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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