Astronomy Hacks

Clean Your Primary Mirror

Eliminate crud and protect against corrosion.

The primary mirror of a Newtonian reflector, exposed as it is to outside air, inevitably gathers dust, water spots, film and haze from air pollution and other contaminants. Depending on how often you use your scope, how well you protect it against dust and other contaminants when it is not being used, and the environment at your observing site, your mirror may become dirty enough to need cleaning every few months or it may remain acceptably clean for years on end.

The only way to determine if a primary mirror needs to be cleaned is to remove it from the scope and examine it. Newbies often shine a flashlight down the tube and are invariably horrified at how dirty the mirror appears, even when the telescope is new. No mirror, even one freshly cleaned, can survive the “flashlight test.” Every speck of dust and every smudge jumps out at you.

Dust and smudges appear ugly, but the truth is that they have little effect on image quality. Consider this: your secondary mirror sits directly in the path of the incoming light, and so is the equivalent of a chunk of dust or a smudge a couple of inches in diameter, and it has almost zero effect on image contrast and clarity. How little, then, will a few small smudges and some dust affect what you see? Not at all.

The real reason for cleaning a mirror is to remove the surface film of air pollutants that may eventually eat into your mirror’s surface, etching it and exposing ...