I see
Spherical abberation is a result of inadequate edge clamping..........that's a known fact, no matter what the material.
If the Mylar is no good as an accurate surface for a mirror, why have there been small working telescopes made and pictures of such published?............I have seen them on the web site of the stretchable membrane mirror under Strathclyde University and Ethereal Technology.
I for some reason at the moment, can't post the evidence of the PDF I have on my work computer, but will get our I.S dept. to assist me next week.
That site also has a graph of the actual curve. It differs from a parabola, but I'm not convinced that it''s totally opposite or wrong to what is required by a telescope??