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In amateur astronomy, observing from through window glass is a definite no-no however a number of years ago observers in colder climes experimented, apparently successfully, with observing through openings covered with Mylar sheeting. The Mylar was thin enough that its optical properties were negligible. Similar Mylar sheeting is aluminized and used nowadays to produce flexible solar filters that cover the objective lens of a telescope for safe solar viewing in white light. (This could presumably be used in sextant sunshades as well).

I might suggest that instead of using the lake as an horizon you try using the image of the Sun reflected of a pan of water. From indoors this could limit you to low altitudes but would ensure that the direct and reflected images of the Sun’s lower limb followed parallel paths through the panes and minimize prismatic effects. Outdoors I routinely use a Davis Artificial Horizon which consists of a pan of water surrounded by glass or plastic wind shields. One challenge faced there it is that, even with the shields in place, a modest breeze still produces surface ripples. Indoors that would not be a problem.

Regards,
Robin Stuart
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