Ultimate Sextant System?
Steven K. Roberts
celestial -- partly as a navigation backup, partly because it's
intrinsically poetic, and partly because, well, it's nifty gizmology. I've
been perusing the catalogs and looking at units, and am about to splurge on
a system. Thought I'd detail the package and solicit comments...
-----
Astra IIIB sextant ($395). Chinese, described as best value in a metal
sextant. Includes lighting of arc and drum. Question: the Celestaire
catalog offers the option of whole horizon or traditional split mirror --
does anyone have experience with BOTH and comments on which is best?
All-weather sealed hard-case for sextant and accessories ($79)
KVH Datascope with mounting bracket to allow use as the sextant's
telescope. This allows pushbutton time-stamping of sights, as well as
direct flux-gate azimuth indication for star-finding (OK, OK, and it's also
an intriguing high-tech/low-tech blend). ($360 plus $70 for the bracket).
Universal sight tube (no magnification) for rough weather and horizontal
angle measurement. ($19)
Prism Level, which eliminates swinging the arc by stealing a slice of the
horizon mirror and exaggerating any error off vertical (half-degree
accuracy). ($58)
Celesticomp V navigation computer, the whiz-bang do-all model that has
almanac for 173 bodies and does running fixes, DR, drift, sight reduction,
etc. ($279 plus $29 for waterproof case)
-----
With GPS, computer-aided celestial, and backup ability to do traditional
sights with short-form annual tables when the Celesticomp crashes, this
would seem a pretty robust package (especially for a coastal cruiser
<grin>). Any suggestions, warnings, or advice on product choices? Money
IS an object, so Plath is not an option...
Thanks,
Steve
PS: these prices are from the Celestaire catalog (800-727-9785). Robert
E. White Instruments (617-742-3045) seems a bit higher price on many items,
though they have other intriguing products.
Paul Kamen
weith for such a weight-sensitive type of vessel (okay, it's not as weight-
sensitive as the bicycle, but weight is still the major limitation).
Go for the top-of-the-line Davis plastic job. Looks and feels kinda cheap,
requires frequent checks of index error, and the plastic shades may introduce
a mile or two of additional error sometimes, but at least it's *light*.
I'm not saying it's not a pleasure to own a "real" sextant, but after you've
had your fun with it I think the weight budget will be better spent on more
useful equipment.
-"Call me Fishmeal"-
AG SHAW
: Even if money is *not* an object, I don't think a metal sextant is worth the
OK I dont know about any body else on this net but I think on
occasions people go over the top about wheight, and how much to cut
down on it, I know for my likings if I had spent a good amount of time
learning how to use a sextant and then taking a sight and finding my
possition I would want to make it pressise so I knew where I was as acuratly
as possible not just in a circle of 2 or 3 mile diameter of dout in case
you get "plastic shades".
This comment is not just about this sextant buisness its about all
the wheight saving that goes on, only just enough fuel, water and food for
the race only 1 spare set of cloths does it all not get abit silly?
Personally Im happyer pushing an extra few kilos hear and there to
know Im going to be warm, safe, fed and maybe even dry although I know that
is asking to much :-).
Gerrit
Peter D. Engels
K. Roberts) wrote:
> Despite the serious niftiness of my Motorola GPS receivers, I'm drawn to
> celestial -- partly as a navigation backup, partly because it's
> intrinsically poetic, and partly because, well, it's nifty gizmology. I've
> been perusing the catalogs and looking at units, and am about to splurge on
> a system. Thought I'd detail the package and solicit comments...
>
> -----
>
> Astra IIIB sextant ($395). Chinese, described as best value in a metal
> sextant. Includes lighting of arc and drum. Question: the Celestaire
> catalog offers the option of whole horizon or traditional split mirror --
> does anyone have experience with BOTH and comments on which is best?
>
> All-weather sealed hard-case for sextant and accessories ($79)
>
> KVH Datascope with mounting bracket to allow use as the sextant's
> telescope. This allows pushbutton time-stamping of sights, as well as
> direct flux-gate azimuth indication for star-finding (OK, OK, and it's also
> an intriguing high-tech/low-tech blend). ($360 plus $70 for the bracket).
I wonder about the fesaibility of using the KVH Datascope for star sights.
It has a 5x30 telescope, which will make the horizon difficult to find on
dim or misty conditions. Also, if this is intended partly as backup for
GPS (electronic) navigation, you are essentially substituting one
electronic gadget for another. Why not get a 4x40 'scope as well, for
those difficult viewing situations, etc. And by all means get the whole
horizon mirror; its much easier to use.
Pete
Mike Waters
> Despite the serious niftiness of my Motorola GPS receivers, I'm drawn to
> celestial -- partly as a navigation backup, partly because it's
> intrinsically poetic, and partly because, well, it's nifty gizmology. I've
> been perusing the catalogs and looking at units, and am about to splurge on
> a system. Thought I'd detail the package and solicit comments...
Well I am a EE working for Motorola and plan to take a Traxar GPS offshore
with me, but I *also* just passed my celestial certification as well.
GPS is great, but batteries can fail, the government can turn the system
off, and many charts were made from celestial observations and so are as
much as 4 miles off the GPS position (the Red Sea being one!).
As every NOAA chart says: "The prudent navigator never relies soley on any
single means of navigation."
> Astra IIIB sextant ($395). Chinese, described as best value in a metal
> sextant. Includes lighting of arc and drum. Question: the Celestaire
> catalog offers the option of whole horizon or traditional split mirror --
> does anyone have experience with BOTH and comments on which is best?
IMHO it is overkill. I have the top of the line Davis Sextant and
consistently get fixes within a mile of my actual (GPS) position.
Both split mirror and whole horizon work well, it is a matter of
preference.
I tried both and prefer the whole horizon 90% of the time. For a shot of
the moon on a hazy day in rough weather recently I could have used the
split mirror to give me better contrast, but I still got the shot within 3
miles (after 15 minutes of trying though). I was the only one on board
who even got a good shot though!
> KVH Datascope with mounting bracket to allow use as the sextant's
> telescope. This allows pushbutton time-stamping of sights, as well as
> direct flux-gate azimuth indication for star-finding (OK, OK, and it's also
> an intriguing high-tech/low-tech blend). ($360 plus $70 for the bracket).
Again overkill, also a favorite item for theives.
Spend the money on good courses on Coastal Navigation and Celestial so you
really know the subject. The instruments are secondary.
With a watch and your eyes you can find your position within 100 miles -
*iff* you have the knowledge!
> Prism Level, which eliminates swinging the arc by stealing a slice of the
> horizon mirror and exaggerating any error off vertical (half-degree
> accuracy). ($58)
Nice, but expensive and one more thing to adjust. The Davis one is a lot
cheaper and does the same job.
Get Baur's (sp) book on sextants, he does an excellent job in removing the
"mystique" and explaining how to adjust and use the instrument.
> Celesticomp V navigation computer, the whiz-bang do-all model that has
> almanac for 173 bodies and does running fixes, DR, drift, sight reduction,
> etc. ($279 plus $29 for waterproof case)
I own one without the waterproof case and am very happy with it. The
downside is that it is complex and you need to have "manual" procedures
"down pat" before it makes a lot of sense. If you just blindly enter
numbers you can foul things up very easily.
If you really know navigation though it can cut the workload and also get
sights which could not be used otherwise. A very useful tool, but still
just a tool!
> With GPS, computer-aided celestial, and backup ability to do traditional
> sights with short-form annual tables when the Celesticomp crashes, this
> would seem a pretty robust package (especially for a coastal cruiser
> <grin>). Any suggestions, warnings, or advice on product choices? Money
> IS an object, so Plath is not an option...
I assume the "short form" tables are HO-249? You should know how to use
all three (Ageton/HO211, HO-249, and HO229), but HO-249 with the Commercial
version of the Nautical Almanac are what I would recommend. 90% of yacht
courses will teach HO249.
>
> PS: these prices are from the Celestaire catalog (800-727-9785). Robert
> E. White Instruments (617-742-3045) seems a bit higher price on many items,
> though they have other intriguing products.
If you really want a metal sextant, look into a used one. I found that the
dealers were extremely helpful and willing to chat for extended periods (on
my phone bill :-), which was a great education on the subject. Most top
quality used sextants are under $500 and come with recertified accuracy.
According to one dealer, using a top quality sextant with HO229 from a
supertanker you should be able to get fixes within 200 yards - thats almost
as good at GPS! On a yacht at sea with HO249 and a Davis sextant you will
typically get within a mile or so, but generally that is all you need.
--
Mike Waters rcr...@email.mot.com AA...@KC7Y.PHX.AZ.US.NA
Paul Kamen
fed, and maybe even dry
Agreed. I'll confess that when I raced my 25' boat to Hawaii I had a metal
sextant on board, and a lot of other stuff that hurt the weight budget.
But that was my *primary* means of position-finding. This boat will probably
have GPS receivers in several devices, and other back-ups as well, and
ocean passages aren't even planned. Also, the "height of eye" for the
observations is going to be awful close to the wave tops, so that mile or
two of dgraded accuracy is lost in the noise from other error sources.
s...@vnet.ibm.com
>Despite the serious niftiness of my Motorola GPS receivers, I'm drawn to
>celestial -- partly as a navigation backup, partly because it's
>intrinsically poetic, and partly because, well, it's nifty gizmology. I've
>been perusing the catalogs and looking at units, and am about to splurge on
>a system. Thought I'd detail the package and solicit comments...
>
>-----
>
>Astra IIIB sextant ($395). Chinese, described as best value in a metal
>sextant. Includes lighting of arc and drum. Question: the Celestaire
>catalog offers the option of whole horizon or traditional split mirror --
>does anyone have experience with BOTH and comments on which is best?
Although I've not had a lot of celestial experience, I have tried
bot a full and split field sextant. The full field makes it a little
easier to find the body (you can't lose it behind the silvered half of
the mirror), but you lose a little light for the benefit, so doing
stars with a mushy horizon (technical term for various forms of
horizon haze, clouds, fog, etc ;-), is a little harder (you lose
at least 5% of the light for the half silver - full field.
The split field is a little harder to learn on, but after a
few hours of practice, it works fine. I have a split field.
I've also heard good things about the Astra IIIB
>
>All-weather sealed hard-case for sextant and accessories ($79)
>
>KVH Datascope with mounting bracket to allow use as the sextant's
>telescope. This allows pushbutton time-stamping of sights, as well as
>direct flux-gate azimuth indication for star-finding (OK, OK, and it's also
>an intriguing high-tech/low-tech blend). ($360 plus $70 for the bracket).
Love those toys! I have a Casio dive watch for time (+.75 sec/day).
>
>Universal sight tube (no magnification) for rough weather and horizontal
>angle measurement. ($19)
Never been in *killer* seas with a sextant, but the 3x has never
been a problem in average stuff in a very little boat.
>
>Prism Level, which eliminates swinging the arc by stealing a slice of the
>horizon mirror and exaggerating any error off vertical (half-degree
>accuracy). ($58)
W. F. Buckley hated this in use after loving the thought (read Airborne).
I've never used one.
>
>Celesticomp V navigation computer, the whiz-bang do-all model that has
>almanac for 173 bodies and does running fixes, DR, drift, sight reduction,
>etc. ($279 plus $29 for waterproof case)
>
>-----
>
>With GPS, computer-aided celestial, and backup ability to do traditional
>sights with short-form annual tables when the Celesticomp crashes, this
>would seem a pretty robust package (especially for a coastal cruiser
><grin>). Any suggestions, warnings, or advice on product choices? Money
>IS an object, so Plath is not an option...
>
>Thanks,
>Steve
>
>
>PS: these prices are from the Celestaire catalog (800-727-9785). Robert
>E. White Instruments (617-742-3045) seems a bit higher price on many items,
>though they have other intriguing products.
Happy starshooting!
Steve Steve Weingart S...@VNET.IBM.COM
IBM, Boca Raton, Fl.
I wouldn't take credit for IBM's opinions, please don't give them
credit for mine ;-)
Steven K. Roberts
Kamen) wrote:
>
> Even if money is *not* an object, I don't think a metal sextant is worth the
> sensitive as the bicycle, but weight is still the major limitation).
<sigh> I know. I keep getting seduced by wondrous gadgets and new
learning curves, and celestial offers delights in both areas. I should
remember my own adage, the Roberts Law of Applied Mobile Gizmology: "An
infinite number of very light things, taken together, becomes infinitely
heavy."
> Go for the top-of-the-line Davis plastic job. Looks and feels kinda cheap,
> requires frequent checks of index error, and the plastic shades may introduce
> a mile or two of additional error sometimes, but at least it's *light*.
Yes... the model 25 is about half the price of an Astra ($175 from
Celestaire, $199 at West, and $236 at White). It is, after all, a backup
for GPS, of which I have two. <sigh> You guys just won't let me blow my
budget on the best toys, willya?
> I'm not saying it's not a pleasure to own a "real" sextant, but after you've
> had your fun with it I think the weight budget will be better spent on more
> useful equipment.
You mean... like anchors? And the CD library?
Thanks for the reality check...
Steve
Roy Smith
an expert on sextant use, but my experience is that the prism level gizmos
aren't worth the effort. I found it just another annoying thing you have to
keep adjusted, protect from breaking, clean, etc. I found swinging the
sextant to find vertical easier than trying to use the prism.
--
Roy Smith <r...@nyu.edu>
Hippocrates Project, Department of Microbiology, Coles 202
NYU School of Medicine, 550 First Avenue, New York, NY 10016
"This never happened to Bart Simpson."
Mike Waters
(AG SHAW) wrote:
> Paul Kamen (fish...@netcom.com) wrote:
> : Even if money is *not* an object, I don't think a metal sextant is worth the
> : weith for such a weight-sensitive type of vessel (okay, it's not as weight-
> : sensitive as the bicycle, but weight is still the major limitation).
> OK I dont know about any body else on this net but I think on
> occasions people go over the top about wheight, and how much to cut
> down on it,
To me the weight argument is not with the boat weight (I agree with you
there), but with the amount of weight you have to hold up to your face with
one hand as you take the sight! After ten minutes even the plastic sextant
gets to be heavy!
>I know for my likings if I had spent a good amount of time
> learning how to use a sextant and then taking a sight and finding my
> possition I would want to make it pressise so I knew where I was as acuratly
> as possible not just in a circle of 2 or 3 mile diameter of dout in case
> you get "plastic shades".
The reality is that you aren't going to get much closer taking sights from
a tossing boat anyway. Nothing stops you from being a purist with a metal
sextant, ultra precise chronometer, assistant to mark the precise sight
time and use the HO229 tables (well ok the UK version :-), but you really
won't get much better accuracy on a small boat!
> This comment is not just about this sextant buisness its about all
> the wheight saving that goes on, only just enough fuel, water and food for
> the race only 1 spare set of cloths does it all not get abit silly?
I agree, but when someone wins a 2000 mile race by minutes then the
competitors will get into that kind of extremes. Some of it helps with the
phsycing up the team too.
> Personally Im happyer pushing an extra few kilos hear and there to
> know Im going to be warm, safe, fed and maybe even dry although I know that
> is asking to much :-).
Well me too, but I'm a cruiser not a racer!
Paul Kamen
small and light book of logarithmic trig tables. The instructions are
nebulous, though. You need help from Letcher's book _Self Contained
Celestial Navigation_ to figure it out. Then make your own work sheets, and
you're all set.
I've carried that little HO 208 book and work forms on every offshore trip
I've made, but never had to use it. First backup to the dedicated nav
computer might as well be one of several DOS (or MAC) based sight reduction
programs that will run on the ship's main system. For that matter, why buy
a dedicated nav computer at all? Something like an HP95 or 100 could also
run that DOS nav program, and serve as a backup for multiple other functions
at the same time. (I don't like the Davis program very much, btw, but there's
one called PC*NAV (or something like that) that's much better.
Uncle Dragon at ronin
> OK I dont know about any body else on this net but I think on
> occasions people go over the top about wheight, and how much to cut
>down on it, I know for my likings if I had spent a good amount of time
> learning how to use a sextant and then taking a sight and finding my
>possition I would want to make it pressise so I knew where I was as acuratly
> as possible not just in a circle of 2 or 3 mile diameter of dout in case
> you get "plastic shades".
You must realize that boat we are talking about here is little more than
three kayak hulls, if that. I really expect that space will more of a
consideration than weight. Secondly, I question if the sextant is at all
necessary since the boat will be doing coastal cruising. The only time
that one is truely lost in coastal cruising is when the visibility is
less than the distance from shore, in which case, the chance of doing
the necessary sights and reductions is much less. I would put the space
set aside for the sextant into stowing a good fog horn. The danger from
other boats is much greater than of being lost. If the GPS fails, it
is probably because it is at the bottom of the bay and getting to shore
will be a much greater concern than who owns the shoreline.
--
\^___^/ ===================================================
(o o) Uncle Dragon: b...@ronin.com || b...@bytepb.byte.com
\|/ Peterborough, New Hampshire Live Free or Die
m ===================================================
Uncle Dragon at ronin
>But that was my *primary* means of position-finding. This boat will probably
>have GPS receivers in several devices, and other back-ups as well, and
>ocean passages aren't even planned. Also, the "height of eye" for the
>observations is going to be awful close to the wave tops, so that mile or
>two of dgraded accuracy is lost in the noise from other error sources.
Totally accurate in my view. Not only that, but since this craft will
be riding the waves rather than displacing them, even standing up on
deck (if there is one) will be a trick. The boat will be bobbing not
rolling.
Mike Waters
> My favorite "short form" sight reduction method is HO 208, which is a very
> small and light book of logarithmic trig tables. The instructions are
> nebulous, though. You need help from Letcher's book _Self Contained
> Celestial Navigation_ to figure it out. Then make your own work sheets, and
> you're all set.
Sounds a lot like the HO211 Ageton's method as found in the commercial
version of the Nautical Almanac. What is the difference?
To me the nicest thing about the Ageeton's method is that it *is* included
with the Almanac - that way you only carry one book with everything. I
have talked to people who prefer that method because it requires fewer
lookups (read time below trying to see the text as it all moves around :-),
but I still find the extra calculation more awkward than HO249.
> I've carried that little HO 208 book and work forms on every offshore trip
> I've made, but never had to use it.
I like to reduce all sights twice - once using the tables and check it with
my Celesticomp. After I get more experience I may slack off some, but at
present I only get the manual reduction correct first try only about 60% of
the time at sea.
> First backup to the dedicated nav
> computer might as well be one of several DOS (or MAC) based sight reduction
> programs that will run on the ship's main system. For that matter, why buy
> a dedicated nav computer at all? Something like an HP95 or 100 could also
> run that DOS nav program, and serve as a backup for multiple other functions
> at the same time. (I don't like the Davis program very much, btw, but there's
> one called PC*NAV (or something like that) that's much better.
My estimate is that losing ship's power is the most likely cause of GPS
failure, so it makes no sense to have your backup affected by the same
failure mode.
I like the redundancy of having something that is totally independent of
everything else including ship's power. On my own boat BTW I plan to take
a laptop with a DOS based nav. program as well.
I don't know about PC*NAV, but the Celesticomp does a whole lot more than
the Davis program. Things like speed/time/distance, rhumb line/great
circle calculations, height of tide at a given time, compass check etc.
About the only thing the Davis program gives you that the Celesticomp
doesn't is a plot of your LOPs. I like to do that if only to visualize the
errors.
JOHN S DREWERY
You can derive it easily from sperical trig. It gets you Ho (without
the need to round the LHA) and Z. Doing the Longpac recently, I had
the formula programmed into a calculator (which was sealed in a
plastic bag) and all worked out well.
But I suppose that's just a convenience - the calculator could fail, so I
carried the tables anyway.
-JD.
Paul Kamen
that would render all solid-state electronic divices kaput. (maybe this
was parnoia, but the cold war was still on and it was hard to get
reliable info.) Of course my digital watch would stop also, so the plan
was to use HO 208 and the method of lunar LOPs I described erlier to find
longitude. That and keeping some extra drinking water in my appartment is
about as far into the survivalist mentality as I ever got.
Steven K. Roberts
OK, I think I've revised the "ultimate sextant system" wish list to
something more modest and reasonable after integrating all this advice and
a bit more via email.
The Astra IIIB -- even though the Davis Mk 25 gets good solid reviews, I've
heard often enough that it A) doesn't give you that quality FEEL of a metal
instrument and B) loses accuracy from heat, etc. The weight is admittedly
an issue, but more the weight of use than that of stowage... but I love
quality tools. I'll probably choose the full mirror, since it is reputedly
much easier for beginners and I probably won't be using this very often
anyway (as long as GPS works). I'll probably skip the prism level as
unnecessary expense, and save the KVH Datascope until after all the real
essentials have been purchased.
Finally, I'll learn with traditional methods (manual reduction), port them
to my HP 48SX or 95LX, then buy myself a Celesticomp someday as a treat. I
agree with the comment that using the ship's computer to do the nav is
unwise when this is a backup mode to begin with. (There will be two GPS
receivers, BTW -- one built-in on the boatnet, the other the Traxar+
handheld in the Abandon Ship bag.)
Hey, I just saved HUNDREDS of dollars! <grin> The motives for acquiring
Celestial capability are twofold: backup and education. Backup, in the
kinds of conditions I expect, is probably going to be a very infrequent
need. But the educational value of understanding the rhythms of the
universe is perfectly in synch with my other motives for this adventure,
and alone justifies the expense of celestial. To that end, automating it
overmuch is at cross purposes with the original goal (though avoiding
arithmetical drudgery is quite acceptable).
By the way, Ben's comments on the size of the Microship remind me that my
Nomadness Notes postings are a bit out of date. It's not really a kayak
anymore... the outriggers are kayaks, and are still detachable, but even
those will probably have to be doubles to provide enough bouyancy to
prevent tripping over a submerged float. The center hull is 28' long and
4' beam... it's a kayacht.
One more thing. Someone passed along the wise sentiment that no boat
should have only one means of navigation, and that reminds me -- in the
literature (Tania Aebi and various others) I see references to RDF's. Are
they still in active use and well supported, and is there any reason to
have one these days with GPS so well-established? Is the hardware smaller
than the boxy unit pictured in Chapman's? RDF seems redundant now, given
GPS and charts, but I've never seen it lambasted as obsolete...
Cheers...
Steve
Fabbian G. Dufoe, III
>Go for the top-of-the-line Davis plastic job. Looks and feels kinda cheap,
>requires frequent checks of index error, and the plastic shades may introduce
>a mile or two of additional error sometimes, but at least it's *light*.
The expensive plastic sextants by Davis and Ebco come off pretty
poorly when compared to metal sextants in reviews I've read. If you're
going to go with a plastic sextant, get the cheap Davis. It's good enough
for sun sights and will provide about as much accuracy as you can expect
from the small platform the Microship will provide.
Any plastic sextant warps when exposed to the sun. The plastic
warms up and throws off your index setting. I've found it a good practice
with the cheap Davis (I believe it's the Mark III) sextant to keep it out
of the sun as much as possible, let it stabilize to ambient temperature
before making observations, and adjust the mirrors to eliminate index
error before taking a round of sights.
--
--Fabbian G.Dufoe, III
350 Ling-A-Mor Terrace South |
St. Petersburg, FL 33705 | internet: fgd3%ni...@palan.palantir.com
813-823-2350 | uucp: ...palan!deep6!nifty!fgd3
Mike Waters
III) wrote:
> In <fishmealC...@netcom.com> fish...@netcom.com (Paul Kamen) writes:
> >Go for the top-of-the-line Davis plastic job. Looks and feels kinda cheap,
> >requires frequent checks of index error, and the plastic shades may introduce
> >a mile or two of additional error sometimes, but at least it's *light*.
>
> The expensive plastic sextants by Davis and Ebco come off pretty
> poorly when compared to metal sextants in reviews I've read. If you're
> going to go with a plastic sextant, get the cheap Davis. It's good enough
> for sun sights and will provide about as much accuracy as you can expect
> from the small platform the Microship will provide.
I *hope* you don't mean the $30 "lifeboat" sextant that Davis makes! That
really *is* a minimal system lacking telescope, vernier dial and most of
the adjustments! They do produce three versions of their "regular" sextant
which are just about equivalent for most purposes.
> Any plastic sextant warps when exposed to the sun. The plastic
> warms up and throws off your index setting. I've found it a good practice
> with the cheap Davis (I believe it's the Mark III) sextant to keep it out
> of the sun as much as possible, let it stabilize to ambient temperature
> before making observations, and adjust the mirrors to eliminate index
> error before taking a round of sights.
No different here from a metal sextant! You should *always* check index
error on any sextant before using it.
Peter Smith
>Despite the serious niftiness of my Motorola GPS receivers, I'm drawn to
>celestial -- partly as a navigation backup, partly because it's
>intrinsically poetic, and partly because, well, it's nifty gizmology.
>whole horizon or traditional split mirror?
Go with the split-image horizon mirror. The single feature that will make
taking good sights easier and more consistant is a sharp horizon. The
double-image mirrors make it easier to get the star down to the horizon,
but don't transmit as sharp a horizon for the final adjustments. Practice
will help with the former, but image quality is a given. Get the biggest
split-image mirror you can and practice, practice, practice.
>All-weather sealed hard-case for sextant and accessories ($79)
I wouldn't. The Astra comes with a decent fitted wooden box which should
protect the instrument from all but the most violent abuse. The foam-lined,
poly-shell cases are huge and to my mind, overkill. Make a place for your
sextant box where it won't fly loose in a knockdown and you'll be fine.
>KVH Datascope...
I'd rather spend this money on a better sextant (R.H.White used to sell
reconsitioned instuments at decent prices), but if this will make celestial
more fun, that's half of why you're doing this, right? Have a ball and
let us know how reliable the "elecric telescope" turns out to be.
---
Peter Smith -- Data General Corp., Westboro, MA
Peter...@dgc.ceo.dg.com or psm...@dg-webo.webo.dg.com
#include <std_disclaimer.h>
Fabbian G. Dufoe, III
>One more thing. Someone passed along the wise sentiment that no boat
>should have only one means of navigation, and that reminds me -- in the
>literature (Tania Aebi and various others) I see references to RDF's. Are
>they still in active use and well supported, and is there any reason to
>have one these days with GPS so well-established? Is the hardware smaller
>than the boxy unit pictured in Chapman's? RDF seems redundant now, given
>GPS and charts, but I've never seen it lambasted as obsolete...
There are a couple of hand-help RDFs floating around, although I
don't know if they are in current production. One of the best is the
Lo-Kata. It includes a digitally tuned radio direction finder and a Sestrel
hand bearing compass in a package you can easily hold in one hand. You
listen for the null with a pair of headphones. The radio receiver is of
excellent quality and the compass is very nice. You could use it for
hand bearings even without the radio.
RDFs have the advantage that you get closer to the radio
beacon the line of position it gives you becomes more accurate. RDF fixes
at sea aren't terribly accurate, but at sea you generally don't need to
know your position as precisely as when you are near hazards.
Whether it is worth the expense and space to carry an RDF is
questionable, though. There are a number of general coverage radio
receivers (the Sangean ATS-803A, for example) which cover all the AM
frequencies between 150 KHz and 30 MHz. It's easy to use one of those
radios to tune in a beacon and find a null. You'll have a little harder
time determining a compass bearing if you want to get an accurate fix, but
it works fine for homing in on a beacon. The general coverage receiver
has the advantage that you can also listen to it for news and entertainment.
Dave Hazen
>
>I'd rather spend this money on a better sextant (R.H.White used to sell
>reconsitioned instuments at decent prices), but if this will make celestial
>more fun, that's half of why you're doing this, right? Have a ball and
>let us know how reliable the "elecric telescope" turns out to be.
>---
Does anyone have an address for R. H. White? (I think I saw one go by
a month or so ago..)
I am taking the Power Squadron JN course this fall and need a sextant.
They are recommending Davis Mark 25's but I would prefer something in
metal, if possible without arranging a second mortgage.
Thoughts appreciated. Will attempt to summarize.
dave
--
Dave Hazen InterNet: ha...@open.dal.ca
Dept. of Oceanography Telemail: dalhousie.ocean
Dalhousie University Voice: (902) 494-3396
Halifax, NS CANADA B3H 4J1 FAX: (902) 494-3877
William Pope
In article <CDGAG...@cs.dal.ca> ha...@server.open.dal.ca (Dave Hazen)
writes:
Does anyone have an address for R. H. White? (I think I saw one go by
a month or so ago..)
That's E. as in Eldridge
R. E. White Instruments Inc.
34 Commercial Wharf
Boston, MA
617 742 3045
--
============================================================================
William Z. Pope Phone: +1 (617) 621-8889
Open Software Foundation Fax: +1 (617) 621-0584
11 Cambridge Center "Dance, Music, Entertainment" Email: zp...@osf.org
Cambridge, MA 02142 OSF DME Integration Project
Paul Kamen
I seem to remember reading somewhere that the old long-wave RDF radio
beacon system is finally being phased out. No great loss, imho, what wiht
cheap GPS on the way. However, one of the most useful features of the
RDF was its ability to take bearing on broadcast AM radio signals (the
"Lo-Kata" and other hand-helds couln't do this - they only tuned in
the long-wave beacons).
The product I'd like to see is a $20 AM radio with a directional antenna
on top. This would still be a lot cheaper than GPS (at least for another
couple of years) and be useful in all sorts of situations (finding the
Hawaiian Islands - even finding the *right* Hawaiian island, for example).
Aeronautical charts, btw, have the most complete list of AM stations and
their frequencies and locations.
Fabbian G. Dufoe, III
>In article <6WqksAH...@nifty.UUCP>, fg...@nifty.UUCP (Fabbian G. Dufoe,
>III) wrote:
>> The expensive plastic sextants by Davis and Ebco come off pretty
>> poorly when compared to metal sextants in reviews I've read. If you're
>> going to go with a plastic sextant, get the cheap Davis. It's good enough
>> for sun sights and will provide about as much accuracy as you can expect
>> from the small platform the Microship will provide.
>I *hope* you don't mean the $30 "lifeboat" sextant that Davis makes! That
>really *is* a minimal system lacking telescope, vernier dial and most of
>the adjustments! They do produce three versions of their "regular" sextant
>which are just about equivalent for most purposes.
That's the one. The reviews I've read over the years indicate the
more expensive plastic sextants, for all their added features, just don't
measure up. The price performance ratio of the $30 sextant is better than
that of the top of the line plastics. The price performance ratio of good
metal sextants (like the Tamaya Jupiter) is better still.
>> Any plastic sextant warps when exposed to the sun. The plastic
>No different here from a metal sextant! You should *always* check index
>error on any sextant before using it.
It has been my experience that the plastic sextant is considerably
more responsive to temperature changes than a good metal sextant.
Paul Kamen
were $12. (Still have it, actually, and it's crossed the Atlantic a few
times since then on a containership.) One day when I must have been bored
I took apart some hopelessly-out-of-alingment binoculars, and the result was
a 7X scope taped to the $12 sextant. The amazing thing was, if I took time
to read the vernier scale carefully, it was accurate to within 2 minutes of
arc, probably even better than that. So for small craft use, accuracy is
simply not a good reason to avoid plastic. Even with a good metal sextant on
big ocean racer, I often prefer the plastic for sun shots because of arm
fatigue. In low light, though, the better optics of the "real" instrument take
over. Also prefer full-field for sun, half-silvered for stars. Tried the
horizon-leveling prism and didn't like it. But mileage varies on all this.
Mike Waters
III) wrote:
> In <rcrw90-140993083645@node_142cf.aieg.mot.com> rcr...@email.mot.com (Mike Waters) writes:
> >In article <6WqksAH...@nifty.UUCP>, fg...@nifty.UUCP (Fabbian G. Dufoe,
> >III) wrote:
> >> The expensive plastic sextants by Davis and Ebco come off pretty
> >> poorly when compared to metal sextants in reviews I've read. If you're
> >> going to go with a plastic sextant, get the cheap Davis. It's good enough
> >> for sun sights and will provide about as much accuracy as you can expect
> >> from the small platform the Microship will provide.
>
> >I *hope* you don't mean the $30 "lifeboat" sextant that Davis makes! That
> >really *is* a minimal system lacking telescope, vernier dial and most of
> >the adjustments! They do produce three versions of their "regular" sextant
> >which are just about equivalent for most purposes.
>
> That's the one. The reviews I've read over the years indicate the
> more expensive plastic sextants, for all their added features, just don't
> measure up. The price performance ratio of the $30 sextant is better than
> that of the top of the line plastics. The price performance ratio of good
> metal sextants (like the Tamaya Jupiter) is better still.
I don't want to get into a flame war over plastic vs metal sextants, but I
disagree with you, especially on the $30 sextant. Even Davis only
advertises it as a "practice" sextant!
My experience with the Davis Mark 25 has been very good, typical sights
have been within .5 min of arc compared to metal sextants when sailing off
SF this fall. The Davis 15 and 25 are made from a composite material which
minimizes the temerature variation. Of course is you leave it on the front
seat of a closed car you will get problem, but you will get problems with a
metal sextant too!
I bought the Davis 25 after recommendations from several sextant dealers,
as well as some very experienced cruisers. I have not regreted it.
If you *do* buy a metal sextant, then look at a good used one. They can be
recertified to "like new" (reputable dealers will routinely do this), and
cost much the same as the Davis Mark 25 new. I wouldn't buy a plastic
sextant used BTW, the metal definitly wears better.
To anyone looking to start using a sextant, I highly recommend Bruce Baur's
(sp) book on Sextants. He even lists about 50 sextant dealers worldwide.
There is *no* magic to a sextant, it is no more than a tool for measuring
angles with great precision. You can always pay more money for more
precision, the real question is how much do you really want to pay for.
the difference between 1nm and .5 nm appears to be about 5x in price, for
about 20X you can get to 2-300 yards - on a big ship!
Capt'n Morgan
Capt'n Morgan
.