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Author
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Topic: Mark 15 Sextant Index Error
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Tedder
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posted July 23, 2011 02:01 PM
I've been using the Index Error determination method provided by Dave Burch's How To Use Plastic Sextants with my Davis Mark 15 sextant. I made a Baadar solar filter and measured the solar diameter both "Toward" and "Away." Accordingly, my IE is about +4' Away and +8' Toward. However, comparing my SD with the Almanac's SD, I get from .8' to 2.0' larger, average about 1.5'. Does this discrepancy indicate poor technique or is it an aspect of the sextant? Does the discrepancy invalidate the index error I have determined?
From: Apalachicola, FL
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David Burch
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posted July 23, 2011 09:08 PM
these index errors are too large. it would be best to adjust the mirrors to get the errors smaller and do it again. also must have all side error out as well.
try to get the IE down to a minute or two... be sure to tap the mirrors to be sure they are seated.
also check that the reveal around the edge of the mirrors is about equal... ie that it is not in the housing crooked.
From: Starpath, Seattle, WA
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Tedder
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posted July 28, 2011 10:23 AM
We're having a lot of cloudy weather in Florida lately, but I have been able to make some solar measurements. Being very careful to not touch the eyepiece and precisely mark the sun's touch, I was able to get the index error to an average of .2' OFF. However, my solar diameter for this set averages 16.5', about .8' too big. Is this acceptable?
From: Apalachicola, FL
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David Burch
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posted July 28, 2011 10:55 AM
might need to post some actual data here so we can see how the analysis is proceeding. in other words, the proper IC cannot be 0.2 if the SD is off by that much. in short, something is still not right.
Also a note on terminology: SD = semi-diameter, not solar diameter, which is roughly factor 2 different, but more important a different concept. solar diameter implies the visual sun is round, but in fact it is an ellipse and the SD is one half the narrow width on that ellipse.
PS, i assume you are not enrolled in our cel nav course -- if you are in that course, this discussion would be better there where others could take part. we do not monitor this section as often.
From: Starpath, Seattle, WA
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Tedder
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posted August 03, 2011 05:01 PM
The current solar semi-diameter is 15.8'.
First I carefully aligned the sextant mirrors so that the sun and reflected sun were superimposed.
Then I took some readings, being as careful as I could to not bump the eye-piece and to mark the limbs as they just touched. Here are some sample data from today:
ON OFF DIFF CHECK SD
37.6 31.4 -3.1' 16.8
36.7 30.0 -3.4' 16.7
So, my semi-diameter measure is about 1' off and my IE is about 3' ON. The weather was extremely hot, BTW. I assume these readings are too high.
However, on a previous, cooler day, I got:
ON OFF DIFF CHECK SD
32.2 33.6 +.7' 16.4
32.8 33.6 +.4' 16.6
I believe this is much better, as I assume that both IE and semi-diameter reading less than 1' is OK.
How do I go about getting consistent readings of less than a minute? Is it the hot weather?
From: Apalachicola, FL
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David Burch
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posted August 04, 2011 11:08 AM
can you please clarify this statement:
First I carefully aligned the sextant mirrors so that the sun and reflected sun were superimposed.
is this using the solar filter, looking directly at the sun?
From: Starpath, Seattle, WA
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Tedder
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posted August 04, 2011 12:52 PM
I looked directly at the sun using the Baadar filter, then I adjusted the horizon mirror so that the reflected sun and the direct sun were almost exactly on top of each other.
From: Apalachicola, FL
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David Burch
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posted August 06, 2011 09:21 AM
OK. i think that is the right way to start. i.e. adjust both screws so suns overlap, nominally no index and nominally no side error... with some light tapping etc as explained in the book.
then set about to measure IC by solar method.
if we get a sun out today (we are overcast here in Seattle for the moment, this Saturday morning) then i will get a Mk 15 and see what i come up with and see if anything rings a bell in the process.
more later.
From: Starpath, Seattle, WA
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David Burch
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posted August 06, 2011 02:21 PM
Here is what i get
ON 60- Off IC SD
32.0 35.8 24.2 3.9 14.1
32.6 39.5 20.5 6.1 13.3
35.4 36.5 23.5 6.0 14.7
AVG= 5.3 14.0
true SD = 15.8
ON 60- Off IC SD
28.4 33.0 27.0 0.7 13.9
31.6 32.9 27.1 2.3 14.7
28.0 32.4 27.6 0.2 13.9
AVG= 1.1 14.1
I think your results are good, now that i return to this issue.
In these measurements i noted that the side error was gradually changing through out the process. it was under a hot sun at noon.
so between these two sets i readjusted side and index error, which accounts for the two different values.
my SD is consistent, but is low. true value is 15.8 which is 1.7 higher. it seems i may have a bias in my alignment of some 0.85'.
Once again, we must stress, to get good at this you must practice and take notes.
It is also clear that if you had a higher power scope you could do better.
It might be useful for us as a class to make an effort to find some small inexpensive scope with more power. My guess is such things exist. we do not need large objective since we have plenty of light.
I recalled two other important points.
You must still rock the sextant, so turn away very slowly, and rock to be sure of alignment.
Also i was reminded that you cannot press the scope against the head while doing this. you have to look through it uncoupled.
all the above were done in the Away direction.
Also learned that practice when the sun is not so high is much easier on the neck. sun here was some 60 high.
From: Starpath, Seattle, WA
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Tedder
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posted August 06, 2011 03:48 PM
Thanks for the careful data--it gives me more confidence that my methodology is reasonable.
I just did some measurements (although the Florida sun is pretty hot these days), and my AWAY results averaged 2.6' ON, STD DEV .6. Semi-diameter readings were quite consistent, from 16.1' to 16.4', and interestingly, increased regularly from reading to reading. The time was 18:30, sun height approximately 20 degrees. Can this SD increase be due to refraction? The IC readings also increased regularly.
From: Apalachicola, FL
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David Burch
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posted August 06, 2011 04:43 PM
as the sun rises, the SD approaches its normal value. but 20° is already up away from most refraction changes.
on the horizon it is some 34', at 5° it is 10' at 10° it is 5', and this effect is the difference between Hs = say 5 and 5 30, etc. so not much.
on the other hand, in the old days navigators who were being careful and were forced to make this check when the sun was low, would hold the sextant sideways and use the width of the sun and not its height.
you might play with that.
and yes, i agree you got the system down.
From: Starpath, Seattle, WA
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David Burch
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posted August 06, 2011 04:45 PM
I think we might be seeing simply a matter of heating.
might test this by just setting the sextant right then watch the sun without changing anything for say 10 to 15 min in direct sunlight.
i think that is what i was seeing.
From: Starpath, Seattle, WA
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