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George

Don't really have the time, interest or energy to do a line by line debate
and/or rebuttal of a discussion that has strayed so far in intent from the
initial post/question.

According to the person with first-hand knowledge, it was a Davis 15, and he
used a Davis (water) artificial horizon with colored glass.  The error was
stated as, "...my observed height is up to 6.5' different from calculated
height....And my observed height is always (with no exceptions yet) less
than calculated height." So he has given us worst case and indeed has
provided the direction of the error-away.  Apparently some sort of personal
or systemic error that we have not solved by moving the discussion towards
defense of our toys in small boats on the water.

Overall, I have no "prejudice" against plastic sextants.  My first sextant
was a homebuilt (from a kit) cardboard unit with the optional bubble horizon
and advertised accuracy of 8 minutes.  After careful construction,
considerable tweaking and calibration, care with technique, and practice I
was able to obtain consistent results with on one intercept in the 2-3
minute range on land, and 5-7 minutes on the water.  I loved it, still have
it, and still use it. (IE is sensitive to humidity;-)  My only "judgement"
here would be that being land locked, the cardboard unit offered a practice
bubble horizon while the Davis line did not, and I chose to learn in the
"usual" positions.  On the downside, the cardboard unit has no horizon
shades, so a poor choice for use with artificial horizons. If I had a
natural horizon, I would have started with a plastic Davis unit.

As a personal "value judgement," bang for the buck, I consider the Astra
IIIB and other metal units in that price-point niche to be an order of
magnitude better than the plastic units in accuracy, precision, and
flexibility.  After that we enter the steep part of the diminishing-returns
curve.  And no George, this does not mean I am bashing upper-end sextants.
I would love to own one, two, or more someday.  I also intend to purchase
the bottom end of the Davis line soon.

David Burch's site was recommended to Asbjorn. Clearly, if you have read
David's data, the Davis sextants suffer from considerable backlash (compared
to a metal sextant). If Asbjorn did not take this into account when setting
IE to zero or in use thereafter, that *may* account for a goodly portion of
his error.  Whether we choose to call it index error, or variable index
error INDUCED by backlash is a reasonable distinction.

As you noted, if there was (variable) index error unaccounted for so not
subtracted from the original angle, this would require approx. a 13' error.
After it was noted that d correction was incorrectly applied, a 10' index
error would be required.  Burch's experiments would indicate approx. 1/2 of
your 10' could be accounted for by backlash. But we don't know that for
certain.

I would suggest:

1. Let's compare apples to apples. Have you made observations with your
Ebbco(s) and an artificial horizon? If so, what are your typical results?

2. Have you read Burch's plastic sextant PDF, as well as sea/land data (Vic
Maui Sextant Sights) or his IE/backlash with plastic sextants PDF?

3.  If yes to #2, do you:

a. Feel Burch is attempting to give an objective overview of the operational
tolerances of Davis plastic units, and presenting methodology to achieve the
best results with these units?

b.  Or, believe Burch is "tarring" plastic sextants. (If yes, as a sanity
check, why does he sell and teach with them?)

As a sidebar it seems to me to fair game to compare a Jaguar against other
automobiles in its price class based on performance and creature comforts,
as it is likewise equitable to compare a Mini-Cooper to its peers.  If a
writer chooses to compare a Mini-Cooper to a Jaguar (for whatever reason)
and concludes the Jag beats the Cooper in most on-the-road performances and
creature-comfort categories; but the Cooper costs less to purchase,
maintain, and operate--is this Mini-Cooper or Jag bashing?

With that low-heat-to-light discussion behind us (hopefully) please allow me
to paint with a broader brush (not the one you would use to tar and feather
me with;-)

In the past I have been chastised for "quoting" without a reference/URL to
source. Note has been taken.

In this case I have provided the sources and URL. It is apparently not
sufficient when the source does not support the first-hand observations of
an exceptional individual with an instrument no longer readily available in
the states--therefore no ability to verify or replicate *unpublished*
results.

Now I find myself like a male dog between four trees--not a leg to stand on.

Point being, are the rules of discussion/debate now reduced to: "Such
messages carry more weight when based on first-hand experience, rather than
being the relayed opinions of others. So I hope that the messenger will
consider himself suitably shot..."

Are credible sources and objective experiments, reports, and field
observations done by others reduced to "relayed opinions?"  Is this research
no longer acceptable?  If so, is it cricket for you to reference Copernicus,
Galileo, Summer, Meeus et al when you were not there, did not do
experiment(s), compile and analyze the data, derive the formulas, or publish
the results?  Is their work also relegated to the area of "relayed
opinions."

The most important point, IMHO, is what are we doing to help Asbjorn?  It's
been a "slow-news" summer and we tend to grasp at any little thing to
discuss. In the global overview it might appear to a beginner/new member
that a couple of guys in advanced curmudgeonhood are ignoring his needs for
our own perverse "debating" pleasure.

All said and done, after the d correction Asbjorn did well for first efforts
given his experience and typical tolerances/performance of his instrument.
The fact that the intercepts were all in the same direction suggest a
personal or systemic error that perhaps the list can help him resolve.  With
that solved and further practice, it would not surprise me to see him back
with intercepts in the 2' range.

Hopefully we will hear back from Asbjorn.

Bill



> When I wrote-
>
>> "I should add that, just like Bill, I have no experience with the Davis
>> model that Asbjorn was using, but Bill's comments tarred all plastic
>> sextants with the same brush; and unfairly so, in my view."
>
> Bill responded-
>
>> It was not my intent to tar all plastic sextants, but merely pass along the
>> feelings/observations of those more qualified then myself and with
>> first-hand experience--like Celestaire and David Burch.  Burch wrote in a
>> PDF on his site http://www.starpath.com: "I would also propose ? as a broad
>> generalization ? that using these procedures one should be able to obtain
>> accuracy?s of some 5 or 6 miles as a general rule with plastic sextants.
>> Maybe better in some cases, maybe a bit worse in others." Celestaire's
>> seminar handout claims 3 to 10 miles.
>
> I wouldn't dispute an assessment of 5 or 6 miles for possible overall error
> of an altitude taken with a plastic sextant on a small craft, in
> less-than-perfect weather. But not all that error is due to the
> plastic-ness of the sextant. Even with the most perfect and expensive
> sextant, observations from a small craft in those less-than perfect
> conditions will show errors of "a few minutes". These are unrelated to the
> accuracy of the instrument itself, but dependent on observing conditions.
> However, we were discussing a 10-minute error, in an on-land measurement
> from a stable platform in Norway.
>
>> In your own words while speaking of
>> your Ebbco and IE checks, "...and it never shows a discernable change
>> (reading to the nearest minute,  which is all the instrument is good to)."
>> Given that is only a minute, not five or ten, it is still greater by an
>> order of magnitude than one would expect from a quality metal sextant.
>
> That may indeed be true: I wouldn't dispute it. But it's not what we were
> discussing, which was Bill's imputing of a 10-minute error in sextant
> reading to variable index error in plastic sextants, in these words-
>
> "If the error is relatively consistent within a batch, perhaps index error is
> the culprit.  I have no first-hand experience with plastic sextants, but
> from what I have read IE appears to vary to much more within a batch, or
> from batch to batch, than does a metal sextant."
>
> I pointed out that IE, when properly allowed for, is NOT a source of error.
> In my view Bill was simply relaying prejudice without first-hand knowledge
> or experience of plastic sextants. It may be true, indeed that some models
> are error-prone. Some may show errors in their scale divisions. I just
> don't know (and neither does Bill). But I can state (and did) from
> first-hand knowledge of two different plastic Ebbco instruments, that it
> would greatly surprise me if defects in that make would give rise to a
> ten-minute error in angle.
>
>> Your
>> results with the Ebbco are a testament both to the Ebbco and your abilities.
>
> Don't get me wrong. I don't claim to be able to take altitudes to within a
> minute; certainly not from my own small boat at sea, not with ANY sextant.
> When I wrote about the Ebbco "reading to the nearest minute,  which is all
> the instrument is good to" I was referring to the precision to which the
> scale could be meaningfully read, not the precision of the resulting sight,
> which can be affected by many other factors.
>
>> But if you are in the mood to shoot the messenger...  
>
> Such messages carry more weight when based on first-hand experience, rather
> than being the relayed opinions of others. So I hope that the messenger
> will consider himself suitably shot...
>
>> George also wrote:
>>
>> "Asbjorn described his technique, which as far as I could tell required
>> adjustment of the index error to zero (using the grub-screws?) on each
>> occasion, which appeared to me misguided. A better procedure would be to
>> leave those grub-screws alone, after an initial  setting, and just accept
>> and allow for any subsequent index error: However large that index
>> zero-error happens to be, if it's allowed for then it doesn't degrade the
>> precision of the measurement AT ALL."
>>
>> I find this interesting.  In my limited experience, adjusting for IE also
>> affects side error.
>
> That depends to a large extent on the layout of the adjustment-points of
> the mirror. If they are delta layout, then it will happen as Bill
> describes. If, better, in a "L" layout, then it won't.
>
>> Again risking reprimand, what I take from the list is
>> that a little side error is not a big problem, and may be useful.
>> (Star-to-star or star-to-planet angular measurements?)  I am not clear
>> Asbjorn checked for side error after his index error adjustments.
>>
>> I suspect side error introduced by adjusting for IE would not account a 5'
>> error.
>
> I agree. And we are discussing a 10' error.
>
>> The problem being I have no idea how to quantify side or
>> index-mirror perpendicularity errors.  My texts explain how to adjust those
>> errors out, and check for side error with a star or horizon, but little
>> more.
>>
>> The texts I have read first have one adjust for index-mirror
>> perpendicularity.  Then there are back-and-forth adjustments for
>> horizon-mirror side error and index error until the index error is close and
>> the side error is eliminated.  One check for side error involves rocking the
>> sextant to see if the horizon remains aligned.  Being landlocked I use a
>> faint star.
>
> Fair enough. Or in daytime you could use a distant rooftop or a flat
> skyline or a distant electrical power-line, with the sextant tilted.
>
> The only reason I can think of for bothering to get the index error near to
> zero is to simplify the arithmetic when subtracting it off. An index error
> has no effect at all on the precision of the final answer.
>
> But going back to Asbjorn's adjustment technique, what worried me was
> allowing for backlash. If you make an index check, then an altitude
> observation, ending each by turning the drum in the same direction, that
> corrects for backlash automatically. If you indulge in the unnecessary
> practice of setting the index error to zero with the grub screws, then you
> would have to be similarly careful about the way the index arm had been
> brought into that zero position. Asbjorn has recently cleared up the matter
> by stating that he checked his index zero with his altitude observations,
> but (I think) didn't state specifically whether he took precautions against
> backlash.
>
>> The questions arise:
>>
>> 1.  How (other than the arc check for rear silvered and block check for
>> front-silvered mirrors) to check for index-mirror perpendicularity error in
>> the field?
>
> The distinction is NOT whether the index mirror is rear silvered or front
> silvered, but whether or not the pivot line passes through the effective
> reflecting surface, wherever that may be. If it does, then you can check by
> looking at the reflected arc. What's the difficulty about doing that, in
> the field? And having made the adjustment initially, is it an important
> matter from then on? Unless the sextant gets dropped, that is...
>
>> 2.  What errors will be introduced, and to what magnitude, if the index
>> mirror is not perpendicular?
>>
>> 3.  What errors will be introduced, and to what magnitude, if the horizon
>> mirror is not perpendicular?
>>
>> It strikes me that by using a star to adjust out side error, it may appear
>> perfect, but still have problems in use if the index mirror is not exactly
>> perpendicularity.
>>
>> Any thoughts or expertise you or the list could offer would be appreciated.
>
> I would guess that any such mis-adjustments would have to be unfeasibly bad
> to give rise to an error in measured angle of ten arc-minutes.
>
>
>> Bill
>
> George.
>
>
> ===============================================================
> Contact George at george@huxtable.u-net.com ,or by phone +44 1865 820222,
> or from within UK 01865 820222.
> Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13
> 5HX, UK.

   

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