Are my filters not optically flat?

[Josh Wilson]

While using my filters on Jupiter last night, I pulled my EP out of the focuser and with out changing the focus, I put my filter on the eyepiece and put it back into the focuser and I noticed that focus is lost even though the eyepiece is in the exact same location.

I have a crayford focuser and anyone who has one knows that the EP does not rest on anything inside the focuser and if filters are added the eyepiece is still in the same position, as mentioned above. So why do I need to change the focus? Are my filters not flat or is it something I'm not considering?

[ronin]

Nothing to do with flatness, the glass has a refractive index, usually around 1.6, so the light path through the glass is 1.6x(the glass thickness).

So if the glass is 4mm thick then the path has changed by 2.4mm and you have to refocus by this amount, if the glass is 2mm then your path has changed by about 1.2mm. The amount of refocus depends on the thickness of the glass filter.