Sextant Use

[kefroeschner]

Adelie, I have not looked into these 'S' tables, but have read of some alternate versions of H.O.211 published recently. Reviews were not totally favourable. I like the old fonts in Ageton, they are much easier on my eye than the kindergarten gothic that the recent stuff is printed in.

The 'perpetual' Almanac is a great idea -- astronomers have been working that way for centuries.

Lunars too. It worked for Slocum. A long term lunar almanac might be difficult -- even a thing like the earthquake and tsunami in Thailand had an effect on lunar dynamics. I also read about using Jupiter's moons as another cosmic clock. It's all clockwork out there -- we just need to learn how to read the dials.

[S/V Alchemy]

To learn a lunar method that my brain can follow is a long-term ambition. Until then, it's the usual almanac and Table 5, etc.

By the way, thanks again to kefroeschner for the gift of the Astra III case and the assorted goodies within. The optics on the scope are slightly better than on my IIIb, at least for noon sights.

[Adelie]

Another method, less accurate(?), but lots faster is the Lopook method (modified Bygrave). All the sight reduction on 1 sheet of paper and a transparency.

https://sites.google.com/site/fredie...ave-slide-rule

As far as Lunars go Letcher developed a variation that is a lot easier to use, shoot a round of stars and the moon. Reduce the sight and plot the fix. Adjust the times used, and replot everything until the moon agrees with the stars. Now you know the correct time. Time consuming, but if you are offshore without any electronics you will have time to burn.

[wsmurdoch]

Originally Posted by kefroeschner

Just finished tricking out an Astra III...
I replaced the scope with a 10x50 prism monocular and re-did the scale lighting with LEDs and a Li coin cell. New grip is hand-carved walnut.

Can you tell us some more about the work you did on your sextant, especially how you mounted the monocular?

[kefroeschner]

wsmurdoch: The first time (the 10x40) I just bandsawed, filed and sanded a jaw-shaped thingy out of black Delrin (acetal) and epoxied it to the monocular (half of a Bushnell Falcon) The pictures show my second version. This is a 7x50 made from half of an Astra binocular which I found cheap on e-bay. I chose it because the eyepieces are of the individual focus type, thus actually attached to the monocular body rather than to a focussing gadget mounted on the hinge assembly as in most binoculars -- this was a problem with the Bushnell 10x40 rig and I had to cobble a not very satisfactory attachment of the eyepiece to the frame. In both cases the binoculars were misaligned (the two views not superimposed) so I did not have any qualms about taking them apart.

So, the mount for the 7x50 was machined from 1/4" Al plate. First bandsawed to the outer shape a bit oversize, then I bored the inner diameter, to match the mono's outer diameter at the optimal attachment point, on my chinese 7"x10" lathe. Left an ear sticking out to attach to the sextant frame like the original. A bandsaw cut parted the circular part that goes around the mono which was then re fastened with a pair of 2-56 SHCS. I got lucky and the fit was perfect.

A lathe is nice but not necessary -- you could do the bore with a hole saw, band saw or whatever and finish it up by hand filing.

The mating part on the sextant has a triangular groove to align with a similar ridge on the original ear or tang of most sextant monoculars. I did not have this on my 1/4" plate, of course, so made do with a pair of 2-56 set screws which protrude into the alignment groove the same way that the ridge of the original part would. Seems to work.

I much prefer the 7x50 to the 10x I started with -- better field of view, higher clarity -- it is and always has been the most recommended style for marine use. The other half is now a very useful and handy monocular independent of the sextant. Incidentally, the hinge parts of the original binoculars were just bandsawed off -- they were an integral part of the body casting, not separate attachments -- then filed down smooth and the bare exposed Al painted over.

The clock is from 'Formotion' originally intended for motorcycles. Its a quartz movement of course and the accuracy claimed is +-2 seconds in a year. I have been checking against GMT from GPS for about a month now and it has never been more than a second or two off. I do wish it had a date function, as I always forget whether it's tomorrow or yesterday in England.

It and the hardware store level are just stuck on with double back foam tape. Not VHB, not yet anyway as I may want to change them some day and VHB (3M's super strong stuff) just will not let go.

The LED lighting was simple enough using Radio Shack parts. I did paint all the solder connections with cyanoacrylate to (hopefully) mitigate corrosion.

And without the need for AA batteries in the hand grip I could indulge my taste for a new one, inspired by the grip I once saw on a Hammerli (?) match pistol. Just wrapped my hand around a chunk of walnut, drew around my fingers, then bandsawed, rasped, filed, sanded, oiled, varnished ... and so on.

And if you are wondering -- didn't all this hacking about knock the sextant hopelessly out of alignment and render it useless for evermore? First part, certainly, but re-alignment is not something to be terrified of. Dutton explains it well and any fool can do it provided sufficient patience.

I hope this helps and was not overly verbose.

[wsmurdoch]

Thanks for the useful reply.

In high school metal shop the instructor had us file for weeks before we ever touched a machine tool. (The joke was that we were using a 9" manual milling machine.) I used that hard won skill to do about the same thing you have done making a mount from 1/4" aluminum flat to hold an older Orion 10x25 monocular on my Freiberger drum sextant in place of the original telescope.

On my 'to do' list is to make a mount for half a 7x50 binocular, but I have not really figured out the design for the mount, especially the best way to attach the forked mount to the binocular half; best being easily attached to the binocular half, stiff, and easy to make.

Also, I have been looking for a stopwatch that works like this to mount on the sextant handle:

It normally shows the date and time to the second.
When a button is pushed it freezes the display.
When a button is then pressed it returns to the date and time.

[Tingum]

I just advance sun lines. The moon moves too fast and requires too many corrections for my old brain, I will admit the new star finder Appbank sure make stars easier but again, too many corrections for me.

[kefroeschner]

wsmurdoch: great idea for a sextant timepiece. Should not be too difficult to put such a thing together using existing clock, gps and LCD chips. The technology is all there -- just needs system integration and package. If anyone out there would like to work with our company (mfaoptics.com) to get it done, please contact me. Probably not much money in it as the market is small, but some things ought to be done, not necessarily for profit, but just because they are the right thing to do.

[Lake-Effect]

Originally Posted by kefroeschner

wsmurdoch: great idea for a sextant timepiece. Should not be too difficult to put such a thing together using existing clock, gps and LCD chips. The technology is all there -- just needs system integration and package. If anyone out there would like to work with our company (mfaoptics.com) to get it done, please contact me. Probably not much money in it as the market is small, but some things ought to be done, not necessarily for profit, but just because they are the right thing to do.

Ok, no GPS chip, that's cheating.

If you're going to go to this length, also incorporate a digital micrometer into the angle adjustment, and onboard or SD-card non-volatile memory that stores the time and angle as data pairs.

[kefroeschner]

Right -- no gps chip (I was thinking of using it only as a time source) I have designed systems with digital angle sensors but none that I am aware of have the accuracy needed. (Could change -- probably has.) The MCP7941X clock -calendar chip looks like a good place to start -- has everything needed including memory and I2C bus.

I think Frieberger built something like this a while ago and it was a complete bust. So, as Gilda Radner used to say: " ... Never mind."

KISS.

[wsmurdoch]

Originally Posted by kefroeschner

wsmurdoch: great idea for a sextant timepiece. Should not be too difficult to put such a thing together using existing clock, gps and LCD chips. The technology is all there -- just needs system integration and package. If anyone out there would like to work with our company (mfaoptics.com) to get it done, please contact me. Probably not much money in it as the market is small, but some things ought to be done, not necessarily for profit, but just because they are the right thing to do.

Google "freeze time of day stopwatch". You will find Fastime. Their current wrist watches are a bit dear and are overly complicated, but they do freeze the time of day display.

Fastime used to make a Fastime 15 which was in the form of a stopwatch and was closer to what I have been looking for. Googling will still find some for sale. They were used in motor rally events to record the time of day of finishes.

In the long distant past there was a mechanical "stopwatch type" watch that when a button was pushed would freeze the clock hands. When reset the hands would revert to the correct time.