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Old 24-05-2019, 02:30   #1
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Davis Mark 15 sextant - is this normal?

Hello,

It seems there are some experienced celestial nav folks here, so hopefully it's the right place to ask this question.

I've recently been learning celestial, primarily for fun / historical interest. I am a coastal sailor (Puget Sound) for whom bluewater cruising does hold some appeal, but retirement is a long ways away, so if I ever really cross an ocean it's still years in my future .

Anyway, I've been learning with a Davis Mark 3 and that's gone well. Using a bowl of water for an artificial horizon I can usually fix myself within 5 miles or so of my back yard. Understanding that any plastic sextant has limitations (I do have Burch's book on using plastic sextants), I still figured that having some magnification and a micrometer would be an improvement, so when I saw Celestaire's sale on the Mark 15 I thought "what the hell".

So I've gotten home and opened it up. Can't take any sights because of a solid marine layer and thunderstorms, but I was messing around with mirror adjustments etc. and discovered something weird: I couldn't get the index arm mark to line up with a whole degree with the micrometer reading anywhere near zero. More fiddling, and I find that the micrometer knob is slipping on its spindle. Eventually the micrometer will no longer adjust elevation at all, the knob is just spinning.

It turns out, the micrometer knob has slipped off its spindle to where it's just loose. I set the index arm to zero degrees, aligned the micrometer to zero, placed my thumb on the micrometer knob and finger on the back of the index arm, and "squeezed" the knob back onto its spindle.

Everything seems to align well now, but I'm wondering: is this normal for the Mark 15, that the micrometer knob can work its way loose like that? I'm inclined to exchange the thing, but if that's just how they are, I guess there's no point. The Mark 15/25 seems like the only intermediate step available between the Mark 3 and a decent metal sextant, and it's hard to justify that cost just for academic interest.

So... has anyone ever seen anything similar?

Thanks...
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Old 24-05-2019, 03:29   #2
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Re: Davis Mark 15 sextant - is this normal?

Not normal. I have had mine for over 25 years and taken sights many hundreds of times. The only downside is the plastic sextants need adjustment about every time you take them out.

For an AF try a 1st surface mirror. Also go to fer3.com: NavList celestial navigation community for folks who are addicted to celestial.

I used mine to take all my sights for the USPS JN course and could get under 2 nm with dip short. I still have it but upgraded to a Astra III when I did the N course.

Celestaire are good people. I have had good luck with them.
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Old 24-05-2019, 03:32   #3
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Re: Davis Mark 15 sextant - is this normal?

Having used and still owning a Mark 15, I'd say it's a defect--never had a knob come loose. However, if it's an easy fix with super glue, it might save you the hassle of returning, etc.
The Mark 15 is a great sextant for learning on, and if you get proficient with it, you'll find your navigation supercharged when you step up to a metal one. It's main deficiency is not the plastic body but the indifferent optics--my humble Astra IIIB has far superior lenses to the Mark 15, allowing shots of fainter stars under worse conditions.
But again, the Mark 15 is great for practice and as a backup (you should never go to sea with just one sextant!)
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Old 24-05-2019, 04:49   #4
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Re: Davis Mark 15 sextant - is this normal?

Greetings and welcome aboard the CF, Stearmandriver.

Bi-plane?
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Old 24-05-2019, 09:54   #5
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Re: Davis Mark 15 sextant - is this normal?

I would contact Celestaire to see if they are willing to take it back. I'm betting that they will as I have found them to be great to work with in the past. This is definitely not normal. If you get serious about celestial and get a quality metal sextant such as the Astra IIIB or a C&P you will be impressed with the difference. When I take celestial sights with my C&P I am consistently within one mile. By the way, when adjusting for index error use a star. First get your scope into sharp adjustment on the star, set your index to 0.00, then look directly at a dominant star. This should be done with all shades out of the way. Initially you will probably have both index error and side error. Adjust index error until it is as close possible (the two images of the star should appear side-by-side) then adjust side error until you still have the slightest amount of side error remaining. At that point you will have two images of the same star essentially side-by-side and very close to each other, then adjust index error again until the two images are absolutely side-by-side. Go back to side error and adjust all side error out. The object here is to end up with one image of the same star rather than two images. The alternative is to leave the absolute minimum amount of side error in the sextant. This will not have any effect on your sights, but will subsequently make checking index error very fast in the future. The advantage of adjusting the sextant in this way is that you are adjusting it against a star rather than objects that are infinitely closer. Using a plastic sextant can still get great results but checking index error has to be done virtually every sight. Do not continue to adjust the sextant as it will wear out the adjustment screws. Instead know how to do the math to mathematically remove however much index error you have on any given sight.
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Old 24-05-2019, 13:04   #6
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Re: Davis Mark 15 sextant - is this normal?

I've navigated to Hawaii twice with only a David Mk 25 sextant and a Casio watch. Both times, I managed to find the northern shore of Kauai, so I am a believer in Davis sextants. Yours has a problem. I would contact Davis customer service, possibly ask for Neils, who is a wonderful guy, and ask for a new sextant.

Incidentally, the guy who invented and patented the full field beam converger mirror on the Mk 25 was an Oakland, CA resident named Angus MacDonald, who lived near the second West Marine store in Oakland. He was (and may still be) an incredibly good inventor, originally in ATMs and associated banking technology, but later in marine products. He was commonly seen riding an electric tricycle around Oakland and visiting the store in 1979.

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Old 24-05-2019, 14:34   #7
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Re: Davis Mark 15 sextant - is this normal?

Thank you for the input, guys.

In the little bit of experimentation I've done today, it happened again that after a coarse angle adjustment (pulling the disengagement trigger and sliding the index arm), I ended up with a real disparity between the degrees mark on the index arm, and the minutes markings on the micrometer scale. I know the degrees marking doesn't usually need to line up perfectly to scale, but when the index arm is basically reading that you're on a whole degree, and the micrometer is sitting at around 35 minutes... it's hard to know what to make of that. I understand that sometimes a little micrometer rotation is necessary to re-seat the worm gear, but several complete rotations of the micrometer failed to change this.

After gently cycling the arm a bit more, things are lining up again, so I take this to mean there's still some slippage. I think I'll exchange it.

Thanks Chuck for the contact suggestion at Davis; I've started by contacting Celestaire (their returns section asks you to start with them before contacting a manufacturer), so we'll see how that goes.

It's good to hear the positive comments about the Davis instruments though; hopefully this is just an aberration. The fer3 navlist was also a great suggestion; I did come across that a while back and have been following it with interest.

GordMay, yeah, biplane. I've instructed in them for about 20 years now; I was doing it full time back when internet forums were new and I needed a handle, and it's just kinda stuck (in other words, I'm too lazy to come up with something new).
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Old 24-05-2019, 17:25   #8
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Re: Davis Mark 15 sextant - is this normal?

Hi, hope you get the sextant sorted and have fun with it. You say you are currently sailing coastal implying you don't really need a sextant. Couple of techneques that you may find useful though when in coastal waters. Using a dipping table you can measure the angle to a point of known hight such as a cliff or lighthouse and calculate distance off. Turn the sextant horizontal and you can get the angle between know points such as headlands. Gives an instant 2 point fix, or with a second sight a 3 point fix. Much quicker and more accurate than running fixes and I find better than a hand bearing compass. At these distances you do not need the vernier, 1deg accuracy is plenty! It is alway ggod the check the GPS against a real fix for each chart. Often the case when in areas with little commercial traffic that the GPS offset is correct.
PS, my home base is Port Renfrew so welcome from a local
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Old 24-05-2019, 17:56   #9
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Re: Davis Mark 15 sextant - is this normal?

Quote:
Originally Posted by GordMay View Post
Greetings and welcome aboard the CF, Stearmandriver.

Bi-plane?
What are you going to do after the lightning strike?
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Old 24-05-2019, 19:36   #10
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Re: Davis Mark 15 sextant - is this normal?

When you are ready to upgrade, a new Astra is economical, but there are also a lot of used Plaths in the wild for under $400. A very nice one can be had for about $600.


When I do an index adjustment, which is seldom, I leave it a full minute on or off the arc. Just check index error before every round of sights, and apply the correction. It is an important correction and should be part of your daily celestial regimen. Constantly fiddling wiht the index adjustment creates unneccessary wear.
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Old 24-05-2019, 22:33   #11
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Re: Davis Mark 15 sextant - is this normal?

This is definitely a defect. As others have stated, Celestaire stands by their products and should rectify the problem. If, for some odd reason, you run into trouble, I recommend you contact Ken Gebhart (CEO) directly. (I'll PM you with his email, if you like.) I have no doubt he will take good care of you.


Regarding NavList: please don't hesitate to post there. I have been a member for some time (as is Ken) and am here to tell you that new members (with as many questions as you like) are more than welcome. Anything that keeps the preservation and practice of "traditional" position finding alive is alright with us. I am probably the youngest member, and have learned more from NavList than any book or YouTube video could ever teach. New members keep old (perhaps stale) discussions fresh and relevant. And don't feel "intimidated" by the knowledge of the other members. All are patient and willing to answer any question you may have. Possibly with much more detail than you could imagine. Lol!



Hope to see you there!


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Old 28-05-2019, 00:27   #12
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Re: Davis Mark 15 sextant - is this normal?

On a ship in most conditions I could get within a mile using a WWII navy style sextant. On my current boat within about 2 miles if the GPS is right.


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Old 28-05-2019, 05:46   #13
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Re: Davis Mark 15 sextant - is this normal?

Again, thanks all for the input. Since many of you have shared experiences on this thread with Davis sextants. I'll pose a follow-up question here: has anyone ever experienced, or heard of, eye injury due to inadequate sun shades on the Mark 15?

I'm still planning on exchanging this sextant, but while waiting to hear back from celestaire over the holiday weekend, I thought I'd take a few sights. I enjoy the magnification of the scope for sun sights, but even with all the index shades down, the sun image was *almost* uncomfortably bright (the horizon shades made the reflection in the artificial horizon comfortable). After a series of sun shots, I had an afterimage in my right eye for a few minutes and today my eye feels just a little irritated - hopefully that's just eye strain.

I'm sure I'm being paranoid but the last thing I want to do is damage my vision, and I did note that the Mark 3 shades are darker. Surely a device sold with the express purpose of staring at the sun has a safe amount of shading, right?
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Old 28-05-2019, 08:20   #14
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Re: Davis Mark 15 sextant - is this normal?

If you are getting dazzled then you need more shading. You will eventually damage your eyes if you keep doing that. Will the Mk3 shades fit on the Mk15? I guess probably not. Maybe you could jury rig something with a piece of a welding shade glass. I am thinking maybe a #6 or so. Or don't use the telescope for sun observations. Or get a spare set of index shades to mess around with, and try applying window tint to a couple. Also you might shoot Celestaire an email. They might have a solution.



It is possible that yours was equipped with the wrong shades. Oh and try mixing and matching. Maybe there is a very dark horizon shade that doesn't need to be there. Swap it for the lightest of the index shades. You might be able to make a difference.
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Old 28-05-2019, 18:11   #15
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Re: Davis Mark 15 sextant - is this normal?

As expected, prompt response from Celestaire with a shipping label for the exchange, on the first business day since my email.

Growley, interesting idea about swapping shades. I have trouble believing that a device sold with the actual intent of using it to stare at the sun doesn't have enough shades, but I think you're right. At first I thought I was probably imagining my eye irritation out of paranoia, but no... it's definitely there. Unless I randomly get a darker combination of index shades on the replacement, I'll not be looking at the sun again until I fix this. Cannot believe, just from a legal standpoint, that they'd sell a sextant with this issue... maybe my eyes are just overly sensitive or something? They never have been... this is just bizarre!

Anyway, good idea on moving shades around, but I think I've got an even safer idea: I've got some eclipse-viewing glasses with foil lenses from the total eclipse a couple years ago. Think I'll cut the foil lens out and stick it on the lightest index mirror. THAT oughta do it!
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