(01-30-2022, 08:32 PM)neil_s Wrote: Hi to all - I am new on the site and a relatively new self taught celestial navigator - I've been hooked for about three years, now. There are a couple of things that are bugging me about using a sextant that the books don't seem to answer. I started out with an Ebbco plastic with bits of film negative where the filters should be and got on pretty fine with that. I coped with measuring ie for each observation, and found that multiple shots and averaging would give LOP's pretty close to where the GPS said. Now, I have a rather better instrument. I don't need to average any more, but am puzzled about the choice of filters. The direct sunlight ones I understand, but what are the green and blue ones for? Another thing is the choice of telescopes. I have only a 4X30 on my sextant but I see that a long thin X12 telescope was also offered. What was that used for? I also see that more modern instruments have telescopes with prisms, maybe 7X30. Do these have any advantage over my 4X30? All comments most welcome!
Neil,
I am newly registered to the forum, but have lurked here for a while. I hope to be able to answer some of your questions regarding the other bits in the sextant box, but as I wrote my reply it got to be rather long for a single post, so I'll break it up into a couple of posts.
Let's begin with the various scopes.
The human eye can resolve to about 1 to 1.5 arc-minutes. An error in the sextant angle of 1 arc-minute is an error in the final line of position of 1 nautical mile. Clearly it would be useful to enlarge the image we see so that we can improve the acuity of this measurement, and that is why we fit optical scopes to our sextants. However everything has a trade-off. The more we magnify the image the harder it is to hold the image steady in our view, and the more brightness is lost to the magnifying process.
The diameter of the front lens of the optical scope, called the objective lens, determines how much light is gathered and how bright the image will be. The larger the objective lens the brighter the image. So if you are going for a lot of magnification you will need a larger objective lens to achieve the same brightness as you would with a smaller objective and less magnification.
There are basically 4 kinds of optics fitted to sextants:
First: a 0-power “Sight Tube.” This offers a very wide field of view and as bright an image as the prevailing conditions permit, but of course at no added visual acuity over your naked eyes. In rough conditions or on bouncy small boats these are the easiest optics to use because it is easy to keep the body in the field of view as you bring it down to the horizon. If the horizon is choppy (because the conditions are rough) you probably can't expect to get superlative results anyway, so the +/- 1.5 nmi is not a terrible trade off, and you can likely improve upon that by taking several sights and averaging them, tossing out the ones that are clearly way off.
SECOND: a “Star Scope” which is usually a straight-through telescope of Galilean optical construction (i.e. like opera glasses) that offers a right-side-up image. Usually star scopes have objective lenses around 40 mm and magnify between 3 and 4 times. This is intended to be a compromise scope that gives some increase in visual acuity while retaining a bright image and a wide field of view. At 3 or 4 power it isn't too difficult to hold the image steady as long as it isn't too rough. At 4X magnification you are starting to “see” to about 3/8 of an arc-minute or 0.4' This is very much within the ultimate accuracy of most celestial navigation fixes from moving boats, which are generally stated to be +/- 1 nmi (+/- 1.0') as being “as good as it gets.”
THIRD: a “Sun Scope” which is a monocular usually of Porro prism design which offers a right-side-up image. The objective lens is usually between 30 to 50 mm diameter and generally about 7 power magnification. Even though your 7X50 binoculars seem dandy at this magnification you are getting to the stage where it is very difficult to hold the sextant steady enough unless you have smooth seas, a crisp horizon, and a steady deck. Also the greater magnification “steals” some of the brightness and field of view, so that could be compensated for by increasing the diameter of the objective, which is not always the case -- 35 mm is common for a sun scope while 40 mm is common for a star scope.
FORTH: a “Collimation Scope.” These are usually long and skinny. They have an optical design of an astronomical refractor telescope, which offers and upside-down and mirror image view. They usually are high power, being in the range of 9 or even up to 12 power magnification. They have very small objective lenses (20 mm?) so the images are not only upside down and backward, they are also generally fairly dim and the field of view is very small. It would be all but impossible to use one of these in daily navigation. It is entirely non-intuitive to aim the scope by pointing it in the opposite directions in which you see your target image, and with such a small field of view you would quickly loose whatever you were targeting with any wrong aiming or with any motion of the boat. These would not have been the only scope sold with the sextant. They would have been included in addition to a sun scope or star scope. The intended purpose of these scopes is for fine adjustment of the sextant, usually done ashore on a steady work bench. Most modern sextants do not offer collimation scopes, generally you only find them with antique sextants. In some cases it is possible to remove the optics without doing any permanent damage to the tube or the optics and thereby re-purpose the collimation scope into a far more useful 0X sight tube.
You can use any one of these optics to take any and all sights that you would take with a sextant. 0X sight tubes are best for rough conditions. Star scopes are good for low light conditions with small targets (stars at twilight) but also work perfectly well for sun and daytime moon sights. Sun scopes are best for bright conditions and large targets, but require a steady platform and a good horizon to get the best out of them. They are a little less ideal for twilight and stars, but can certainly serve in that capacity. The worst case would be a collimation scope which would be essentially useless for day-to-day navigation.
Of course you can have more than one set of optics with each sextant. It might be prudent to acquire a 0X sight tube, (or just buy a Davis Mk 3 to serve as as a backup /teaching /spray proof/ wet day/ sextant with its 0X sight tube,) but you also might be inclined to augment a sun scope with a star scope etc. if you can find a suitable one. If the sextant originally was sold with more than one scope then the certificate will probably state the errors for each scope separately. If additional scopes were added by owners after the original sextant purchase those probably will not be included on the certificate.
Many sextants seem to be sold to people who are planning to learn celestial, but have not yet learned it. They immediately fall prey to the “bigger is better” trap and buy a sextant with a sun scope “upgrade” over the star scope. If you peruse the used sextant market you will find many examples offered with only a sun scope and no other optics, however if you had to choose only one set of optics for your sextant you probably should select a 3X or 4X star scope for its greater versatility.
One last comment about optics: The overall quality matters – a superb 2X optical scope will easily beat-out a cheapo 4X scope for ultimate accuracy and performance. More magnification through inexpensive optics is not a functional upgrade. If you have ever looked though a bargain-basement pair of binoculars you know what I mean.
PeterB
(01-30-2022, 08:32 PM)neil_s Wrote: Hi to all - I am new on the site and a relatively new self taught celestial navigator - I've been hooked for about three years, now. There are a couple of things that are bugging me about using a sextant that the books don't seem to answer. I started out with an Ebbco plastic with bits of film negative where the filters should be and got on pretty fine with that. I coped with measuring ie for each observation, and found that multiple shots and averaging would give LOP's pretty close to where the GPS said. Now, I have a rather better instrument. I don't need to average any more, but am puzzled about the choice of filters. The direct sunlight ones I understand, but what are the green and blue ones for? Another thing is the choice of telescopes. I have only a 4X30 on my sextant but I see that a long thin X12 telescope was also offered. What was that used for? I also see that more modern instruments have telescopes with prisms, maybe 7X30. Do these have any advantage over my 4X30? All comments most welcome!
Now to continue about the shades and other bits:
Usually there are several horizon shades and several index shades. Since we often take sun sights clearly the index shades have to be dark enough to block all dangerous light from the sun. This is much more important when you have a scope fitted. The larger diameter of the objective lens gathers a lot more energy in the form of light than your naked pupil, and the lens of the scope concentrates that energy into a focal point. Just as you can set a leaf on fire with a magnifying glass, you can cook your retina with an insufficiently shaded scope – and it only takes a moment or two to cause permanent damage. Furthermore you won't know it is happening because there are no pain receptors in the retina.
There may be one specialty index “shade” which isn't a shade at all but looks like slightly distorted clear glass. This is an astigmatiser. It renders circular images into horizontal lines. If you look at a pinpoint image of a star with this in place it will look like a fine horizontal line of light. Align the star's horizontal line with your horizon. If you look at the sun it will appear as a fat horizontal line. For bubble sextants you can center this fat line on the bubble and record Hs to the center of the sun's disk. For marine sextants using a natural horizon I'm not sure it is of any particular help whatsoever.
By contrast the horizon shades are just there for comfort sake, and so they are not generally as dark as the index shades. Often, even on very fine sextants, the entire stack of horizon shades is not enough to protect your vision from a direct image of the sun. This becomes a serious issue in three instances:
ONE: when using a reflecting liquid artificial horizon
TWO: when using a sight of the sun onto itself to determine index error
THREE: when looking through the sextant set to 0° to use the technique (often taught) to see the sun simultaneously in both the index and horizon views to acquire it and bring it down. (this is DANGEROUS! DON'T DO IT!)
If you intend to engage in any of those three activities get yourself a piece of Baader Solar Film and make a supplemental shade for the horizon view, or make a “lens cap” with the solar film to fit over the objective of the scope which will simultaneously filter both views.
For the modern sextants which I have seen most of these shades, either horizon or index, are “neutral gray” which attenuates all wavelengths (colors) evenly. To my eyes these shades look a little gray-green because the human eye is most sensitive to yellow and green light.
On some sextants the shades have various colors.
A GREEN shade blocks both red light and blue light, passing mostly yellow and green light, which are in the middle of the spectrum. Probably a shade that looks green to your eyes actually is more neutral, blocking all wavelengths to some extent.
A YELLOW shade cuts out blue light. Since blue is the most easily scattered wavelength of visible light, a yellow shade cuts though haze and increases contrast of a murky horizon.
A RED shade also cuts out blue light and so works much the same as a yellow shade but even more so, however the overall image may be darker
A BLUE shade cuts out red and yellow light. I'm not sure why you would want to do that – but maybe it helps to show up stars if there is a reddish twilight.
If you combine BLUE and YELLOW you cut out nearly all the light. In fact on the Davis MK 3 sextant you can flip down both blue index shades and the one yellow sight tube shade and you will 100% eliminate the image of a bright noon-day sun! This is probably also true of RED and BLUE shades but I have never had occasion to try it.
Since sextants have been around a long time you can't always be certain that an antique sextant has scientifically proven shades. I have an Italian sextant that has a red shade for the index view. This seems intriguing, but I can't help but wonder, since it passes red light, if it is in fact effective for blocking highly damaging infra-red light
Often the different colors for the shades can be helpful when using a liquid artificial horizon for practice ashore. When using these sorts of artificial horizons you align the image of the sun as seen reflected off the liquid through the horizon glass with the direct image as seen through the index mirror. If the two images can be rendered in different colors it can be less confusing.
If I had my “wish list” I'd always have a yellow shade for one of my horizon shades for murky days, but as far as I know no sextant maker allows you to pick and choose your shades, so I have to satisfy myself with yellow lens sunglasses.
There is often another sort of shade sometimes included with antique sextants. This is a small shade that screws onto the eyepiece-end of the scope, and if the sextant has a collimation scope that is the one this shade usually fits. This is for using when sighting the sun to itself for determining index error. It is supposed to be the only necessary shade to attenuate the sunlight in both the horizon and index view, and since it is only one shade, and it is the same for both views, there is no worry about other shades adversely effecting this measurement. Unfortunately this shade is fitted at the end of the scope where the sunlight is already concentrated by the optics which can cause these sorts of sun-filters to heat up and crack in use. Such filters are considered by astronomers to be inherently unsafe.
I have one last sort of shade to comment on that you may never see. I have a small antique quadrant with a 0 power peep sight, and at the peep sight there is a tiny sun shade to flip into place. This is for sighting the sun onto itself to determine index error, but in this case the peep sight has no magnifying optics so it is not as dangerous as that just discussed above.
Other stuff you are likely to find in a sextant box are small specialty wrenches for adjusting the mirrors. Perhaps a soft brush for cleaning the rack (use sparingly!) A key to lock the box from curious fingers. Spare light bulbs and batteries if the arc has illumination, and maybe a small screwdriver. Some sextants have a provision to stash a spare mirror in the box. To help set the perpendicularity of the index mirror there might be two precision machined cylinders. (I use casino dice instead which are even more precisely made and available as inexpensive souvenirs from any casino.) Lastly, for every serious metal sextant you should also always look for a certificate stating the errors over the full range of the sextant. It was issued when the sextant was made and it should stay with the sextant FOREVER. (Plastic sextants don't get certificates.)
In my personal sextant boxes you will also find a few notes stating things like the index error and when I last measured it (to see if it changes) and any pertinent notes about the sextant, the accessories, or the adjustments.
I hope you find this helpful
PeterB