When I worked at Advanced Optics in 1983, I helped make an optical window for my boss. It was a a lot of fun! I don't think I saw the final result, but I remember him telling me to make it a wedge instead of two parallel faces so you don't get an interference pattern when you look through it. It was a square piece of glass about 1/4" to 1/2" thick and about six inches on a side.
We looked at it with a collimated light source (He had a fairly large sperical or paraboloidal mirror on the floor and a cardboard tube over it and a light shining on it so the rays coming up were parallel). After the initial polish, we saw that there were a lot of closely spaced fringe lines because I ground it so it was a slight wedge, which is what he wanted.
So if I made a window for a Newt. would it have to be a wedge or should the faces be parallel?