NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Observations with pocket sextant in the Baltic
From: hellos
Date: 2006 Jul 10, 18:41 -0500
Bill, your natural horizon was probably 3-4 miles away, similar to the stack at
5 miles, and both subject to some haziness from atmospherics near the ground.
The sun brings its own problems from being so bright and begs the questions of
affects of the filter, your eye watering/squinting, etc.
As Fred mentions "Fourth, the 4x telescope for my C+P shows changes in side
error
depending upon the focus setting." I would note that I've found some sextant
telescopes to be rather sloppily made. That is, a loosely fitted tube which only
appears to be smoothly fit because they have used heavy optical grease in it.
That's surprisingly not unusual in the optics business. Again, one hopes that
setting it rigidly to the stop (infinity) would at least keep it in the same
relative position each time.
I found that checking IE by sighting on a single bright star on a clear night
was inevitably faster and more precise than any other target. It is at
"infinite" distance, so parallax is not an issue, and being "up" it is further
removed from ground effects on the atmosphere. As a pinpoint it makes a very
tiny mark to align and compare. Although I must admit, here the C&P optics have
a further advantage, since one light path is tinted green and the other red, the
star really snaps into "white" when it is precisely aligned in both. (I'm not
aware of any other maker using tinted optics that way but it certainly is
possible.)
It is also possible that some of your changing error was was thermal effects on
the sextant as it heated in the sun. Another variation found much less often at
night.<G>
"My left eye distorts a sphere, with the vertical axis longer than the
horizontal axis. My right eye distorts a sphere with the vertical axis
shorter than the horizontal axis."
Ah, welcome to the world of ASTIGMATISM. It is common but many folks simply
never notice it since they are not using their eyes for anything critical. It
can easily be corrected by glasses, less accurately by contact lens. You should
also be able to have a corrective filter fitted to the eyepiece of your sextant
to match the correction needed by your better/preferred eye, although I have no
idea who would deal in those matters. An optician who loves a challenge, I
suppose.
As a practical matter...use one eye and stick with it, then select a method of
observations which is best suited to correcting that error. I am very much
familiar with the frustration of having eyes that are not as precise as the
sextant I'm trying to use with them.
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
From: hellos
Date: 2006 Jul 10, 18:41 -0500
Bill, your natural horizon was probably 3-4 miles away, similar to the stack at
5 miles, and both subject to some haziness from atmospherics near the ground.
The sun brings its own problems from being so bright and begs the questions of
affects of the filter, your eye watering/squinting, etc.
As Fred mentions "Fourth, the 4x telescope for my C+P shows changes in side
error
depending upon the focus setting." I would note that I've found some sextant
telescopes to be rather sloppily made. That is, a loosely fitted tube which only
appears to be smoothly fit because they have used heavy optical grease in it.
That's surprisingly not unusual in the optics business. Again, one hopes that
setting it rigidly to the stop (infinity) would at least keep it in the same
relative position each time.
I found that checking IE by sighting on a single bright star on a clear night
was inevitably faster and more precise than any other target. It is at
"infinite" distance, so parallax is not an issue, and being "up" it is further
removed from ground effects on the atmosphere. As a pinpoint it makes a very
tiny mark to align and compare. Although I must admit, here the C&P optics have
a further advantage, since one light path is tinted green and the other red, the
star really snaps into "white" when it is precisely aligned in both. (I'm not
aware of any other maker using tinted optics that way but it certainly is
possible.)
It is also possible that some of your changing error was was thermal effects on
the sextant as it heated in the sun. Another variation found much less often at
night.<G>
"My left eye distorts a sphere, with the vertical axis longer than the
horizontal axis. My right eye distorts a sphere with the vertical axis
shorter than the horizontal axis."
Ah, welcome to the world of ASTIGMATISM. It is common but many folks simply
never notice it since they are not using their eyes for anything critical. It
can easily be corrected by glasses, less accurately by contact lens. You should
also be able to have a corrective filter fitted to the eyepiece of your sextant
to match the correction needed by your better/preferred eye, although I have no
idea who would deal in those matters. An optician who loves a challenge, I
suppose.
As a practical matter...use one eye and stick with it, then select a method of
observations which is best suited to correcting that error. I am very much
familiar with the frustration of having eyes that are not as precise as the
sextant I'm trying to use with them.
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
