Index Error Sextant

S

Sailwahine

New Member
Joined
29 Aug 2022
Messages
11
Visit site
Hello, I have pulled my sextant out of its box after a few years of non-use. After correcting for perpendicularity and side errors I have found that I cannot get index error down to fewer than 20' on the arc. Generally I've had 3-4' max. Further fiddling re-introduces side error and it's a woeful vicious cycle.
My question: Is an index error of 20' acceptable (correcting for it of course en route from Hs to Ho) or do I really need to keep fiddling and get it down to acceptible levels?!
 
Feels a bit high, but if that's the best you can get then yeah.

Might be worth checking collimation, since it's possible that the 90 deg rotation for Perp. and Side might be accentuated by a bit of that.

How accurate are the fixes? If you can get good ones, then that's what matters (needn't be a full fix, even a Mer Pass should be informative, assuming you have a spare horizon to hand).
 
If you have taken out all perpendicular and side errors 20' index error sounds excessive as I have always understood that its OK to accept up to 5' index error before correcting the instrument. I can not think of a practical reason why you could not work with an IE of 20', but would also suspect some other instrumental misalignment.
What method are you using when correcting the sextant?
 
I would have thought 20’ a bit high and would tentatively suggest that the index mirror needs adjustment, as porthandbuoy has questioned how far way is the horizontal datum you are using. Unless something dramatic has happened to your sextant since you put it away then I would not really expect the IE to have changed from the 4-5’ you were used to.
Prompted by your OP, just dug my 1973 Zeiss Freiberger Drum sextant out of the cupboard (haven’t use it for about 10 years), to check it over, we are lucky in that from our balcony the horizon is about 60 miles across Cardigan Bay, so did an IE check and it is still the same as the 1973 factory certification of 0’ IE.
 
I don't think a 20' ie is acceptable. The index error adjustment (nearest the frame on the horizon mirror) does push out the side error - but a few goes around the loop should reduce both errors to very little. Maybe your horizon mirror glass is not moving freely in it's frame. Another possibility - on my Tamaya, there is no key on the micrometer dial, so if it has slipped around a bit, that will result in a permanent ie offset. For 20', though, it should be obvious that the 0' position does not align with a division on the arc.
 
DFL1010 said:
Feels a bit high, but if that's the best you can get then yeah.

Might be worth checking collimation, since it's possible that the 90 deg rotation for Perp. and Side might be accentuated by a bit of that.

How accurate are the fixes? If you can get good ones, then that's what matters (needn't be a full fix, even a Mer Pass should be informative, assuming you have a spare horizon to hand).
Click to expand...
I haven't done a sight yet (since pulling it back out of the box, but a few years ago at sea I was able to get fix within a nautical mile of my GPS position) so if there is an issue it's recent. I'll take a sight and see how it comes out allowing for the 20' and at least see if I can get an LOP that lands on my position.
 
neil_s said:
I don't think a 20' ie is acceptable. The index error adjustment (nearest the frame on the horizon mirror) does push out the side error - but a few goes around the loop should reduce both errors to very little. Maybe your horizon mirror glass is not moving freely in it's frame. Another possibility - on my Tamaya, there is no key on the micrometer dial, so if it has slipped around a bit, that will result in a permanent ie offset. For 20', though, it should be obvious that the 0' position does not align with a division on the arc.
Click to expand...
Now you might have something there, I did notice that just generally when the micrometer drum shows exactly 0.0 minutes the arc degree of 0 on the degree scale is not exactly lined up. So it could be the micrometer drum? Is it possible for me to take this off and realign? I don't mind experimenting a bit because I've had this sextant since 1992 and if I end up having to get a new one I will well and truly have gotten my money out of it, although if I can avoid that I will.
 
Praxinoscope said:
I would have thought 20’ a bit high and would tentatively suggest that the index mirror needs adjustment, as porthandbuoy has questioned how far way is the horizontal datum you are using. Unless something dramatic has happened to your sextant since you put it away then I would not really expect the IE to have changed from the 4-5’ you were used to.
Prompted by your OP, just dug my 1973 Zeiss Freiberger Drum sextant out of the cupboard (haven’t use it for about 10 years), to check it over, we are lucky in that from our balcony the horizon is about 60 miles across Cardigan Bay, so did an IE check and it is still the same as the 1973 factory certification of 0’ IE.
Click to expand...
I'm at the beach looking at the sea horizon so it's probably about fifteen nm away? I've read the sun is a better way to get a more accurate index error but haven't tried that yet, I feel like even accounting for the horizon method being a bit less accurate than the sun method 20' is pretty high.
 
DFL1010 said:
Feels a bit high, but if that's the best you can get then yeah.

Might be worth checking collimation, since it's possible that the 90 deg rotation for Perp. and Side might be accentuated by a bit of that.

How accurate are the fixes? If you can get good ones, then that's what matters (needn't be a full fix, even a Mer Pass should be informative, assuming you have a spare horizon to hand).
Click to expand...
Good point I'll check about the collimation error, I haven't done that. I haven't done a fix since breaking the sextant back out of the box so I'll do a sight and see how it goes allowing for the large index error, just to see.
 
srm said:
If you have taken out all perpendicular and side errors 20' index error sounds excessive as I have always understood that its OK to accept up to 5' index error before correcting the instrument. I can not think of a practical reason why you could not work with an IE of 20', but would also suspect some other instrumental misalignment.
What method are you using when correcting the sextant?
Click to expand...
For perpendicularity I'm just making sure the arcs line up (the real one and reflection) when it's on it's side, just a tiny adjustment with 1st adjustment screw to correct for this. For side error I'm tilting the sextant to look for a break in the horizon and using the 2nd adjustment screw to correct until they line up. For index error I'm using the horizon (actual horizon while I'm at the beach) which I know is not as accurate as using the sun, but still I figure even using the horizon that's a lot?!
 
  • Like
Reactions: srm
Sailwahine said:
Now you might have something there, I did notice that just generally when the micrometer drum shows exactly 0.0 minutes the arc degree of 0 on the degree scale is not exactly lined up. So it could be the micrometer drum? Is it possible for me to take this off and realign? I don't mind experimenting a bit because I've had this sextant since 1992 and if I end up having to get a new one I will well and truly have gotten my money out of it, although if I can avoid that I will.
Click to expand...
On the Tamaya sextant - mine is an old brass framed model - there is a screw at the centre of the thumbwheel. If you undo this, the thumbwheel comes off and there is a nut that clamps the micrometer dial to the worm shaft. You simply slacken this and then the micrometer dial will rotate freely. I don't know how other sextants do this though!
 
neil_s said:
On the Tamaya sextant - mine is an old brass framed model - there is a screw at the centre of the thumbwheel. If you undo this, the thumbwheel comes off and there is a nut that clamps the micrometer dial to the worm shaft. You simply slacken this and then the micrometer dial will rotate freely. I don't know how other sextants do this though!
Click to expand...
OK thank you so much! I just have a Davis Mark 15 plastic sextant so I'll check it out. It could just be that the old girl has warped over the years (the sextant, not me!) but I'll definitely check this out.
 
Sailwahine said:
OK thank you so much! I just have a Davis Mark 15 plastic sextant so I'll check it out. It could just be that the old girl has warped over the years (the sextant, not me!) but I'll definitely check this out.
Click to expand...
I wouldn't run down the Davis - I got on well with an Ebbco plastic sextant (I still have it!). I found that the secret to getting good fixes was to use averaging - maybe four or five sights. I got the Tamaya because I wanted to take star sights and my eye-sight wasn't good enough to see the stars with the little plastic telescope.
 
A Davis mk15? I suspect that’s the problem. You may find a difference when you turn the drum clockwise vs anticlockwise toward zero. Whether the difference accounts for your 20' error I don't know. I'll dig mine out tomorrow and give you my results.
 
Sailwahine said:
For index error I'm using the horizon (actual horizon while I'm at the beach) which I know is not as accurate as using the sun, but still I figure even using the horizon that's a lot?!
Click to expand...
Sea horizon is the norm, as that will be your reference when observing.

Am a bit puzzled about using the sun for IE, as I have not come across any references to that technique. I have used a bright star though, which is probably a bit kinder on the eyes should you inadvertently look over the telescope.

Many years ago I did try correcting one of the early plastic sextants for someone. Mirror screws were turned with a hex key. I brought the index mirror into line, relaxed the pressure on the key and it returned to the original position. Mirror was not moving but the frame was bending!
 
Porthandbuoy said:
A Davis mk15? I suspect that’s the problem. You may find a difference when you turn the drum clockwise vs anticlockwise toward zero. Whether the difference accounts for your 20' error I don't know. I'll dig mine out tomorrow and give you my results.
Click to expand...

Just checked the IE on my Davis Mk 25.
Winding the drum down gives an IE of 0.6' Off the arc
Winding the drum up gives an IE of 7.2' Off the arc.

Not as bad as your 20' but still significant compared to a metal sextant.
 
You must log in or register to reply here.

Share this page

Top