Thanks ged, you know, I'm gonna give it a go dammit. I always thought I should at least learn how to use the thing, even if only a rough idea.
Transit Compass
- Thread starter Wayland
- Start date
As well as the sextant, you need a clock and a nautical almanac to get a fix. However, IIRC there are programs for handheld computers that work as an almanac for celestial navigation (there was certainly one for the Palm Pilot).
I suspect celestial navigation has been all but forgotten by professional mariners. LORAN was hugely easier to use, and GPS easier still. Makes me wonder how they'd cope without it now.
I suspect celestial navigation has been all but forgotten by professional mariners. LORAN was hugely easier to use, and GPS easier still. Makes me wonder how they'd cope without it now.
I've got the book - it has some useful stuff. I bought it because it claims to tell you how to service a sextant - it doesn't. Calibration needs collimators and other optical trickery and is very expensive and completely unnecessary unless you think the sextant has been dropped or massively mishandled. If it has, it probably needs retiring to the mantelpiece as once the arc is damaged the cost of repair will likely exceed the cost of a new sextant.
To check your sextant out is easy, shame you are oop North, as it is much easier to show than describe!
Basically you want to do the following:
1. To check for perpendicular error (of the index mirror) set the sextant to about 35 degrees and hold it out horizontally so you can see the arc and the reflected arc in the index mirror. There should be no step in the arc as it passes from direct view to reflected view. Adjust the index mirror (side screw) until the arc lines up. If it won't line up then the arc is bent or the index mirror frame is not perpendicular to the sextant body. It will look like this:
2. Then check for side error (of the horizon mirror) by setting the sextant to 0 degrees and viewing the horizon with the sextant horizontal or a vertical edge (I often use a lighthouse) with the sextant vertical. The further away the better. You are looking to see if there are two images of the edge, one from the direct view the other from the reflected view. These should be co-incident and I find swinging the sextant from side to side to watch the edge move over the direct/reflected boundary works well. Adjust the horizon mirror until the images line up.
3. Finally of the ones you can do anything about, there's index error. Set the sextant to 0 degrees and sight a far away body, don't forget the shades on both paths if you use the sun. The images should line up. If they don't, you can adjust the index mirror (top screw) to make them do so.
You may find that adjusting the index error throws out the perpendicular and side errors so re-visit these checks. On my sextant it tends not to, but you may find you need to iterate around the loop once or twice especially if the sextant has gone way out of alignment.
Anything less than 3 or 4 minutes of error is traditionally left as on old sextants the adjustment screws work against a solid mounting and can crack the mirrors so you adjust as little as possible - this may be the case on your old sextant so be very careful! On a modern sextant, the opposite side of the mirror from the adjustment screw is held by a spring clip and so you are not creating any stress in the mirror by adjusting it so I tend to adjust for zero error.
The final check is for collimation error and you better hope it's fine because you can't really do anything about it! You sight on two stars at least 90 degrees apart using say the left hand side of the field of view. Then move the sextant so you are using the right hand side of the field of view. If the stars separate, you have a collimation error (the mounting of the telescope is not parallel with the sextant frame).
Hope this helps.
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Want!
Lovely instrument Andy, I did a writeup on the M88 (lightweight version of the M73) here...
http://www.britishblades.com/forums/showthread.php?t=82843
I've been away for a few days so sorry for the late reply.
No specific link but if you type in DQL-8 on ebay you get a couple of listings.
I have to add for the record, that the Brunton has a vernier scale but this Chinese copy does not. I do not know about the damped version, if anyone gets one it would be great to get some feed back here.
No specific link but if you type in DQL-8 on ebay you get a couple of listings.
I have to add for the record, that the Brunton has a vernier scale but this Chinese copy does not. I do not know about the damped version, if anyone gets one it would be great to get some feed back here.
Wow, great review Martyn. I need one even more now. But which one to get?
On a boat I'm not too worried about the baseplate as I have a plotter with me. I generally use a Plastimo sighting compass but this looks way more fun. Doubt I'll get half a degree though, 5 is usually good enough!
Achh, man with a Franny Barker you'd get 2 degrees in a force 10 gale, the speed and damping on em is very impressive.
If you are going to lug it round with you, get the M88, if you are keeping it on a boat or in a vehicle, get the M73.
Be sure to check this website out...
http://www.trademarklondon.com/
...and say goodbye to your bank balance.
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If I'm close enough to land to use a sighting compass in a F10 gale I shall probably be worrying about other things!
Thanks for the link - fortunately (for my bank balance) he doesn't have an M73 or M88!
Any idea how do they compare to the Brunton that Wayland mentioned at the beginning of the thread?
Thanks for the link - fortunately (for my bank balance) he doesn't have an M73 or M88!
Any idea how do they compare to the Brunton that Wayland mentioned at the beginning of the thread?
Price wise the M88 can be found for about £200 and the Brunton Composite at about £170 at the time of writing.
Thanks Gary. Any advantage to one over the other?
I think a comparison is in order
I wonder if they'll supply review samples?
I think a comparison is in order
I wonder if they'll supply review samples?
Brunton is a much bigger geological/surveyors compass with a load of bells and whistles, the Franny Bee is a military marching compass - it points north.
Did you see that little pocket sundial on the trademarklondon site? How cool is that thing?
You can occasionally drop on a good, used M88, I got mine off ebay for £100 on BIN. It's in perfect condition. A lot of em are in mils though, if you spot a cheap one in degrees, it's worth thinking about. Though Psyer-SGI will re-card em for a fee.
Brunton is a much bigger geological/surveyors compass with a load of bells and whistles, the Franny Bee is a military marching compass - it points north.
Did you see that little pocket sundial on the trademarklondon site? How cool is that thing?
Maybe I need both then.
Pocket sundial - very nice. Shiny shiny. Must not buy.
On the subject of converting a FB from mils to compass, and at the risk of running into the same criticisms from the mil purists, I used a guy called sea to science. I've no connection just a happy customer. I had an FB but had to sell it when times where hard, later had a little more money I bought a mils FB from eBay and got it converted. Conversion and compass cost about £110
Well as an experiment I've just taken my first star shot of Polaris from my back yard.
It wasn't easy, in fact I had to make a slight estimation as I couldn't really see the star and illuminate the level at the same time.
Although the UV led light worked well enough I don't think my eye was dark adapted enough.
The reading I got from the scale was 53° or maybe just a smidge under.
My location here according to the 25K OS map on Memory Map is 53°34'8"N 2°5"02.1"W So roughly 39 miles out on a first try. Make of that what you will.
I have an idea that might improve that, but I'll have to try that another night.
It wasn't easy, in fact I had to make a slight estimation as I couldn't really see the star and illuminate the level at the same time.
Although the UV led light worked well enough I don't think my eye was dark adapted enough.
The reading I got from the scale was 53° or maybe just a smidge under.
My location here according to the 25K OS map on Memory Map is 53°34'8"N 2°5"02.1"W So roughly 39 miles out on a first try. Make of that what you will.
I have an idea that might improve that, but I'll have to try that another night.
Just a thought - the florescent dye in the bubble level might well be flourescein: I use this to examine the eye for corneal abrasions. When doing this I use blue light, not UV. Might be worth trying.
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