NavList:

Greg R wrote:

> I think the best way to answer the question "scientifically" (at least
> in the limited case of my 2 sextants) is to take a series of sights
> with some done in the same direction, and some done "up and down" like
> I mentioned earlier and compare the results.

Some of this has been covered a year or two ago on the old list (check the
archives) but there seem to names new to me on the list, so here goes.

You can that with one or both limbs of the sun or moon, or superimposing a
star if you are good at that.  (Probably more accurately than on a bouncing
boat taking a series of traditional sights.) The dip part of a power or
phone cable, flat roof top etc also work well.  You do not have to worry
about distance of the object as all you are doing is comparing the figure
arrived at in one drum rotation direction as opposed to the figure in the
other. 

Actually it is not quite that simple unless your eye and sextant are precise
and accurate to the sub-0.1' level. A better, IMHO, method is to establish a
range where you perceive alignment (touch and leave) for each each direction
and establish the midpoints as absolute alignment. Use a figure just below
"leave" as the upper range limit along with touch to establish alignment
range and midpoint for each direction. Compare the midpoint from each
rotation direction.

You might be surprised (or not) with the range that a 3.5-4 power scope and
your eye perceive as aligned even with a black power line a against a light
blue sky, or roofline on the side in shade vs. a bright sky.

Following is an  excerpt from a PDF (or at least it once was a PDF)
available on David Burch's Starpath web site:

http://www.starpath.com/index.htm

Path to >celestial navigation >Instruments >Davis Plastic Sextants >How to
Optimize Plastic Sextant Sights

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

"An ideal sextant has a very positive action of the micrometer drum, meaning
no slack in the gears. Turn it to the right by 1¹ and immediately the angle
increases by 1¹. Stop and turn it to the left and it immediately starts to
go down. A good metal sextant in good condition will behave properly in this
regard. Plastic sextants, on the other hand, tend to have a bit of slack in
this mechanism, consequently we get slightly different results when turning
to the right to achieve alignment as opposed to turning to the left to
achieve the same alignment. This is a well known issue with plastic sextants
and it is mentioned in the manuals for the Davis Mark 15 and Mark 25 plastic
sextants (it does not apply to the more basic Mark 3 model which does not
have a micrometer drum.

But there is more to this story. We cannot investigate slack in the gears
without some means of observing the effects of our rotation of the drum. In
other words, we have to decide what is or is not in alignment once we rotate
the drum. An obvious time to study this effect is during the index
correction (IC) measurement, which is typically done with the sextant set to
0° 0.0¹ while viewing a distant sea horizon. (Note that there are other,
probably even more accurate, means of measuring the IC ‹ and gear slack ‹
but for now we discuss only the more common IC method of using the horizon.)

The sea horizon is the most convenient and most commonly used method, but
for precision work it has the limitation of not often presenting a perfectly
sharp line between sky color and sea color. Look very carefully at the best
horizon and you often see ‹ or at least appear to see ‹ a very narrow line
of some other color right at the horizon, or some other slight disruption of
a perfect line. Consequently, even when we have a perfect sextant with no
gear slack at all, we can still get the appearance of a slight gear slack
because the imprecision of the reference line leads to some variance from
sight to sight in what the observer might call "perfectly aligned." The
amount of this variance will depend on the nature of the horizon, the skill
of the observer, the power of the telescope, and with the sextant model. A
6- or 7- power scope is better for IC checks than the 4-power scopes which
are standard on most sextants, and this effect is naturally larger when
viewed in the 2-power scopes on plastic sextants.

Here is a procedure for investigating this effect:

First remove the side error of the sextant by adjusting the horizon mirror
until you can rock (roll) the sextant set at 0°0¹ and not detect any
splitting of the horizon. Many texts (and Bowditch, of course) explain the
procedure. This may also require some collateral adjustment of the index
mirror. With plastic sextants we have found that it is often useful to give
each mirror housing (not the mirror itself) a bit of a flick with the finger
to help the seating of the mirrors before and after the adjustments. If the
flick changes things, you have to keep working on it. (Don¹t flick it any
harder than you would flick your own nose!)

Then with the sextant set to 0° 0.0¹, view the horizon and turn the drum
"toward" you (clockwise, angle decreasing) to clearly separate the two
horizons viewed directly and by reflection. Then slowly turn the drum "away"
from you (counterclockwise, angle increasing) until the horizons just first
appear as a smooth straight line, which is what we call in alignment. Be
sure to sneak up on this very slowly so you do not overshoot the alignment.
We want the reading just as they first become aligned.

Confirm that you are aligned by panning (yawing) the sextant right and left
a bit to verify that there is no motion along the horizon. This is a more
accurate method than just looking straight at it and concluding it is
aligned. If you are just very slightly unaligned, you will notice a slight
bump moving right and left at the intersection of the two views, direct and
reflected. Once confirmed, record the IC reading to the nearest 0.1¹ and
label this IC measurement with an "A" to note that you were turning the drum
in that direction and a "touch" to note that this was the setting for the
first touch of the two horizon views in alignment. If you have overshot the
alignment, start all over again.

Now to continue, first double check your notes to confirm which way you are
turning and think through the motion, then very slowly and carefully
continue turning in the away direction until you can first detect that you
are no longer aligned. Again, this is best done by doing a slight rotation
then panning the horizon, then another and another pan, until you can detect
some motion along the horizon which indicates that you are no longer
aligned. Then read and record the new IC and label it with "A" and "leave,"
meaning this was the value when you left the alignment.

Repeat this 5 or 6 times in the away direction and then do the same in the
toward direction. This type of measurement will show what we are up against.
You have effectively measured the angular width of "perfect alignment." With
a metal sextant and a sharp horizon, the touch and leave values will
typically differ by only a few tenths, which reflects our limits on locating
the horizon precisely. Put another way, if we just randomly set the sextant
to alignment on a series of sights, we could fairly expect to get at least
this level of spread in the values we measured, since anywhere between
"touch" and "leave" gives the same appearance of alignment.

More to the point at hand, however, is that with a metal sextant, the spread
in the touch and leave values will show little if any difference when
measured in the toward or away direction. With a typical plastic sextant
this is not the case. Not only will you detect larger spreads in the touch
and leave values, you will most often note a significant difference in the
IC values measured in the toward and away directions, which is a measure of
the slack in the gears ‹ or, if not that, at least some measure of the
general behavior of the device (the actual worm gear in the plastic sextants
is metal, but it seats into notches in plastic)." --David Burch

Bill




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