NavList:

Greg for your information,

A few years earlier, I have used the sextant indoors looking through the double glazing window
because it was raining. But was not satisfied with the result. (cannot remember the difference).
From that time on. I did not make more, observations through double glazing windows.
In principle the glasspanels off the double glazing units should be parallel. But a lot are not.
I have been working with these units and remember that the total thickness off these units
can differ 1½-2  mm from one to the other side. This can  introduce a error int the sight.
Modern ships have also double glazing units in the wheelhouse, i have delivered replacement
units in the Rotterdam harbour,  the biggest units had 19mm thick glass on the outside, 15mm spacer,
and 6mm glass on the inside. Be aware that all these glasses are thermical thougened.(heated to
650° C and the chilled with air) for safety reasons. But the thougening proces diforms the glasses
they are not more a flatt as original. The old production facility where i bought thougened glasses
had a tolerance off 1½ mm per meter witdh. So the glass could be curved 1½ mm depth  over one
meter that gives optical distorision.

So be suspicious with observations through double glazing units and even single pane thougened glass.

regards,
HermanD