reflecting telescope
Instead of bending light through a lens, what if you bounced it off a curved mirror? That is the idea behind the reflecting telescope, first built by Isaac Newton in 1668 and the design behind essentially every great observatory today, from the giants on mountaintops to the Hubble and James Webb space telescopes.
A reflector uses a large concave primary mirror, shaped like a shallow bowl, at the bottom of the tube. Light falls onto it and is reflected back up to a focus. A small secondary mirror usually redirects that converging beam out to a convenient spot — to the side (the Newtonian design) or back through a hole in the primary (the Cassegrain design) — where a camera or detector sits. Because the mirror reflects light from its front surface, all colours bounce the same way, so there is no chromatic aberration at all.
Mirrors win for big telescopes for plain engineering reasons: a mirror can be supported across its entire back, so it does not sag like an edge-held lens, and only one surface must be ground perfectly. This lets mirrors grow enormous. The largest single mirrors are about 8.4 m across, and segmented designs that tile many hexagons together — like Webb's 6.5 m mirror or the planned 39 m Extremely Large Telescope — push far beyond. Every research telescope of the last century has been a reflector.
The Hubble Space Telescope's 2.4 m primary mirror was ground so perfectly that, after a famous flaw was corrected in orbit, its images stay sharp to the diffraction limit — far better than any ground-based view of its era.
Mirrors are why telescopes can grow huge.
A reflector's mirror does not 'use up' light it absorbs; modern coatings reflect over 90% of visible light. The small secondary mirror and its support blocking the centre cause only a tiny loss, not a hole in the image.