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Re: Question about Davis Mk 25 sextant beam converger
From: Greg R_
Date: 2006 May 13, 11:28 -0700
> GregR wrote-
>
> "Fred: I hear you on being careful not to
> "over-adjust" it for errors, but I did need
> to get a good initial calibration (something that
> I couldn't seem to do in the backyard),
> and from what I understand plastic sextants are a
> lot more susceptible to IE than
> the metal ones. In fact, the Davis instruction
> manual seems to recommend adjusting
> it whenever it's used ("Adjusting your sextant is
> easy and should be done each time
> it is used."). Maybe that's why they put knurled
> adjustment screws on it instead of
> regular screws?"
>
> There are a number of things wrong with what GregR
> says here.
>
> First, in no way does such a zero adjustment get
> any sort of "calibration" of the sextant, which
> would require a check, by some means, of the arc
> divisions right around the scale, not just at the
> zero point. In general, one normally has to accept
> that the initial calibration, of the markings
> around the scale, is approximately correct.
> Certain sextants will have had that calibration
> checked by some authority, in which case there
> will be a table of known errors at different
> angles tacked to the inside of the box. It is
> possible, but rather difficult, for a user to make
> his own calibration checks by star-to-star angles,
> as Alex Eremenko, in particular, has written about
> in the list's archives. All such calibrations are
> made on the understanding that any index offset
> has been allowed-for first. You will therefore
> never find any offset noted for zero degrees, in a
> calibration certificate.
>
> Second, obtaining zero index offset, to high
> accuracy, by adjusting the fiddly mirror screws is
> a difficult task, nearly an impossible one. When
> you have made your best attempt, what do you do
> next? You check it out, by viewing a distant body,
> to see what (exactly) the scale reads when the
> images align. What if they don't exactly align at
> zero on the scale, but at a small offset of the
> order of a minute or so? Do you go back and do
> another round of mirror adjustments? No, you just
> accept what you get, and allow for that initial
> offset by a simple bit of arithmetic. In fact, no
> matter how big the zero-offset is, it does not
> affect the result in any way, as long as it's
> allowed for. What's wrong here is the mind-set, in
> thinking of such an offset as a sextant "error",
> which has to be minimised or zeroed-out if the
> sextant is to do its job properly. Nothing of the
> sort.
>
> Third, it's true that plastics expand with heat
> more than metals, and this, depending on the
> design details may make the zero offset less
> stable. I have not found that to affect the
> performance of my own plastic sextant (Ebbco), in
> practice. I always check the zero before a set of
> observations, and after, and sometimes
> interspersed between observations, but never find
> a significant change. For other plastic sextants,
> I have seen accounts of significant short-term
> intability, and wonder if that may be related to
> looseness somewhere in the mirror mountings. I am
> careful, when I am using the sextant, not to put
> it down where a shadow-edge falls across the
> mirror mountings, to avoid thermal gradients in a
> sensitive spot, but am no more careful than that.
>
> Fourth, if a user keeps readjusting the zero
> offset by tweaking the screws, he has lost the
> opportunity to monitor the stability of his
> sextant by noting such changes.
> If he records it, and allows for it, never
> adjusting it, he will get a feel for its long-term
> reliability.
>
> Fifth, what's been missed is how quick-and-easy
> such an index check is. Simply the work of a
> moment, to point to the horizon, or Sun or star,
> align the images, note the reading. Much less
> time-consuming than the business of tweak, then
> look, then delicate tweak again, that is involved
> in trying to zero it out. It's easy to check it
> before and after, and between, altitude
> observations.
>
> A trick of the old-salt navigators, once they had
> adjusted any tilt out of their mirrors, and got
> the zero roughly right, was to introduce a spot of
> salt-water, or urine, to initiate a bit of
> corrosion of the adjustments, so that they would
> never be touched again.
>
> GregR is new to the sextant game, but he will
> learn as he goes.
>
> =================
> He added-
>
> "BTW, don't know if this has been done before, but
> I came up with a slightly
> off-the-wall method of "faking" a horizon since
> the sun's dec is now too high
> to get a LAN shot with an artificial horizon at my
> latitude (34°14.9' N):
>
> I took a length of surveyor's string and attached
> one end to the side of the
> house at my eye height, with the other end
> attached to a tripod also set at
> my eye height (and checked for horizontal with a
> carpenter's line level).
>
> From across the backyard (~20') I'm able to get
> "reasonable" LOPs - the
> intercepts on those are running anywhere from 3.7
> to 8 miles (though I do
> get the occasional one that's way out of the
> ballpark, so this method isn't
> perfect. In fact, I can induce a several-minute
> error by slouching slightly
> vs. standing up straight).
>
> Even if it's not accurate enough for real
> navigation, it did serve its purpose
> in giving me something to practice bringing sights
> down with (and it's long
> enough to be able to rock the sextant to find true
> vertical). Now that I've got
> that part down, time to work on improving the
> accuracy with a real horizon. :-)"
>
> ===========
> Fair enough, as long as GregR is aware of the
> inaccuracies involved. Bending his knees to change
> his height by one inch will change the angle by
> about a quarter-degree, which would be enough to
> shift his position by about 15 miles
>
> --
> GregR
>
From: Greg R_
Date: 2006 May 13, 11:28 -0700
George wrote:
> First, in no way does such a zero adjustment
get
> any sort of "calibration" of the sextant,
> any sort of "calibration" of the sextant,
Just for my own further education (and not
wanting to duke it out with you over semantics, it's a lot more fun to do that
over a sextant auction... ;-)), what is the "proper" term for the initial
accuracy checkout of a sextant when it's first acquired?
I probably used "calibration" in the wrong
context here (wasn't referring to an optical lab collimation at all), but what
would you call the initial "shakeout cruise" testing - or is there even such a
thing?
--
Thanks,
GregR
----- Original Message -----
From: "George Huxtable" <george@HUXTABLE.U-NET.COM>
Sent: Monday, May 08, 2006 10:39 AM
Subject: Re: Question about Davis Mk 25 sextant
beam converger
>
> "Fred: I hear you on being careful not to
> "over-adjust" it for errors, but I did need
> to get a good initial calibration (something that
> I couldn't seem to do in the backyard),
> and from what I understand plastic sextants are a
> lot more susceptible to IE than
> the metal ones. In fact, the Davis instruction
> manual seems to recommend adjusting
> it whenever it's used ("Adjusting your sextant is
> easy and should be done each time
> it is used."). Maybe that's why they put knurled
> adjustment screws on it instead of
> regular screws?"
>
> There are a number of things wrong with what GregR
> says here.
>
> First, in no way does such a zero adjustment get
> any sort of "calibration" of the sextant, which
> would require a check, by some means, of the arc
> divisions right around the scale, not just at the
> zero point. In general, one normally has to accept
> that the initial calibration, of the markings
> around the scale, is approximately correct.
> Certain sextants will have had that calibration
> checked by some authority, in which case there
> will be a table of known errors at different
> angles tacked to the inside of the box. It is
> possible, but rather difficult, for a user to make
> his own calibration checks by star-to-star angles,
> as Alex Eremenko, in particular, has written about
> in the list's archives. All such calibrations are
> made on the understanding that any index offset
> has been allowed-for first. You will therefore
> never find any offset noted for zero degrees, in a
> calibration certificate.
>
> Second, obtaining zero index offset, to high
> accuracy, by adjusting the fiddly mirror screws is
> a difficult task, nearly an impossible one. When
> you have made your best attempt, what do you do
> next? You check it out, by viewing a distant body,
> to see what (exactly) the scale reads when the
> images align. What if they don't exactly align at
> zero on the scale, but at a small offset of the
> order of a minute or so? Do you go back and do
> another round of mirror adjustments? No, you just
> accept what you get, and allow for that initial
> offset by a simple bit of arithmetic. In fact, no
> matter how big the zero-offset is, it does not
> affect the result in any way, as long as it's
> allowed for. What's wrong here is the mind-set, in
> thinking of such an offset as a sextant "error",
> which has to be minimised or zeroed-out if the
> sextant is to do its job properly. Nothing of the
> sort.
>
> Third, it's true that plastics expand with heat
> more than metals, and this, depending on the
> design details may make the zero offset less
> stable. I have not found that to affect the
> performance of my own plastic sextant (Ebbco), in
> practice. I always check the zero before a set of
> observations, and after, and sometimes
> interspersed between observations, but never find
> a significant change. For other plastic sextants,
> I have seen accounts of significant short-term
> intability, and wonder if that may be related to
> looseness somewhere in the mirror mountings. I am
> careful, when I am using the sextant, not to put
> it down where a shadow-edge falls across the
> mirror mountings, to avoid thermal gradients in a
> sensitive spot, but am no more careful than that.
>
> Fourth, if a user keeps readjusting the zero
> offset by tweaking the screws, he has lost the
> opportunity to monitor the stability of his
> sextant by noting such changes.
> If he records it, and allows for it, never
> adjusting it, he will get a feel for its long-term
> reliability.
>
> Fifth, what's been missed is how quick-and-easy
> such an index check is. Simply the work of a
> moment, to point to the horizon, or Sun or star,
> align the images, note the reading. Much less
> time-consuming than the business of tweak, then
> look, then delicate tweak again, that is involved
> in trying to zero it out. It's easy to check it
> before and after, and between, altitude
> observations.
>
> A trick of the old-salt navigators, once they had
> adjusted any tilt out of their mirrors, and got
> the zero roughly right, was to introduce a spot of
> salt-water, or urine, to initiate a bit of
> corrosion of the adjustments, so that they would
> never be touched again.
>
> GregR is new to the sextant game, but he will
> learn as he goes.
>
> =================
> He added-
>
> "BTW, don't know if this has been done before, but
> I came up with a slightly
> off-the-wall method of "faking" a horizon since
> the sun's dec is now too high
> to get a LAN shot with an artificial horizon at my
> latitude (34°14.9' N):
>
> I took a length of surveyor's string and attached
> one end to the side of the
> house at my eye height, with the other end
> attached to a tripod also set at
> my eye height (and checked for horizontal with a
> carpenter's line level).
>
> From across the backyard (~20') I'm able to get
> "reasonable" LOPs - the
> intercepts on those are running anywhere from 3.7
> to 8 miles (though I do
> get the occasional one that's way out of the
> ballpark, so this method isn't
> perfect. In fact, I can induce a several-minute
> error by slouching slightly
> vs. standing up straight).
>
> Even if it's not accurate enough for real
> navigation, it did serve its purpose
> in giving me something to practice bringing sights
> down with (and it's long
> enough to be able to rock the sextant to find true
> vertical). Now that I've got
> that part down, time to work on improving the
> accuracy with a real horizon. :-)"
>
> ===========
> Fair enough, as long as GregR is aware of the
> inaccuracies involved. Bending his knees to change
> his height by one inch will change the angle by
> about a quarter-degree, which would be enough to
> shift his position by about 15 miles
>
> --
> GregR
>
