A large concave mirror, even of low quality, has lots of light-collecting power. Would such a mirror suffice for low-power views of extended deep-sky objects, even if it didn't show stars as neat dots? For example, a 20-inch plastic mirror might be fairly inexpensive.
My 7th-grade science teacher once let me borrow a very homemade 6-inch reflector that had been left to the school. I was thrilled with the deep-sky views it gave, since all Iâ€™d used up to that point was a 3-inch reflector. Then a knife-edge test revealed the truth: The 6-inch primary mirror had a hideous figure, nowhere near a sphere (let alone paraboloid).
So you're right: Optical quality is not essential for low-power enjoyment of the sky. The quest for cheap alternatives to fine optics is as old as telescope making itself.
Nearly 30 years ago English amateur Maurice Gavin made an experimental 21-inch mirror of aluminized Mylar (S&T: May 1979, page 489), so I e-mailed him for an update. "Unfortunately, the figure is very crude and will not, in my experience, produce viewable images — each pointlike star source is diffused over maybe a whole degree or more. When the membrane stretches under partial vacuum, the cross section of the surface is too steep just inside the rim and too shallow elsewhere — the complete reverse of a parabola!" He summarizes his findings at home.freeuk.com/m.gavin/flux.htm.
Maybe there's a better way using other materials or techniques. Find it, and you'll be world famous.
— Roger W. Sinnott