Maintaining and Cleaning Your Telescope

Telescopes are more robust than their owners fear and more delicate than their owners assume. Most maintenance is boring and preventive, and most damage is done during cleaning rather than during use.

Routine checks

Before each session: check leg locks, mount bolts and the finder's ring screws are snug.

Check collimation on a reflector — two minutes with a Cheshire or laser collimator.

Check the focuser moves smoothly through its full travel with the imaging load attached.

On a GoTo mount, check the clutches hold and that cables are routed so they cannot snag during a slew.

After each session: cap everything, let the telescope dry fully before storing, and store it capped.

Collimation

Collimation is aligning a reflector's optical axes. Get it wrong and stars show a flared, asymmetric coma instead of tight points, and planetary detail vanishes. The good news is that it takes about two minutes once you have done it a few times.

Rough check without tools: defocus on a bright star. The shadow of the secondary should sit centrally in the out-of-focus disc.

Align the secondary first — tilt and rotation, so the primary's reflection sits centrally in it.

Then align the primary, using the Cheshire ring or the laser's return dot.

Confirm on a star at high power: an in-focus Airy pattern should show concentric rings, and defocusing should show concentric circles.

Tip

Collimation tolerance tightens dramatically at fast focal ratios. An f/4 Newtonian is roughly four times more sensitive than an f/8, which is why fast imaging reflectors need a coma corrector as well as careful collimation.

Refractors and catadioptrics do not need routine collimation. An SCT has secondary tilt screws, but they rarely need touching, and a refractor's lens cell is set at the factory and should be left alone.

Cleaning optics: mostly do not

Dust on an objective or primary mirror costs a fraction of a percent of contrast. Wiping it off risks dragging grit across the coating and leaving fine scratches that scatter light permanently. That trade is not close: leave dust alone.

Clean when there is actual contamination — fingerprints, pollen residue, or dried dew deposits. A primary mirror might need washing once every three to ten years. An eyepiece's outer eye lens needs it far more often, because it meets eyelashes and skin oil every session.

If you must clean a lens

Blow off loose dust with a bulb blower. Never use canned air — the propellant can condense on the glass.

Brush gently with a camel-hair brush, from the centre outwards, lifting rather than sweeping.

Dampen lint-free lens tissue with optical cleaning fluid — never household glass cleaner, whose ammonia attacks coatings.

Wipe once, gently, in a single stroke, using fresh tissue for each pass. Do not scrub.

Let it dry in a dust-free indoor space, not outside where dust is blowing.

Washing a primary mirror

Only when genuinely necessary. Remove the mirror cell, rinse with distilled water, wash with a drop of mild dish soap using clean cotton balls rolled gently across the surface — never pressed and dragged — then rinse with distilled water and let it air-dry at an angle.

Note

A mirror's reflective coating sits on the front surface and is far softer than a lens coating. Never wipe a mirror dry. If your mirror needs attention more than once a decade, something else is wrong — usually dew or a storage environment problem.

Storage

Store capped, in a dry place, with the tube horizontal or pointing slightly downwards so dust does not settle on the optics.
Use silica gel in the case if you store in a damp garage or shed.
Never store a telescope while it is still wet from dew. Let it dry fully first, or you will grow mould on the coatings.
Store eyepieces capped and upright in a case; the eye lens is the surface that gets dirty.
Remove batteries from finders, red-dot sights and mounts before long storage.

Mechanical care

A Dobsonian's altitude bearings need occasional tensioning; the tube should hold position at any angle but move with one finger. Mount gears should not be regreased unless you are prepared to reassemble them correctly — on most mounts that is a manufacturer service job rather than a home task.

Tripods deserve more attention than they get. Check leg locks, tighten the spreader, and keep the feet clean. A tripod that has been dragged through mud will not damp vibration the way it did when new.

Tip

The most valuable maintenance habit costs nothing: let the telescope reach ambient temperature before observing and let it dry completely before storing. Both prevent more damage than any cleaning regime.

Note

These are general guidelines for common amateur equipment. Always follow your manufacturer's instructions where they differ, particularly for coated optics and for catadioptric corrector plates.

Disclaimer

This article is educational reference material. Figures are typical approximations, and no observing result is guaranteed — your equipment, sky and conditions will differ.

Tobias Lindqvist

Optics and instrumentation. Writes about what telescopes actually do rather than what their marketing says.