Question in Detail:
Can I stretch the film between 2 Plexiglas panes? (5mm each) The two panes are then placed in a plywood frame. (several 10mm layers glued on top of each other) I put the whole thing over my Bresser 8″.
I mainly want to protect the foil from contact and damage. Whether this makes sense optically or photographically – no idea.
Answer: technically this is of course possible – but even if you could find planeoptically polished Plexiglas of the highest optical quality, the picture would be much worse than without it..
On the one hand, the film is designed to sit loosely. If it is put under tension, the contrast drops. This is also the reason why we offer our ASTF filters with temperature compensated filter holders: If you glue the film e.g. into a black frame (as is often done with cheap holders) and this cheap holder heats up during observation, the image will also deteriorate during longer observation and become „muddy“. Of course, then the film will be blamed and not the frame.
And on the other hand one does not observe (hopefully) through the closed windows of an apartment… the effect would be the same: a muddy sun as a white ball without structure..
To find two planeoptically polished Plexiglas panes without wedge errors (i.e. with exactly parallel surfaces) and to mount them perfectly parallel is an impossibility. This is because flat surfaces are harder to produce than sphere/parabolic mirrors. Each such Plexi precision disk would be more expensive than the mirror of your telescope.
Our many years of experience have shown that if you store the film filter in a dustproof and moisture-protected cardboard box, it can be used for many years. As long as the film is not processed with pointed objects, the film coated on both sides is surprisingly robust. If you install it in a slightly wider frame, you can handle the sun filter well without worrying about accidentally touching the coated film.
Therefore we can only advise you to use the film as it is and to avoid any additional „protective layer“.