Quote Originally Posted by BetterSense View Post
But you need an angle-measuring device to measure the
angles and see if they are the same.
A square or triangle, draftsman's or grade schooler's, is
needed. Ninety degrees is THE angle. Only TWO checks of
the internal angles need be made. If the object projected
is a rectangle and the projected image tests square at
two corners the enlarger is well aligned.

The same test will do to confirm progress when actually
working to align an enlarger. After all, the finale proof
of alignment is on the baseboard. Dan