NavList:

Bill wrote-

| Your math is correct.  My input was not.  A blunder on my part.
| On the arc is correct at 31.625.  Off the arc was a typo.  Reading
average
| 55d 28!595 off.  60 - 28!595 = 31.405 (not 30.405 as stated).  So
0.11' IE
| was correct.

All.right. Moral; it shows the advantage of providing "raw data",
rather than partly-processed data. But now, I don't understand where
the 55d comes from. By the way, why is an exclamation mark used, where
a decimal point is appropriate?

===============================


| > I'm aware that Bill has discounted gravity-flexure because of
| > differences in behaviour seen by left or right eye, but he has not
| > spelled out those differences for us in detail, so I am
disinclined to
| > accept those assurances at face value. More information about that
| > please, better specified. One test for flexure would be to make
the
| > same measurement with the sextant vertical (in its normal
| > orientation); then inverted.
|
| This I have already done in the vertical versus horizontal tests
posted if I
| understand you correctly. All published results were from the left
eye only,
| with the exception of noting the reversal of trend with the right
eye. I am
| sorry for any confusion I may have caused. Horizontal tests were
done in two
| stages, one set with handle up and one with handle down.  No
noticeable
| difference between the two sets, so they were lumped together in the
data
| posted.
|
| Regarding rigorous right-eye testing, at this juncture I have not
done that.
| I determined early on that my left eye was more precise.  I made a
dozen or
| so observations with the right eye to establish a baseline this
time, but
| the results were poor--as usual. Despite being able to focus to the
point
| that I could see a sunspot clearly with the left eye, I could not
bring it
| into focus (or even see it) with the right eye after repeated
attempts.
| Standard deviations of the observations I did make with the right
eye
| vertically and horizontally were 0.25' and 0.3' respectively.  As a
layman,
| I could not justify comparing the left eye numbers with 0.1'SD to
the right
| eye numbers
|
| What I can propose regarding gravity on the frame or mirrors is that
I would
| expect changes from vertical to horizontal to trend in the same
direction
| for both eyes or any  observer.  That was clearly not so.  Achieve
tangency
| (observed) with the left eye and sextant vertical, then rotate the
sextant
| to horizontal position, handle up then down, and the images clearly
| separate.  Achieve tangency (observed) with the right eye and
sextant
| vertical, then rotate the sextant to horizontal position, handle up
then
| down, and the images clearly overlap.

Well, that's just the woolly sort of statement that I was asking for
an improvement on. If Bill trusts his right-eye observations
sufficiently well to use them to discount the possibility of
gravity-flexure, then why not produce some numerical results
(corresponding to his left-eye observations) to prove it? We need
clarity and rigour here. If his right-eye vision is so poor that he
couldn't do that, then I distrust his badly-specified statements about
image separation and overlap.

============================

We have discussed irradiation, and Bill has assured us that he has
used similar shades for his two views of the Sun, and both images are
equally bright. In which case, we can, as I see it, exclude
irradiation as a contribution.|

However, he has asked-

Which brings up a question.  Before it was determined that personal
differences, scope and filter differences etc. made in impossible to
accurately calculate and adjust for irradiation, what were the publish
magnitude(s) for irradiation?

It was presumed that there would be no irradiation correction for
measurements of the lower-limb of the Sun against the horizon. This
was because the shaded Sun image was brighter than the sea image, and
the sky image above the horizon was also brighter than the sea image,
so both of those interfaces would be displaced (downwards) by
irradiation, and by (roughly) the same amount. However, with an
upper-limb Sun observation, the bright upper edge of the Sun is
displaced upwards by irradiation, the horizon interface as before
being displaced down. So there's a double error introduced into
upper-limb observations. The almanac correction tables, between 1955
and 1970, presumed a combined correction of 1.2 arc-minutes for
upper-limb observations, which may have represented a shift of 0.6'
for each interface. Experimental fieldwork by the Nautical Almanac
Office here in Britain showed that irradiation was so variable that
the assumptions were untenable, which is why it was dropped from the
corrections. But that figure of 0.6' gives an idea of the
order-of-magnitude you might expect, and it's just the same sort of
magnitude as the discrepancies Bill is chasing up. Where it applies,
such as in precise lunar distance observations between Sun and Moon
edges, or Moon edge to star, it's worth taking seriously , even though
it is so hard to account for.

=====================

I remain somewhat sceptical about these accounts of index error and
side error changes as the focus is adjusted, and their attribution to
eccentricity in the optics; though just because I can't imagine how
they might arise doesn't imply that they don't exist.  Bill wrote-
"Any eccentricity in the focusing mechanism or lens(es) not
parallel to the permanently mounted lenses could throw it off.", but I
find that hard to accept: that there should be any such significant
eccentricity, and if there was, that a shift of telescope aiming point
could affect such observations.

If you take a look at Frank's recent proposal for checking index error
with a laser (in Navlist 679, "Table top" index error measurement) he
simply says- "Set your  sextant on its side on a table and aim the
laser through the telescope." Note that he suggests no precautions for
putting the laser beam exactly on the optic axis of the telescope, or
parallel to the plane of the sextant, and to me, that seems to be
right. Collimation error comes in only when measuring wider angles,
not when checking index error. As long as the mirrors are truly
parallel, it doesn't matter how off-axis the light-beam is, the beams
from the two mirrors will remain  truly parallel. Frank's method
presumes that to be the case.

However, that table-top method, as described, measures index error
when the sextant is placed on its side, and if there's any flexure,
will give wrong answers for use in normal orientation. All it needs is
some sort of lab-clamp to grip the handle, and the job could be done
the same way in any orientation.

=======================

I am puzzled about the reluctance to use the horizon for checking
index error. "Because the books say so", says Alex, but why should
they say so? It's something of a tradition for us to question what the
books say. OK, the horizon isn't usable in many circumstances (from
inland, at night, when it's hazy, when it's turbulent).  But when it
is sharp and clear and reasonably smooth, why not use the horizon? The
horizon is far enough away that parallax, between the views via the
two mirrors is not a problem. Dip change, between the heights of the
two mirrors, needs to be thought about, but standard dip changes only
by 0.2' for a foot change in height, so for the 3 inches or so between
the mirrors, should be negligible.

And if no horizon, inland, what about some other horizontal feature,
at least a couple of miles away? A rooftop, a flat or rounded skyline,
a power line at the bottom of its sag?

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---