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George wrote:

> 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.

"Wooly?"  You want right eye numbers, you shall have them.  It may be days
before the skies clear.  What I can tell you, and you can see from the data,
is the left eye vertical average was 0.43 and -0.42' higher than the left
eye horizontal average at 63d elevation, and dropped to 0.25' and -0.3' at
32d elevation.  With the 3.5X scope I can readily see overlap or gap with a
0.2' change from observed tangency.  While the right eye is soft around the
edges, there is no doubt in my mind that that alignment changed from
vertical to horizontal positions and overlap is introduced.  I can see it
with the left eye, and the figures support it.  I can see it with the right
eye, so all we are really talking about is hard numbers to quantify the
magnitude of the shift IMHO.  Once that, and inverted test are completed, we
may be able to take rigidity off the table and place part of the blame on
the left eye. Or not.

As we cross off possible causes for the natural horizon IE vs sun-limb IE,
we can also move closer to the original goal. That being a constant to
account for systematic errors if there prove to be any.

Regarding the shades, if testing proves them to be a problem, then we are
faced with determining to what degree each grouping contributes to the
problem.  For sun shots, the index shades will remain in place but the
horizon grouping will not.

it seems like a lot of fuss, but at this point in my development under
static conditions and good on-the-water conditions, IE is becoming a large
limiting factor in accuracy.

> 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.

Thank you, nice to get some feel for possible magnitude.

> 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.

What can I say?  Fred has seen it with his C&P, GregR has seen it, Alex has
seen it, Red points out a possible cause, and I have seen it in four scopes
(Astra star, 2 prism scopes, and Alex's SNO-T star scope).  If you doubt the
possible amount of slop in the focusing mechanism, ask Alex about his SNO-T
Galileo.  He had a very thick lubricant put on the focusing mechanism last
year (in Germany if I recall) as it was so loose.  As to possible chromatic
problems, that would require bench testing in a lab.

Do I claim all scopes exhibit this behavior. No. Do I claim it will always
change along the X axis?  No. I claim every scope I have handled (with the
possible exception of Alex's inverting scope, where focus is changed by
sliding a press-fit tube in and out) exhibits a shift.  I do believe it is a
variable that I need to be aware of with my equipment.

Step outside and try it for yourself. If your scope(s) don't exhibit any
shift, then we must all be wrong. 

I would encourage other readers to try the experiment and report in.

Bill


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