Should you clean it at all?
Cleaning is the riskiest routine job an owner does to a reflector, so the first question is whether it is needed. A thin, even layer of dust on a primary mirror has only a small effect on what you see, mostly a slight loss of contrast. Many experienced observers clean their mirrors only every few years, or less, and some advise that a mirror should only be cleaned when contamination is clearly affecting the image.
Reasons you might decide to clean include a heavy coat of dust or pollen, insect residue, finger marks or a hazy film that has built up over time. If you are unsure, compare the view with and without the dusty mirror, or ask at a local astronomy club. Regular care, such as keeping the telescope covered and capped, reduces the need.
Try dust removal first
For light dust, you may not need to wash the mirror at all.
- Hold the mirror facing down and use a rubber bulb blower to blow dust off.
- Avoid canned compressed air, which can spit propellant liquid onto the surface.
- Avoid dragging a brush or cloth over dry dust, which can scratch the coating.
If a bulb blower clears it, stop there.
What you need
- Screwdrivers or hex keys to remove the mirror cell, as needed for your model.
- A clean sink or plastic tub, lined with a soft towel.
- Mild, fragrance-free liquid dish soap, with no added lotions or moisturisers.
- Distilled water for the final rinse, since tap water can leave mineral spots.
- Fresh cotton balls or pads, used once each.
- Clean, lint-free paper towels or a clean towel for the surroundings, and nitrile gloves if you prefer.
Some guides add 99% isopropyl alcohol as an optional final step. That choice is debated, so go by your telescope maker’s advice. Never use window cleaner, abrasives, or household sprays on a mirror.
Removing the mirror
Many mirrors are held in a cell with clips and screws. Before removing it:
- Take photos of the cell and mark the orientation, for example with a note on the tube, so you can put things back the same way.
- Note the positions of collimation screws. Do not change them while removing the cell.
- Handle the mirror by its edges only, and never touch the reflective surface.
- Remove the mirror carefully from the cell, since clips and a small amount of silicone may hold it in place.
If you are unsure, an experienced club member may be able to show you the process in person.
Washing step by step
- Let the mirror come to room temperature. Putting a cold mirror into warm water can stress glass.
- Rinse gently with lukewarm distilled or tap water to remove loose grit before touching it.
- Add a few drops of dish soap to water in the tub, and submerge the mirror, or pour soapy water over it.
- Use fresh cotton pads and wipe lightly in a single straight line across the surface with almost no pressure, letting the soap do the work. Use a fresh pad for each pass, and do not rub circles.
- Rinse thoroughly with clean water until all soap is gone.
- A final rinse of distilled water helps prevent spots.
Rinse and dry
Tilt the mirror so the water runs off in a sheet, and let it air dry standing on a soft towel at an angle, face away from dust. You can also use a bulb blower to remove the last beads. Do not wipe the surface dry, and leave it until it is completely dry before moving anything.
Reinstall and collimate
Refit the mirror into the cell as you found it, without over-tightening the clips, which can distort the mirror and cause pinched images. Once it is back in the telescope, you will need to check and likely adjust collimation. The wider telescope troubleshooting guide covers common alignment symptoms.
Coatings and recoating
Most telescope mirrors have a thin reflective layer, commonly aluminium, with a protective overcoat on many modern ones. Coatings wear over years, and can show pinholes, tarnish or stains. If a mirror’s coating is badly degraded, a professional recoating service can re-aluminise it. Recoating is typically done by specialist companies, and costs vary, so it is worth requesting a quote. Cloudy or stubborn film that does not wash off may be damage to the coating itself rather than dirt.
Mistakes to avoid
- Cleaning too often or as a routine.
- Touching or breathing on the surface.
- Using paper tissue, kitchen towel or microfibre cloth directly on the mirror.
- Scrubbing or circular wiping.
- Using hot water on a cold mirror.
- Cleaning the secondary mirror in place without support. It is small and delicate.
- Forgetting to mark orientation and collimation.
Humid nights lead to dew, which can leave marks on the mirror; see dew on a telescope. For refractor and eyepiece lenses, see cleaning telescope eyepieces.
Frequently asked questions
Will isopropyl alcohol damage a mirror? Pure isopropyl alcohol is commonly used on optics, but it may not suit all coatings and it can streak. Check your manufacturer’s advice, and never use it on a mirror with a damaged or fragile coating.
What are telescope mirrors coated with? Most are coated with a thin layer of aluminium, often with a protective overcoat such as silicon dioxide. Some use enhanced or dielectric coatings.
How do you get cloudy film off a mirror? Some film washes off with soap and distilled water. Persistent haze may be coating damage that only recoating can fix.