Why your telescope's coatings deserve respect

Your telescope's lens might look like a simple piece of glass, but it is a precision optical surface covered in thin layers of anti-reflective coatings. These coatings are designed to let more light through and reduce ghosting, but they are soft and easily damaged by abrasive dust, harsh chemicals, or even repeated wiping. In the UK, where damp air, pollen, and coastal salt spray are common, it is tempting to clean your lens often. Resist that urge. A little dust has almost no effect on the view, but a single scratch in the coating is permanent and will scatter light every time you look at the Moon or a distant galaxy. The goal is not sterile perfection; it is careful, infrequent maintenance that preserves your optics for years of stargazing.

First, remove loose dust without touching the glass

Before you apply any liquid or cloth, you must remove every particle of grit, sand, or dried pollen. If you wipe a lens with dust still on it, you are effectively grinding those particles into the coating. Start with a good quality rubber-bulb blower. Never use canned compressed air, as it can spit propellant or liquid onto the lens. Hold the telescope so the lens faces slightly downwards, then puff air across the surface from the centre outwards. For stubborn dust, use a soft camel-hair brush or a dedicated lens brush. Brush very gently, without pressing the bristles against the glass. If you are at a dark-sky site in the Yorkshire Dales or on a beach in Cornwall, do this before packing up, so grit does not get a chance to settle and rub during transport.

Safe solutions for smudges and fingerprints

After dusting, if you still see fingerprints, dew spots, or a hazy film, you need a safe cleaning solution. The best choice is a 70% isopropyl alcohol (IPA) solution diluted with distilled water—or a ready-made optical cleaner from a camera shop. Avoid acetone, ammonia, window cleaner, vinegar, and household polish. These can strip coatings or leave residues. In hard-water areas, tap water leaves mineral spots, so always use distilled or deionised water. A very mild alternative is one tiny drop of washing-up liquid in 500 ml of distilled water, but this must be rinsed with distilled water afterwards, which is risky for a telescope lens. For most UK observers, a simple 50/50 mix of 70% IPA and distilled water is ideal. Never spray solution directly onto the lens; apply it to a clean microfibre cloth or a fresh lens tissue instead.

The gentle cleaning method, step by step

• Wash your hands thoroughly to remove natural oils, then work indoors at room temperature—never in a cold shed or damp garage.
• Blow and brush the lens as described above, checking with a bright torch held at an angle.
• Fold a clean, lint-free microfibre cloth into a pad, or use a fresh lens tissue. Put two or three drops of your solution onto the cloth, not the lens.
• Wipe in a single, very gentle circular motion from the centre outwards. Do not scrub back and forth. Use a fresh part of the cloth for each pass.
• If a smudge remains, breathe lightly on the lens and use a new dry portion of cloth. Avoid pressing hard; let the solvent do the work.
• Inspect again with the torch. If you see streaks, repeat once with a fresh cloth. Allow any remaining moisture to air-dry.
• If dew has formed on a cold lens, let it evaporate naturally before cleaning. Wiping a dewed lens can drag dust across the surface.

Remember that a faint mark that refuses to shift is far less harmful than aggressive rubbing. Leave it alone and enjoy the night sky.

When to leave your lens alone, and how to store it

Overcleaning is the most common way UK astronomers damage their optics. Clean your telescope lens only when views are noticeably dim or hazy—perhaps once or twice a year for a frequently used scope. Between sessions, keep lens caps on, and store the telescope in a dry, dust-free cupboard with a sachet of silica gel. Avoid leaving it in a cold conservatory or damp outbuilding where condensation cycles can attack coatings. Before observing, let the scope adjust to outdoor temperature to reduce dew, and use a dew shield. After a session, bring the scope indoors, remove the caps, and let any condensation evaporate fully before capping it again. Your lens will reward this care with crisp, bright views of the Orion Nebula, the rings of Saturn, and all the quiet wonders of the UK night sky.