A telescope mirror is a piece of glass with a microscopically thin layer of aluminium on the front, and almost everything owners worry about with mirrors turns out to be either harmless or fixable.
What the coating is and how it ages
A few tens of nanometres of aluminium, usually with a protective overcoat, deposited in a vacuum. It reflects about ninety per cent of the light when new. Over ten to twenty years, humidity and pollutants dull it, and reflectivity drops noticeably. Recoating is a service several firms offer and it costs a fraction of a new instrument.
Dust, which matters far less than it looks
A visibly dusty mirror loses well under one per cent of its light and scatters slightly more. Cleaning it badly, by wiping it dry or with the wrong cloth, leaves permanent fine scratches. The right approach is to clean rarely, only when there is a visible film, and by soaking and rinsing rather than rubbing.
Replacement parts and what breaks
Focusers, finder shoes, collimation screws, springs and the plastic parts of mounts are what actually fail, and all are available separately from the major brands. The optics themselves almost never fail, which is why a twenty year old telescope with a tired coating is usually a bargain rather than a risk.
Buying a mirror on its own
Primary mirrors are sold separately, mostly to people building or refurbishing dobsonians. It is a genuine and rewarding path, and it is a project rather than a purchase: the mirror is the cheapest part of what follows, which is a tube, a cell, a secondary, a focuser and a base.
Questions people ask about telescope
How long does a telescope mirror coating last?
Ten to twenty years in ordinary use before reflectivity falls noticeably. Recoating services exist and cost far less than a new telescope.
Should I clean my telescope mirror?
Only when there is a visible film, and by soaking and rinsing rather than wiping. Dust costs almost no light; scratches are permanent.
Can you buy telescope mirrors separately?
Yes, mostly for building or refurbishing dobsonians. The mirror is usually the cheapest part of the resulting project.