It seems to me, that a bi-convex lens could be replaced by two plano-convex lenses, positioned plano-to-plano with zero separation, to the same effect.
Is that true? What are the downsides? Should they be glued or the gap oiled and taped? I assume there is some reduction of transmitted light.
I ask, because I am trying to assemble a telescope that calls for a lens thicker than any affordable glass, but less than twice as thick as what I have.