NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: sextant for use on land
From: Robert Eno
Date: 2006 Aug 13, 09:10 -0500
If you can, get hold of an RAE Mk IX-A bubble sextant. It is simply the best
aircraft bubble sextant ever made. The bubble system and in particular, the
way in which it is illuminated is second to none. The only drawback of the
MK IX-A is that you have no way to determine index/instrument error except
to take dozens of careful observations from a known location and try to
determine a constant error (if any). I have three MK IX's. I got lucky on my
first one: no errors, however in the others there are large errors.
Unfortunately, I know of no one who can repair or adjust these instruments.
It took special equipment to do this. I recall many years ago that
Celestaire sold MK IXs but I have not seen them in their catalogue since the
early 1990's.
I can only comment on the periscopic and A-10 sextants, both of which I
would not recommend only because of mechanical problems associated with
them. The periscopic sextant is, understandably, awkward to hold and the
reflecting pellicle must be kept bone dry or it will disintegrate. I have
yet to find a replacement pellicle for mine. The A-10 is a beautiful,
compact instrument and were it not for the pain-in-arse bubble system, I
would be more kindly disposed towards it. I still have mine and would not
part with it, however, I seldom use it for its intended purpose. It is just
part of my ever-expanding collection.
As for attachments for a marine sextant: I have tried a lot of them. The
best, in my opinion, is the old C.Plath attachment (based on their
successful WWII SOLD aircraft sextant) which has a provision for adjusting
the size of the bubble. This is an important consideration for accuracy.
These are no longer manufactured but there are still a lot of them floating
around out there. The drawback of the C.Plath is that it is very difficult
to use for stars that are less than first magnitude. The illumination system
(even though it is fitted with a dimming rheostat) is still too bright at
its lowest setting and tends to wash out dim stars. Still, for practice on
land, I use the C.Plath bubble system almost exclusively. With a lot of
practice, you can nail your position down to 1 minute or less on sun shots.
I get very upset with myself if my intercept does not fall within 0.0 - 0.9
minutes of arc. I get upset a lot these days.
Both the C.Plath and the RAE MK-IXA have spherical bubble chambers, which
means that the star/body will remain in coincidence with the bubble even
when the sextant is tilted up or down (to a point) and without introducing
errors in the measurement. A side to side tilt will, however, result in
errors. The instruction booklet for the MK-IXA discusses this principle in
detail.
For external artificial horizons, I use a Freiberger. For all star/planet
observations, I mount the artificial horizon on a tripod and get up as close
as possible when observing dim stars. In otherwords, the horizon would be
set at chest level and you would move in as close as you possibly can to
view the object of interest. I use a 6 x 30 scope for this purpose. I have
used mercury and in fact still have a small stockpile of the stuff. It is a
better reflector than the black glass which (it seems to me anyway) tends to
have an attenuating effect on the brightness of the stars. Nevertheless,
mercury is more trouble than it is worth and further, it volatilizes at room
temperature and so presents a health hazard. Best stay away from that stuff.
That you live in New Hampshire and are surrounded by trees is a difficulty
which a few of us might wish we had.
My two bits' worth.
Robert
----- Original Message -----
From: "James R. Van Zandt" <jrvz@comcast.net>
To: <NavList@fer3.com>
Sent: Friday, August 11, 2006 8:10 PM
Subject: [NavList 1050] sextant for use on land
>
>
> I have gotten interested in celestial navigation, and would like some
> advice in choosing a sextant.
>
> I would like to be able to take sightings from my New Hampshire home,
> which is some distance from the water. That pretty much rules out a
> regular sextant.
>
> We are also surrounded on most sides by tall trees, which would make
> it hard to use an artificial horizon (limited to 60 degrees
> elevation). Besides, I want to make star sightings, which seem pretty
> difficult with reflections off water. (I could try to get hold of
> enough mercury to fill an artificial horizon, but it would probably
> not be worth the hassle.) [1]
>
> My first feasible alternative would be a bubble horizon for a regular
> sextant:
>
> http://www.celestaire.com/catalog/Marine_Sextants/Cheap_Sextant/ $27
> http://www.celestaire.com/catalog/products/0511.html practice bubble
> horizon $49
> http://www.celestaire.com/catalog/Marine_Sextants/Cassens_and_Plath/ $900
>
> The last one is definitely out of my price range.
>
> I have a couple of questions:
>
> 1) I'd like to know if these levels are coordinated, in the following
> sense: suppose I start with both a star and the bubble centered in
> the image, then I lower the sextant so the star rises half way to the
> top of the image. Does the bubble rise to the same height? That
> would greatly simplify the measurement. Of course, the bubble would
> still be sensitive to linear acceleration, which would make no
> difference to a regular sextant.
>
> 2) Does Celestaire's "practice" bubble horizon fit on any of the Davis
> sextants?
>
>
> My other alternative is a bubble sextant, e.g. one of those at eBay
> designed for aircraft:
>
> Kollsman MS28011-S
> Kollsman 1471-01
> Bendix AN-5851-1
> Link A-12
>
> 3) Are these set up for star sights, as well as sun and moon?
>
> 4) At least some of these call for 28 VDC. Is that only required for
> the averager, or also to illuminate the bubble or for some other
> function?
>
> 5) I can't judge size very well from the pictures. Are these sextants
> too heavy or bulky to be hand held? (Although I think it would be
> cool to mount one in a sun roof :-)
>
>
> I'd appreciate any pointers.
>
> - Jim Van Zandt
>
>
> [1] I see Lewis and Clark sometimes used a carefully leveled mirror
> http://www.davidcortner.com/2822/mandan_moon.html
> I suppose I could try that.
>
>
> >
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
From: Robert Eno
Date: 2006 Aug 13, 09:10 -0500
If you can, get hold of an RAE Mk IX-A bubble sextant. It is simply the best
aircraft bubble sextant ever made. The bubble system and in particular, the
way in which it is illuminated is second to none. The only drawback of the
MK IX-A is that you have no way to determine index/instrument error except
to take dozens of careful observations from a known location and try to
determine a constant error (if any). I have three MK IX's. I got lucky on my
first one: no errors, however in the others there are large errors.
Unfortunately, I know of no one who can repair or adjust these instruments.
It took special equipment to do this. I recall many years ago that
Celestaire sold MK IXs but I have not seen them in their catalogue since the
early 1990's.
I can only comment on the periscopic and A-10 sextants, both of which I
would not recommend only because of mechanical problems associated with
them. The periscopic sextant is, understandably, awkward to hold and the
reflecting pellicle must be kept bone dry or it will disintegrate. I have
yet to find a replacement pellicle for mine. The A-10 is a beautiful,
compact instrument and were it not for the pain-in-arse bubble system, I
would be more kindly disposed towards it. I still have mine and would not
part with it, however, I seldom use it for its intended purpose. It is just
part of my ever-expanding collection.
As for attachments for a marine sextant: I have tried a lot of them. The
best, in my opinion, is the old C.Plath attachment (based on their
successful WWII SOLD aircraft sextant) which has a provision for adjusting
the size of the bubble. This is an important consideration for accuracy.
These are no longer manufactured but there are still a lot of them floating
around out there. The drawback of the C.Plath is that it is very difficult
to use for stars that are less than first magnitude. The illumination system
(even though it is fitted with a dimming rheostat) is still too bright at
its lowest setting and tends to wash out dim stars. Still, for practice on
land, I use the C.Plath bubble system almost exclusively. With a lot of
practice, you can nail your position down to 1 minute or less on sun shots.
I get very upset with myself if my intercept does not fall within 0.0 - 0.9
minutes of arc. I get upset a lot these days.
Both the C.Plath and the RAE MK-IXA have spherical bubble chambers, which
means that the star/body will remain in coincidence with the bubble even
when the sextant is tilted up or down (to a point) and without introducing
errors in the measurement. A side to side tilt will, however, result in
errors. The instruction booklet for the MK-IXA discusses this principle in
detail.
For external artificial horizons, I use a Freiberger. For all star/planet
observations, I mount the artificial horizon on a tripod and get up as close
as possible when observing dim stars. In otherwords, the horizon would be
set at chest level and you would move in as close as you possibly can to
view the object of interest. I use a 6 x 30 scope for this purpose. I have
used mercury and in fact still have a small stockpile of the stuff. It is a
better reflector than the black glass which (it seems to me anyway) tends to
have an attenuating effect on the brightness of the stars. Nevertheless,
mercury is more trouble than it is worth and further, it volatilizes at room
temperature and so presents a health hazard. Best stay away from that stuff.
That you live in New Hampshire and are surrounded by trees is a difficulty
which a few of us might wish we had.
My two bits' worth.
Robert
----- Original Message -----
From: "James R. Van Zandt" <jrvz@comcast.net>
To: <NavList@fer3.com>
Sent: Friday, August 11, 2006 8:10 PM
Subject: [NavList 1050] sextant for use on land
>
>
> I have gotten interested in celestial navigation, and would like some
> advice in choosing a sextant.
>
> I would like to be able to take sightings from my New Hampshire home,
> which is some distance from the water. That pretty much rules out a
> regular sextant.
>
> We are also surrounded on most sides by tall trees, which would make
> it hard to use an artificial horizon (limited to 60 degrees
> elevation). Besides, I want to make star sightings, which seem pretty
> difficult with reflections off water. (I could try to get hold of
> enough mercury to fill an artificial horizon, but it would probably
> not be worth the hassle.) [1]
>
> My first feasible alternative would be a bubble horizon for a regular
> sextant:
>
> http://www.celestaire.com/catalog/Marine_Sextants/Cheap_Sextant/ $27
> http://www.celestaire.com/catalog/products/0511.html practice bubble
> horizon $49
> http://www.celestaire.com/catalog/Marine_Sextants/Cassens_and_Plath/ $900
>
> The last one is definitely out of my price range.
>
> I have a couple of questions:
>
> 1) I'd like to know if these levels are coordinated, in the following
> sense: suppose I start with both a star and the bubble centered in
> the image, then I lower the sextant so the star rises half way to the
> top of the image. Does the bubble rise to the same height? That
> would greatly simplify the measurement. Of course, the bubble would
> still be sensitive to linear acceleration, which would make no
> difference to a regular sextant.
>
> 2) Does Celestaire's "practice" bubble horizon fit on any of the Davis
> sextants?
>
>
> My other alternative is a bubble sextant, e.g. one of those at eBay
> designed for aircraft:
>
> Kollsman MS28011-S
> Kollsman 1471-01
> Bendix AN-5851-1
> Link A-12
>
> 3) Are these set up for star sights, as well as sun and moon?
>
> 4) At least some of these call for 28 VDC. Is that only required for
> the averager, or also to illuminate the bubble or for some other
> function?
>
> 5) I can't judge size very well from the pictures. Are these sextants
> too heavy or bulky to be hand held? (Although I think it would be
> cool to mount one in a sun roof :-)
>
>
> I'd appreciate any pointers.
>
> - Jim Van Zandt
>
>
> [1] I see Lewis and Clark sometimes used a carefully leveled mirror
> http://www.davidcortner.com/2822/mandan_moon.html
> I suppose I could try that.
>
>
> >
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
