Davis Mark 15 sextant - is this normal?

[Stearmandriver]

Replacement sextant in hand. I haven't had the chance to take any sights yet, but quick inspection shows the following:

1. The micrometer mechanism is much more positive, not showing any of that slop and slipping I had in the first one. When I release the squeeze trigger on the index arm, it jumps to and locks into a position that is appropriate for the micrometer position (ie, if the micrometer is set to 30 minutes, the index arm locks roughly between two degree marks). So, improvement on this front.

2. Shades situation is exactly the same. Looking at the bright midday sun (too high for a sight with an artificial horizon), the direct reflection on the right side of the horizon mirror is too bright even with all shades in place. I'll need to rig up a better filter.

When I get the chance, I'm curious to take a series of sights with both the Mark 15 and the Mark 3. My suspicion is that there will not be an appreciable difference in accuracy, making the additional expense of the Mark 15 something of a waste. But I'm hoping to be wrong about that; will report back.

[Stearmandriver]

Hmmm I guess I can't edit a response after a certain amount of time.

So I unexpectedly DID have time for a couple rounds of sun sights today - 4 shots each time, a few hours apart.

What I found was that the shade situation is ok, if I keep the sun reflection on the left side of the horizon mirror - the glass side, not the silvered side. It's an acceptably dim red ball there (full sun in a clear sky). If I rock the sextant to see the sun on the silvered side, it's too bright... but keeping it on the non-silvered glass instead is fine, so this is the technique I'll use. I know I'm supposed to keep it split down the middle for the most accurate shot, but rocking the sextant just enough to keep it on the left side of the horizon glass seems like a negligible difference.

Accuracy is within 5 miles, so entirely acceptable (though maybe not better than the Mk 3, still have to run that experiment with a series of shots from each. )

So, before doing that experiment my thoughts on the Mk 3 can be summed up as: functional as-is, decently accurate, and while maybe not more accurate than what can be achieved with the Mk 3, there's probably value in getting comfortable with a scope and micrometer. Just in case anyone reads this who is interested in getting started and trying to decide which sextant...

[SeanPatrick]

Originally Posted by Stearmandriver

Shades situation is exactly the same. Looking at the bright midday sun (too high for a sight with an artificial horizon), the direct reflection on the right side of the horizon mirror is too bright even with all shades in place. I'll need to rig up a better filter.

I'm very surprised by this. You must have sensitive eyes. But then, the video you posted did seem to show a brighter than usual image. It's tough to fully understand what's going on without seeing it directly.



If you happen to be near the Norfolk/Hampton Roads area we could meet up somewhere and do some testing if you like.


As for just getting started: I think either the Mk.3 or Mk.15 would be a good choice. They both allow you to familiarize yourself with the mechanics of taking a sight and to practice technique. Although the Mk.15 (having a micrometer as opposed to a vernier) is more similar to a modern metal sextant, which I would highly recommend saving up for if you find you have more than a passing interest in celestial.

[Stearmandriver]

Originally Posted by SeanPatrick

If you happen to be near the Norfolk/Hampton Roads area we could meet up somewhere and do some testing if you like.

As for just getting started: I think either the Mk.3 or Mk.15 would be a good choice. They both allow you to familiarize yourself with the mechanics of taking a sight and to practice technique. Although the Mk.15 (having a micrometer as opposed to a vernier) is more similar to a modern metal sextant, which I would highly recommend saving up for if you find you have more than a passing interest in celestial.

Thanks, I'm on the opposite side of the country though, near Seattle.

I meant to say that the Mark 15 was probably worthwhile for practice because of the micrometer and magnification (I erroneously said the Mark 3), so I do agree with you. And yes, I see myself buying an Astra IIIB or used decent metal sextant at some point, but I'll enjoy working with this one in the meantime. I can say from my couple rounds of practice sights that the LOP clustering from decent sights is definitely tighter than with my first Mark 15, so I think this micrometer mechanism is definitely in better shape. Thanks to everyone who has chimed in on this thread!

[Jim Cate]

FWIW:

when I started celestial, I bought a Mk 25 and eventually used it on our first offshore passage, a round trip SF to Hawaii and back. Both landfalls were within reason. Thereafter, I purchased a Frieberger (sp?). I enjoyed the better optics and better stability of the metal sextant, but found that in practice, taking sights from an eight foot height of eye on a bouncing 30 footer had such built in errors that there was no real difference in accuracy at sea. I still liked the better optics, and there was a real difference in star sightings, but nothing that vastly improved my navigation.

For those that navigate from a stable bridge 50+ feet up, a great instrument will give better results. Our reality is different!

Jim

[GrowleyMonster]

I am glad you seem to have found your solution.



Regarding accuracy with a plastic sextant... there are a few things that will help. First, realize that actual potential accuracy is quite usable, if you do your part.



First of all, remember, you do not use a height of eye, or dip, correction. Second, refraction, parallax, and other optical errors are based on HALF the Ha, which is the angle reading of the sextant corrected for index error, and dip, which is not used with an artificial horizon. Don't just take the Hs and apply the altitude corrections based on the whole angle. This is a big difference from taking a normal sight from the sea horizon. Applying the corrections wrong can introduce several miles of error.


Second, always when you adjust the micrometer knob position, let the final movement be always in the same direction. This way, the real world gear lash will be somewhat consistent and will just be lost within the index error. Even better, make your final tweak of the knob set the image and reflected image slightly apart, so that within a half minute or so, the motion of the sun brings them together, and instantly note the time. A hack watch is useful for this, but you can simply count three or five or whatever seconds as you bring your watch into viewing position and note the time, then subtract those few seconds before correcting time to actual GMT. Best practice is to note the watch error and apply it, rather than to be constantly setting the watch exactly to GMT. For purposes of backyard celestial, it is okay to use a radio controlled "atomic" watch but you do need to still do a radio time check.


Try to keep your sextant in the shade, if possible even when you make your observation. Give it time to stabilize in outdoor temperatures and check your index error immediately before and after the sight. Temperature changes can affect a sextant's reading to a sometimes measurable amount, but particularly so with plastic sextants. The Davis sextants are surprisingly well made and accurate, but temperature sensitivity is one of their major weaknesses. If you make an effort to minimize the effects of temperature, you will be rewarded with more accurate sights.



IMHO it is important to be in the habit of applying an index correction, so setting the index on the sextant itself should be done only ONCE, and you should make the resulting error always be in the same direction so you don't inverse it by mistake. So set it up so that you read a full minute of angle greater than the actual angle of zero when observing this. So if your Hs when taking a sight is say a half minute off, there will be no ambiguity. It will be obvious whether to add or subtract index error.



If you do your part, you should be able to compute and plot an LOP that is within a mile of your actual position, and maybe better than that, with practice. That's not too shabby, huh? A quarter mile accuracy is perhaps attainable.



At sea, a lot of your errors will be lost in the noise of the irregular horizon, the bouncy boat and resulting error in dip correction, and other things. So backyard practice in a controlled setting will help you to avoid making errors you might not notice at sea, giving you a slight edge in accuracy. FWIW, on a small boat I always endeavor to take the actual sight from the top of a wave, since the horizon is made up of the tops of waves.



In the initial stages it is okay to be a one trick pony, working just on say the mid morning and mid afternoon sun lines. Sun lines are pretty basic, and you have a very wide time window for observing. Other types of sights can wait until you have achieved a good familiarity with the sextant and the math involved. Unfortunately LAN can be problematic, with the artificial horizon, due to the very large doubled angle, but it is nice and simple with the natural horizon. So since you are practicing so much in the backyard, I suggest concentrating on those Sun LOPs first, at home and on the boat, THEN learning to do LOP on the boat, and only then graduate to stars, planets, etc, maybe leaving moon for last, even though it is daytime visible and can give you a nice crossing with a Sun line. Moon is slightly more complex and difficult to get right.