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Old 27-11-2013, 08:33   #1
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Check your sextant against the stars

So I ran a few test sights with my new plastic Davis Mark 25 sextant with disappointing results. I'm new to all this, but I can say I read the instructions pretty thoroughly!

I'd just like to see if anyone has any tips. Is it the sextant? Or is it me? I hope it's me, cause then I can fix whatever I'm doing wrong

First I calibrated the sextant per the manual (1. Adjust the index mirror while sighting the reflection against the sextants graduated arc. 2. Sight the horizon and align the reflection with the actual horizon. 3. Turn the thing sideways without adjust anything, then adjust the knob closest to the frame until the horizons match again. 4. Set the sextant to 00°00.0', then adjust the knob furthest from the frame until the horizons align. 5. Rotate the sextant around a star without any misalignments. Perfect!)

I then calculated the distances between a few star-pairs (Calculating the Angular Distance Between Stars (for Focal Length Measurement) - Bob Atkins Photography) and compared them to my sights. I double-checked the calibration after the readings and it was still spot-on:
Star-Star: Calculated / Observed / Diff
Capella-Mirphak: 20°42.3' / 19°06.7' / +1°35.6'
Capella-Vega: 95°54.3' / 93°22.0' / +2°32.3'
Vega-Deneb: 23°22.8' / 22°52.9' / +0°29.9'
Caph-Vega: 59°08.9' / 54°06.2' / +5°02.7'
Caph-Deneb: 37°58.9' / 33°40.0' / +4°18.9'

I read the numbers directly off the sextant and made no corrections. The temp outside was 25F and all the stars were between 30° and 50° above the horizon (trying to minimize refraction). I noticed the differences were greater with greater angles (except the 95° reading which was a bit better than the 60° reading -- weird) which would maybe mean the graduation of the sextant was off? Not sure.

Anyway, thanks for reading. Let me know if anyone has any idea how to better match the numbers to reality!

Thanks!
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Old 27-11-2013, 12:49   #2
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Re: Check your sextant against the stars

You've done a lot more calibration with your Davis than most follks do with any sextant. I'd say you are good to go and don't worry about it. Just a little wiggle or movement of any kind can make a big change and from a rolling deck you'll not only consider coming close as a great sight but very much better than no sight at all.

kind regards,
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Old 27-11-2013, 14:23   #3
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Re: Check your sextant against the stars

The problem with plastic sextants is, their shape changes slightly with the heat from your hand. Then, the calibration isn't the same when the sextant is "cold" and when it is "warm".

Alain
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Old 27-11-2013, 15:52   #4
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Yeah, that doesn't sound good. Temp changes aren't going to give you 2deg errors this is either a problem calcing the angular distances or a manufacturing problem.

I would try to check the sextant directly. You will need a 20' 2x4 and a 100' tape. Stand the 2x4 up plumb in both directions. Pull the tape out fully and shoot the top and bottom of the board standing at varying distances.
The ground should be level and you should sit to make the shot.

If your height of eye were at the ground the following would be correct, sitting will get you close enough. There will be some parallax error too it once again close enough. If you are still seeing errors over a degree the was a fabrication problem.
100 -- 11-18.6
80: 14-02.2
60: 18-26.1
40: 26-33.9
30: 33-41.4
20': 45-00.0
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Old 27-11-2013, 17:18   #5
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Re: Check your sextant against the stars

Check your calculations.

Caph-Vega 54d 07.38'
Caph_Deneb 33d 38.74'
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Old 27-11-2013, 19:11   #6
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Re: Check your sextant against the stars

Take a bunch more sights over a period of time, and realize that a new navigators technique may not be exact. If your errors come out consistent, then just keep track and figure it into every shot, just like you would height of eye. Practice is good, but the first time you do it on a rolling deck, you will wonder if you are in the correct ocean. It is a great skill, dont give up. _____Grant.,
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Old 27-11-2013, 19:53   #7
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Re: Check your sextant against the stars

I would advise you to practice taking sun and star shots as though for navigation, from a known location, to see whether you're doing things right. What the link describes is something meant for photographers to do that is not navigation, and worrying about angular distances between stars may only serve to confuse your learning efforts in future. Not knowing how (or really understanding why, in my case) the photographer calculated his angular distance, there's no need to worry if you can't duplicate the results with a sextant. I found it harder, BTW, in my learning days to take accurate horizontal angle measurements than vertical ones--maybe it's more awkward to hold the sextant sideways.
As for the plastic sextant, if you've calibrated it per instructions, there's not much more accurate things can get. I've had very good results with several different brands of plastic sextants--distortion due to temperature is not really great enough for the novice to worry about. The greater disadvantage of plastic sextants for the learner is that the optics are not as good as those of more expensive metal sextants, so getting a body accurately to the horizon is more difficult.
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Old 27-11-2013, 21:07   #8
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Re: Check your sextant against the stars

Quote:
Originally Posted by ChuckSK View Post
Check your calculations.

Caph-Vega 54d 07.38'
Caph_Deneb 33d 38.74'
Just in case anyone missed it, ChuckSK has nailed the biggest part of the problem right here.
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Old 06-12-2013, 03:41   #9
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Re: Check your sextant against the stars

Wow, thanks for all the advice.

First things first, looks like I better check my star-star calculations.

The 2x4 idea looks interesting. Testing the sextant from only 100' away seems like there would be some huge parallax errors, but I'll try it out just for kicks.

I on deployment with very little to do, so this sure helps kill the time, haha!

Thanks all!
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Old 10-12-2013, 21:20   #10
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Re: Check your sextant against the stars

Quote:
Originally Posted by beebopbogo View Post
Wow, thanks for all the advice.

First things first, looks like I better check my star-star calculations.

The 2x4 idea looks interesting. Testing the sextant from only 100' away seems like there would be some huge parallax errors, but I'll try it out just for kicks.

I on deployment with very little to do, so this sure helps kill the time, haha!

Thanks all!

For the 2x4 trick I did the math and figured out that for a 3" separation between the direct view and the mirror there is a 10 arc-min error when measuring an 11deg angle. Height of eye is also a source of error. I put everything in an Excel spreadsheet that you just have to plug the numbers into. If you want it I will send it to you.
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Old 11-12-2013, 09:22   #11
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Re: Check your sextant against the stars

Quote:
Originally Posted by beebopbogo View Post
---The temp outside was 25F ---
Brrr! lol
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