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Dear Greg, dear Brad,

Here I reported my failed attempt to replace the most transparent horizon shade with a photo-grade polariser filter (49mm diameter, of similar density). That particular piece of glass occurred to be a wedge actually.

In some literature I met an advise to check and create a correction table for IE depending on the set of shades used - the idea was each shade could have some (very little) but non-zero wedginess which could become noticeable in some shades' combination.

Yes, every high-quality recent metal sextant is fitted with practically flat shades - that's why it is difficult to notice any change with/without them. It is achieved through some significant effort on the side of a manufacturer. Still, no matter how hard he tries - every piece of a glass is a wedge and/or a lens. All the difference is if we can notice that distortion.

The only reason to have one shade - on the scope, rather than regular shades, when doing an IE check - less risk to point an unprotected scope at the Sun.

But my strong confidence is - the visual path should be the same whether we do IE check or we take a celestial body sight. All error conditions must be controlled/accounted for. My sextant (СНО-М) allows me to view the Sun with the index arm at 0° - the darkest horizon shade is OK for that.

Warm regards,

Tony

60°N 30°E