Detailed question:
I have constructed a lot of solar filters using your AstroSolar® film. I have always used the system with 2 cardboard rolls. Recently I tried the system described with the film with two cardboard rings and additional PVA glue near the edges (1 cm) of the bi-adhesive film. For that I’ve used a very stiff cardboard. When the glue dried, the film was stretched almost wrinkle-free. Only around the edges there is still a small wave. Is this a safety risk (if the film isn’t wavy), or is that something merely optical, so the picture won’t be as perfect as it could be (that’s no problem for me)? Please advise if the filter is risky to use, or can I use this filter without hesitation?
Answer: If the film is stretched, that makes no real security concern. It only affects the image quality. At magnifications higher than 60 times, this doesn’t allow focusing and will increase the blur. Therefore we recommend for those who make their own filters to drop the cardboard film holder on top of the film so it remains relaxed and slightly wavy.
Since the optical performance is not important to you, and since you can’t break it apart because of the PVA glue, you can use your mounted filter without any concerns. We recommend the method you described to construct a filter, especially avoiding a metal frame, so that the extensive tension won’t be a problem as the cardboard is more flexible and won’t let the film tear apart.
Where we don’t want to see such tension is when using metal frames for the film holder. Thermal expansion could create more tension and since the film is definitely not stronger than the metal frame, it would break. That is the reason why we decided to produce our own filters after 15 years.