NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Observations with pocket sextant in the Baltic CROSS POST
From: hellos
Date: 2006 Jul 4, 18:43 -0500
Alex-
> Huh, so the companies are measuring the arc--
> and presumably certifying their own
> instruments based on that certification
> --without acutally measuring the *system*
> error which would include the teeth?
"I am not sure that I understand what you wanted to say here.
In a modern sextant "measuring the arc" is the same as
"measuring the teeth."
I could see them measuring by several ways, i.e. suppose they clamp the sextant
into a machine, and the machine swings the arm 10 degrees, measures the
deflection at the scope as being 10d "from" a target on a poster across the
room. Step and repeat nine times, at each step measuring to see if a target that
was 10d further down the poster was imaged correctly. Or any scenario, where
they swing the arm a measured number of degrees, and compare the results of that
swing--without actually engaging the gear teeth at all! You can, after all, use
the arm and read the scale without using the gear teeth that have been cut into
it. That would, in a sense, be measuring error on the arm and arm alone. Or,
they could check to see that every five degrees *indicated* on the arm, matched
a 5 degree swing of it. Again, without engaging the "trommel" on the teeth at
all.
Without knowing what they really are checking for, it is hard to say what they
actually measured, much less how or why that might still leave systemic errors.
"They can always say that their test is more precise than my test."
Undoubtedbly, but I have corresponded with them in the past and the brief
impression I got was of folks who take price in their craftsmanship--what we
used to call "professionalism" before that just meant "folks who got paid for
it". Or perhaps the folks at Robt.White in Boston would know more about this.
White & Co. are most definitely old world gentlemen in the best meaning of the
phrase, with experience in repair and calibration if I recall correctly. And no
axe to grind since they aren't Freiberger agents.
> > I cannot coat the eyepiece. The filter has to be detacheable.
"You propose that I go around with my sextant and a bottle of
some chemical. Cover the eyepiece with the chemical to measure
my index correction, then remove the coating to measure a Sun altitude,
then cover again etc.?"
Not quite! Nothing so rash. I'm suggesting that you can take a hole puncher
from a stationery store, punch out a small circle of the tint material, and
simply place it on the eyepiece lens. No chemicals needed, not even hydrogen
hydroxide. Just press the tint in place, at that substantially non-critical
point in the optical path that should be enough to let it work as a shade for
you. When you are done, just pluck it out and cast it away.
If you need a larger piece...a scissor will do. If the piece come out still
good, by all means, don't litter.<G>
"How to attach the film to the eyepiece?"
Same thing, just cut it out and plunk it in there. Fully exposed b&w film is
used to safely view solar eclipses, and no one bothers about trying to keep it
flat, etc., they just hold it up or stick it on something. Sometimes "rough
science" is good enough for the task at hand, I suspect it would be here.
> to let you try sun sights you mentioned you weren't able to try yet.
"The glass of the art horizon can be blackened with a candle."
Candle soot...well, that's traditional but do you really want to put a flame or
soot near your sextant?? At that point, barbarian that I am, I'd just take a
lens from a cheap plastic set of sunglasses, and clip it to the glass. (Scotch
tape and Velcro, the prize accomplishments of the 20th century. Hmmm.)
Or a polarizing filter for any similar sized camera lens, that'll be optical
quality and infinite control over the darkness for you.
"I am not "dedicated" to THAT extent."
Oh, just a hobbyist, eh?<VBG>
If you've got a high school with a metal shop someplace in the area, or an adult
vocational metal shop...I'd bet someone could make up a proper double threaded
eyepiece for you for $20. Double threaded, so you can sandwich a piece of filter
glass into it. They might be tickled pink to be fabricating sextant parts.
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
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From: hellos
Date: 2006 Jul 4, 18:43 -0500
Alex-
> Huh, so the companies are measuring the arc--
> and presumably certifying their own
> instruments based on that certification
> --without acutally measuring the *system*
> error which would include the teeth?
"I am not sure that I understand what you wanted to say here.
In a modern sextant "measuring the arc" is the same as
"measuring the teeth."
I could see them measuring by several ways, i.e. suppose they clamp the sextant
into a machine, and the machine swings the arm 10 degrees, measures the
deflection at the scope as being 10d "from" a target on a poster across the
room. Step and repeat nine times, at each step measuring to see if a target that
was 10d further down the poster was imaged correctly. Or any scenario, where
they swing the arm a measured number of degrees, and compare the results of that
swing--without actually engaging the gear teeth at all! You can, after all, use
the arm and read the scale without using the gear teeth that have been cut into
it. That would, in a sense, be measuring error on the arm and arm alone. Or,
they could check to see that every five degrees *indicated* on the arm, matched
a 5 degree swing of it. Again, without engaging the "trommel" on the teeth at
all.
Without knowing what they really are checking for, it is hard to say what they
actually measured, much less how or why that might still leave systemic errors.
"They can always say that their test is more precise than my test."
Undoubtedbly, but I have corresponded with them in the past and the brief
impression I got was of folks who take price in their craftsmanship--what we
used to call "professionalism" before that just meant "folks who got paid for
it". Or perhaps the folks at Robt.White in Boston would know more about this.
White & Co. are most definitely old world gentlemen in the best meaning of the
phrase, with experience in repair and calibration if I recall correctly. And no
axe to grind since they aren't Freiberger agents.
> > I cannot coat the eyepiece. The filter has to be detacheable.
"You propose that I go around with my sextant and a bottle of
some chemical. Cover the eyepiece with the chemical to measure
my index correction, then remove the coating to measure a Sun altitude,
then cover again etc.?"
Not quite! Nothing so rash. I'm suggesting that you can take a hole puncher
from a stationery store, punch out a small circle of the tint material, and
simply place it on the eyepiece lens. No chemicals needed, not even hydrogen
hydroxide. Just press the tint in place, at that substantially non-critical
point in the optical path that should be enough to let it work as a shade for
you. When you are done, just pluck it out and cast it away.
If you need a larger piece...a scissor will do. If the piece come out still
good, by all means, don't litter.<G>
"How to attach the film to the eyepiece?"
Same thing, just cut it out and plunk it in there. Fully exposed b&w film is
used to safely view solar eclipses, and no one bothers about trying to keep it
flat, etc., they just hold it up or stick it on something. Sometimes "rough
science" is good enough for the task at hand, I suspect it would be here.
> to let you try sun sights you mentioned you weren't able to try yet.
"The glass of the art horizon can be blackened with a candle."
Candle soot...well, that's traditional but do you really want to put a flame or
soot near your sextant?? At that point, barbarian that I am, I'd just take a
lens from a cheap plastic set of sunglasses, and clip it to the glass. (Scotch
tape and Velcro, the prize accomplishments of the 20th century. Hmmm.)
Or a polarizing filter for any similar sized camera lens, that'll be optical
quality and infinite control over the darkness for you.
"I am not "dedicated" to THAT extent."
Oh, just a hobbyist, eh?<VBG>
If you've got a high school with a metal shop someplace in the area, or an adult
vocational metal shop...I'd bet someone could make up a proper double threaded
eyepiece for you for $20. Double threaded, so you can sandwich a piece of filter
glass into it. They might be tickled pink to be fabricating sextant parts.
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
