextant wise, the
latest tests have been done in very similar temperature conditions, with the
sextant and scope brought to ambient temperature.
>
> "My left eye distorts a sphere, with the vertical axis longer than the
> horizontal axis.  My right eye distorts a sphere with the vertical axis
> shorter than the horizontal axis."
> Ah, welcome to the world of ASTIGMATISM. It is common but many folks simply
> never notice it since they are not using their eyes for anything critical. It
> can easily be corrected by glasses, less accurately by contact lens.  You
> should
> also be able to have a corrective filter fitted to the eyepiece of your
> sextant
> to match the correction needed by your better/preferred eye, although I have
> no
> idea who would deal in those matters. An optician who loves a challenge, I
> suppose.

My eyeglass prescription is old, but does correct the problem (mostly).  It
does not seem to help with the scope, and using glasses with any light
pollution, coupled with the eye relief of the scope, causes more problems
than it solves.

> As a practical matter...use one eye and stick with it, then select a method of
> observations which is best suited to correcting that error. I am very much
> familiar with the frustration of having eyes that are not as precise as the
> sextant I'm trying to use with them.

I did settle on one eye very soon after purchasing the sextant.  Not only by
"feel" but later by standard deviations in observations.  Besides the first
two points, when using the left eye my right eye is shielded from the sun by
the scope, so a big plus for sun observations (at least IMHO). I had
introduced the right eye as a control only for the latest series of tests.
(frame rigidity).

You also touched on being near land, and possible affects from that, which
has been covered in recent list postings.  Out of my control, but over a
year of undershoots (with lunars, sun and moon with natural horizons, sun
and artificial horizons etc) seems to indicate personal error--be it pilot
error or an eye distortion.  Especially when compared to star-to-star (point
sources) tests.  I tend to discount pilot error at this point, as once I
achieve what I perceive to be tangency I can move the drum +/- 0.2' and
achieve clear overlap or separation.

Another very fuzzy variable not touched on is irradiation which hopefully
cancels out with bright sun LL observations against a sky lighter than the
water. When the water is no longer darker than the sky above the horizon,
what happens?  It appears attempts to quantify irradiation have been
abandoned.

You and Fred have jogged the old memory banks.  One recollection relating to
questions by Alex about shades.  Keep them out of the path and filter the
scope to see if that changes things.

The other, at least when using a natural horizon, is doing a series of
upper-limb and lower-limb observations in hopes upper-limb observations will
null out the eye problems (irradiation again).  I need to give that some
thought to determine if it might help or hurt--at least in theory.

Thanks

Bill




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