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» Online Classroom   » Celestial Navigation   » Public Discussion of Cel Nav   » Sextant scopes and sextant piloting

   
Author Topic: Sextant scopes and sextant piloting
Chris


 - posted January 04, 2006 01:28 PM      Profile for Chris           Edit/Delete Post 
Dear Starpath, I would like to know the merits of a 4x40 telescope & 6x30 monocular. I will be using the sextant for piloting, however, also to explore the different aspects of celestial sights for the intellectual rewards.

Also is it true that a monocular such as 8x30 power is beyond utility for use aboard small cruising sailboat?

I am aware that plastic sextants are quite adequate for piloting chores but ask this question as someone that desires an instrument for all around use & prefers to buy, for reasons other than pure utility, one of the more expensive split mirror metal sextants.

Many Thanks

From: Seattle
David Burch


 - posted January 04, 2006 10:00 PM      Profile for David Burch           Edit/Delete Post 
There are a lot of aspects to this question. But the executive summary to all is, the monocular will noticeably aid several operations, but is not strictly required for any. Another way i often describe it is, we do have one here, and when heading off to do cel nav or piloting or to head offshore, i am willing to spend some time to look for it, but if i cannot find it, i am not upset. it makes some things better, but none are really crucial (compared to the basic, general purpose 4x40 scope).... will give some details below to be more specific

First a note on the use of the sextant for piloting.

and first to this is the point implied in your last sentence.... wanting a (traditional) split mirror instrument. this is easy to address in that for sextant piloting it is essentially required to use this type of sextant and not the "full view" or "whole horizon" type horizon mirror. these latter options do not work well at all for piloting... (see faq on sextant mirror choices.) in rare cases they might work, but more often than not they simply will not do the job.

As you have pointed out, but may not be so well appreciated by those who not have worked with this application, there are numerous nuances to sextant piloting... enough that i have taken the liberty to increase the title of your question by adding "sextant piloting" to facilitate search engines.

sextant piloting can be done with vertical angles or horizontal angles. the former are used for finding distance off, as we cover in our home study courses. This application is a topic in itself, as there are 3 methods, ie measure (1) tip of (say) a hilltop to the shoreline below it, when the shoreline is well within the distance to the horizon, (2) hill top to visible horizon when the actual base of the hill is below the horizon, and (3) measure the angle between the visible shoreline below the peak and the visible horizon that is higher.... an aspect that is more common when you view the horizon from a higher eye height.

You can use a plastic sextant with low or no power for method 1 at all times, and indeed could do this with a ruler or finger without losing much accuracy. methods 2 and especially 3 require a good sextant, with method 3 usually needing a good metal sextant since the angles are very small. The monocular would help very much with 2 and 3.

the main virtue of the monocular is its higher magnification which lets you see more crisply what you are sighting.

Horizontal angles can be used for distance off alone, or more commonly for a very accurate piloting fix--the so-called 3-body fix (see our home study course)--that is indeed the most accurate type of piloting fix, so much so that it is often referred to sextant surveying.

for most horizontal angles, the accuracy obtainable with a simple davis mark 3 sextant ($40, no scope at all) will generally be better than you could obtain by other piloting methods (ie 3 compass bearings, depth and something, distance off and something) with the possible exception of a fortuitous intersection of two crisp ranges.

You will do better with a metal sextant and maybe even better with a monocular, but generally the differences will be small gains. The exceptions occur when you are measuring, say, the distance from a dark brown rock or beach head, against a dark background, or both object and back ground are gray, etc. In this case the monocular helps ensure what you are seeing. But if you can discern well what you are seeing with the naked eye, then you do not gain much.

The price one pays for the monocular is it makes the sextant heavier, and sometimes makes the sights a bit harder since a crucial step in the process is to be sure the sextant is really horizontal.... or you won't be able to find the other reference point. Since the field of view is so limited with the monocular, and you must hold this up with one hand, it can be tedious to get it tilted just right to see what you want in the direct and reflected views.

This points out the value of the simple Davis mark 3... it is super light and has no telescope at all to limit field of view.... but it does limit you to what you can see unaided, and if one is to do this a lot, that will show up as a limitation sometime.

So those are some points related to sextant piloting.

As for cel nav.... the monocular is only used for sun or moon... and possibly some planet sights. The magnification does not help at all with star sights, it just makes them harder since they have less light gathering and less field of view.

With sun and moon you get a sharper image of the limbs and hence, probably, in the long run get slightly more accurate sights... maybe. There are many keys to accurate position fixes that far outweigh the scope type. It is much more important to know your average boat speed to a tenth of knot, for example, than to use a monocular.... a good exercise for cel nav students to demonstrate.

On the other hand, when it comes to measuring the index correction, the monocular will give better results and more quickly, almost regardless of your method... again though, not on par with other important procedures when it comes to final fix accuracy. (Some older sextants came with an inverting 10x20 scope for index correction measurments.)

And finally, when doing lunar distance measurements, it is almost fair to say that you need a monocular to obtain good results... you need a lot more, but the monocular is definitely valuable for this very special application.

For the last point about use of 8x30. This is a more rare type. Typical sextant monoculars are (6 or 7) x (30 or 35).

I would not offhand see any reason that the boat size matters(i presume the implication is the smaller vessel might be bounding around more than the deck of a ship).... at least for cel nav. For this ap you would be doing sun or moon sights or index correction and these are not going to be much different with the higher power (8 vs 6 or 7). As for piloting aps, the cautions mentioned above are just that much more enhanced to deal with.

From: Starpath, Seattle, WA


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