01-02-2025, 05:26 PM
I have an older Simex Mark I sextant with a 4X star scope and a traditional horizon glass. I acquired it used off of E-BAY. The sextant was in good condition, but the box had a rotted leather handle and a balky latch – both easily replaced.
I took it to a known location on land with a good sea horizon to the south and did a few practice sun sights. I came out 2 miles off toward. I figured it was probably operator error, so on subsequent days I did additional sights. Each time they came out to be just about the same two miles toward from my GPS location. I have other sextants and I had not run into any similar problems with those, so I figured it wasn't a matter of me “seeing” the sight incorrectly or not getting the right index error or time hack. It had to be something about that particular sextant with that scope. Perhaps a collimation issue since the mirrors checked out fine?
I carefully collimated the scope to the best of my abilities since I have no special equipment for the task. Frankly things didn't change very much so I was unsure if this was the needed fix.
I returned to the same spot and did a series of 7 sun sights using the newly collimated scope. While I was there, being uncertain if the repair had worked, I did two other series of sights. One of 7 sights with a 0X sight tube and one of 3 sights with no optics whatsoever.
Back at my desk I plotted each of the sight series series on their own graphs. I laid down “best fit” lines by eye to the 7 sight data sets and tossed out obvious outliers. There is no good way of doing that with the 3 sight series.
The remaining “good” sights with the 4X scope were averaged for time and Hs. The average sight plotted directly onto my best fit line, as expected.
For the sight tube sights I picked three sights that both laid directly on my best fit line and which in my notes I marked as particularly “good” and reduced each of these separately
The three no optics sights were averaged for time and Hs.
The 4X star scope LOP fell exactly through my GPS position. (well... after I worked out a couple of blunders...) Hurrah!
The sight tube sights were surprisingly more difficult to “see” than I would have expected. A long narrow sight tube, no magnification, and a limited field of view. One LOP plotted 1/4 nmi away from my GPS; another 1/2 nmi toward; and a third about 4.25 nmi away. I suppose you could find an island with those errors, but since I was standing on terra firma in ideal conditions I was a bit disappointed at the 4.25 nmi away result from a sight that I had remarked as being “good” in my notes.
Lastly the no optics whatsoever sights turned out to be very easy to do. Obviously a super-wide field of view contributed to the ease. I was pleasantly surprised upon plotting to find that the LOP ran directly through my GPS position. Spot on! Easier than using a sight tube and, at least in this case, more accurate results.
I was not expecting that result. But it is good to know and it is a confidence builder that when things get bumpy you can just leave the optics off the sextant altogether and get acceptable results. Maybe not “spot on” as I did this time, but not dozens of miles in error ether.
PeterB
I took it to a known location on land with a good sea horizon to the south and did a few practice sun sights. I came out 2 miles off toward. I figured it was probably operator error, so on subsequent days I did additional sights. Each time they came out to be just about the same two miles toward from my GPS location. I have other sextants and I had not run into any similar problems with those, so I figured it wasn't a matter of me “seeing” the sight incorrectly or not getting the right index error or time hack. It had to be something about that particular sextant with that scope. Perhaps a collimation issue since the mirrors checked out fine?
I carefully collimated the scope to the best of my abilities since I have no special equipment for the task. Frankly things didn't change very much so I was unsure if this was the needed fix.
I returned to the same spot and did a series of 7 sun sights using the newly collimated scope. While I was there, being uncertain if the repair had worked, I did two other series of sights. One of 7 sights with a 0X sight tube and one of 3 sights with no optics whatsoever.
Back at my desk I plotted each of the sight series series on their own graphs. I laid down “best fit” lines by eye to the 7 sight data sets and tossed out obvious outliers. There is no good way of doing that with the 3 sight series.
The remaining “good” sights with the 4X scope were averaged for time and Hs. The average sight plotted directly onto my best fit line, as expected.
For the sight tube sights I picked three sights that both laid directly on my best fit line and which in my notes I marked as particularly “good” and reduced each of these separately
The three no optics sights were averaged for time and Hs.
The 4X star scope LOP fell exactly through my GPS position. (well... after I worked out a couple of blunders...) Hurrah!
The sight tube sights were surprisingly more difficult to “see” than I would have expected. A long narrow sight tube, no magnification, and a limited field of view. One LOP plotted 1/4 nmi away from my GPS; another 1/2 nmi toward; and a third about 4.25 nmi away. I suppose you could find an island with those errors, but since I was standing on terra firma in ideal conditions I was a bit disappointed at the 4.25 nmi away result from a sight that I had remarked as being “good” in my notes.
Lastly the no optics whatsoever sights turned out to be very easy to do. Obviously a super-wide field of view contributed to the ease. I was pleasantly surprised upon plotting to find that the LOP ran directly through my GPS position. Spot on! Easier than using a sight tube and, at least in this case, more accurate results.
I was not expecting that result. But it is good to know and it is a confidence builder that when things get bumpy you can just leave the optics off the sextant altogether and get acceptable results. Maybe not “spot on” as I did this time, but not dozens of miles in error ether.
PeterB