Some camera stands for reproducing documents use an inflated pillow or a layer of synthetic foam at the back of the document to press it against a glass plate. This would allow you to use a thinner glass, anchored to a frame along its edges so that it does not lift up. This is an interesting idea, ...

As you already suspect, 20 mm of glass is going to introduce strong spherical aberrations. It will also introduce axial chromatic aberration, although I am not sure whether the amount will be enough to notice, and lateral chromatic aberration (probably to a much larger extent). The problems can be ...

I am looking to scan opaque diagrams/documents, SD card to 6x8 film size, at 0.8:1 to 2:1 magnification. As you can imagine this requires me to flatten them out, and so I have been using some float glass taken out of an old scanner, that's abit larger than A4. I'm now looking to replace this as I di...