  The Cherenkov Telescope Array (CTA) observatory for very high-energy gamma
rays will consist of about a hundred of imaging atmospheric Cherenkov
telescopes (IACTs) of different size with a total reflective area of about
10,000 m$^2$. Here we present a novel technology for the production of IACT
mirrors that has been developed in the Institute of Nuclear Physics PAS in
Krakow, Poland. The mirrors are made by cold-slumping of the front reflecting
aluminium-coated panel and the rear panel interspaced with aluminium spacers.
Each panel is built of two glass panels laminated with a layer of a fibreglass
tissue in between for reinforcement of the structure against mechanical damage.
The mirror structure is open and does not require a perfect sealing needed in
closed-type designs. It prohibits water to be trapped inside and enables a
proper ventilation of the mirror. Full-size hexagonal prototype mirrors
produced for the medium-sized CTA telescopes will be presented together with
the results of recent comprehensive optical and durability tests. Their design
will be compared to the earlier technology developed at INP PAS that used a
rigid flat open support structure with a reflective layer made by cold-slumping
of the coated glass panel to the cast-in-mould spherical epoxy resin layer.
