NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Index correction sun sights
From: Peter Hakel
Date: 2012 Feb 20, 11:31 -0800
From: GregR <gregr_ingest@yahoo.com>
To: NavList <NavList@fer3.com>
Sent: Monday, February 20, 2012 11:47 AM
Subject: [NavList] Re: Index correction sun sights
Your experience mirrors mine (pardon the pun..) with a Davis Mk 25, and
I've also heard/read the same advice about not turning the adjustment screws
on a plastic sextant very often. The solution for me was to upgrade to a
metal one (Astra III B) which solved all of those issues nicely, plus
increased my accuracy to where I can usually get within 1 NM compared to a
GPS position. They show up on EBay fairly regularly, seems like I paid
around ~$400 for mine back in 2006.
--
GregR
----- Original Message ----- From: Randall.F.Morrow@kp.org
To: NavList@fer3.com
Sent: Monday, February 20, 2012 7:51 AM
Subject: [NavList] Index correction sun sights
I spent some time yesterday doing repeated index correction sun sights
with my Davis Mk 15, using the techniques described in a Starpath article.
In more than 20 observations I got 20 different values, both on and off the
arc before I finally gave up. Previously I had posted that sun and moon
sights using the appearance of tangency always give me trouble due to the
distortion I observe as the edges approach. But in addition to this, when
the images do not fall on a vertical line I adjust the horizon mirror as the
article suggests, but when I repeat the sight for verification the images
are again off line. It has been suggested that I do not turn these screws
on the mirror often but they do not stay in line from one sight to the next
so re-adjusting seems necessary. When turning the horizon mirror finger
screws the image seems to be affected by movement of the plastic frame of
the mirror by the pressure from my fingers as I turn the screw. Is lack of
rigidity in this component a factor? Before making the sights I removed the
telescope and looking at the reflection of the arc, brought the direct and
reflected images in line for perpendicularity. Am I missing something?
Please advise.
Randy
Randall F Morrow PT
Kern County - Bakersfield
From: Peter Hakel
Date: 2012 Feb 20, 11:31 -0800
Same here with my Mark 15. Last July I wrote:
"The main difficulty I had was establishing the index error of the instrument. The index mirror held perpendicular to the frame very well but I spent a lot of time tinkering with the horizon mirror. I repeatedly measured and adjusted the index error by looking at the horizon, overlapping the two sun disks, and also using the "Solar IC procedure" from David Burch's book on plastic sextants. The IE tended to gravitate toward 8' to 9' off the arc but I would have to attach an "error bar" of a few arcminutes to that value from what I saw. Using this Solar IC method to recover the Sun's semidiameter also showed differences of the order of 1'. Between sights I turned my back to the Sun and shielded the sextant from direct
sunlight."
The full posting is here:
The full posting is here:
http://fer3.com/arc/m2.aspx/Santa-Barbara-sights-PeterHakel-jul-2011-g16765
Peter Hakel
From: GregR <gregr_ingest@yahoo.com>
To: NavList <NavList@fer3.com>
Sent: Monday, February 20, 2012 11:47 AM
Subject: [NavList] Re: Index correction sun sights
Your experience mirrors mine (pardon the pun..) with a Davis Mk 25, and
I've also heard/read the same advice about not turning the adjustment screws
on a plastic sextant very often. The solution for me was to upgrade to a
metal one (Astra III B) which solved all of those issues nicely, plus
increased my accuracy to where I can usually get within 1 NM compared to a
GPS position. They show up on EBay fairly regularly, seems like I paid
around ~$400 for mine back in 2006.
--
GregR
----- Original Message ----- From: Randall.F.Morrow@kp.org
To: NavList@fer3.com
Sent: Monday, February 20, 2012 7:51 AM
Subject: [NavList] Index correction sun sights
I spent some time yesterday doing repeated index correction sun sights
with my Davis Mk 15, using the techniques described in a Starpath article.
In more than 20 observations I got 20 different values, both on and off the
arc before I finally gave up. Previously I had posted that sun and moon
sights using the appearance of tangency always give me trouble due to the
distortion I observe as the edges approach. But in addition to this, when
the images do not fall on a vertical line I adjust the horizon mirror as the
article suggests, but when I repeat the sight for verification the images
are again off line. It has been suggested that I do not turn these screws
on the mirror often but they do not stay in line from one sight to the next
so re-adjusting seems necessary. When turning the horizon mirror finger
screws the image seems to be affected by movement of the plastic frame of
the mirror by the pressure from my fingers as I turn the screw. Is lack of
rigidity in this component a factor? Before making the sights I removed the
telescope and looking at the reflection of the arc, brought the direct and
reflected images in line for perpendicularity. Am I missing something?
Please advise.
Randy
Randall F Morrow PT
Kern County - Bakersfield