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Re: Accuracy: main shades vs eyepiece shades?
From: Brad Morris
Date: 2019 Jul 25, 18:03 -0400

Hi Michael

I knew the topic of plastic sextants would come up. I am glad you mentioned them.

They are awesome training aids. Incredibly useful for acclimating a student to practices, performance and procedure. What better way to get a beginner trained than with a sextant that, even if dropped or abused, really won't hurt it. Its plastic. Its relatively inexpensive. In exchange for that, compromises must be accounted for. Those compromises will show in all different insidious and subtle errors. For example, the index error is going to wander like a sailor on shore leave, with perhaps a few adult beverages too many imbibed.

To expect a training aid to perform like a quality metal sextant is perhaps unreasonable. Yes, we can save the money, but at what cost. After being trained, an enthusiast is well advised to get a quality metal sextant that can be relied upon without those compromises.

A cheap plastic training aid didn't perform as well as the real thing? Shocking.

Brad

On Thu, Jul 25, 2019, 4:47 PM Michael Bradley <NoReply_Bradley@fer3.com> wrote:

There are shade set ups which cripple the sextant. Sextants of decent quality won't show these bad effects.
However, in the case of the Ebbco plastic sextant, shade construction being two pieces of glass sandwiching a film, all three items free to rotate against each other, cheapo glass elements themselves prismatic, you get a crippled sextant.
A single shade assembly like that can end up with two glass prisms set at different angles in one shade holder causing the reflected image to move diagonally in the field of view as the sextant is rocked in pitch. Very disconcerting, the effect makes one think that one or more of the mirrors is bent. Seeing the effect, I eventually measured up the glass shade items, and they were indeed prismatic. The problem was solved by replacing the original shades' glazing with plastic single element spare shades sold for a Davis Mk 15. The rectangular Davis shade elements need trimming to fit into the circular Ebbco shade holders. Each final shade element is shaped in a 'race track' oval whose straight edges don't show, being hidden under the circluar Ebbco shade holder pop in fastener ring.
I've copied Greg Rudzinski, and in that way made a very useful refurbished Ebbco sextant for the extra 18 monies the spare set of Davis shades cost. Far better than any other plastic sextant I've checked out.
It's best to check out sextant arc errors before spending any money, using the star to star distance method ( no shades involved ) to compare with a 'good' sextant, or by appropriate calculations if no reference sextant is available.
It's true, I was born very close to the border of Yorkshire, and now live very close to the border with Scotland. Both sets of folk have a reputation for minimising cash outflow. It's an observable fact that Ebbco sextants on a well known auction web site are much cheaper in the northern winter than in the middle of the northern hemisphere sailing season.
The modified Ebbco satisfies me greatly, providing consistent results, combined with 4 place Hav Doniol, 2 sides of paper each for HO 249 Long Term Almanacs for Sun and Airies, on star or sun sights. Very seldom do I get intercepts outside 5'. Sold.
Michael Bradley

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