There is an old adage that the best way to clean any piece of optical equipment is to prevent it getting dirty in the first place. Obviously this isn’t practical if you plan on doing something as reckless as actually using your binoculars, but you can certainly reduce the frequency of cleaning if you’re careful.
Almost all binoculars come with cases and a set of lens caps; keep them clean, inside and out, and make use of them whenever your binoculars aren’t in action.
Please don’t assume that lens caps alone are sufficient; they are not.
While it’s essential to keep the lenses as clean as possible, you also need to reduce the amount of dust that penetrates the various mechanical parts of your binoculars, especially the focusers and hinges, where it can increase the rate of wear.
Also, any grime that falls off the body of the binoculars could fall into their case or onto the inner surface of a lens cap, from where it can be transferred to an optical surface.
Your fingers might also pick up dirt from the body of the binoculars that could end up on the lenses.
In practice it is the optical surfaces, particularly the eyepieces, that are the most susceptible to damage.
Never be tempted to leave your binoculars uncapped, standing on a windowsill.
The eyepieces will immediately begin to accumulate dust, which has the potential to damage the lens coatings when it’s wiped off, particularly if done inexpertly.
You will probably find that your eyepiece lenses get dirty much more quickly than the objective lenses do.
This is largely because binoculars are eyepiece-up when you hang them from your neck, making the eyecups perfectly positioned receptacles for dust, pollen, cake crumbs, drips of coffee or any other contaminant that seems to be attracted with magnetic precision towards any exposed optical surface.
Standard individual eyepiece caps are not very convenient to use during an observing session, but a rain guard, tethered to the neck strap, is ideal.
If you make a habit of covering the eyepieces when you take the binoculars from your eyes and let it hang, it will soon become second nature, and you’ll find your eyepieces stay much cleaner.
There’s not a lot you can do about the objective lenses.
They accumulate little grime when they’re hanging downwards from the neck strap, but when you’re using them, they’re inevitably directed upwards, in a detritus-collecting orientation.
No matter how careful you are, your lenses will eventually need cleaning.
Small amounts of ordinary dust are best left; this doesn’t noticeably affect the image and, as long as it is not wiped, it won’t harm the lens.
However, there are some offenders that should be removed as soon as possible: pollen, fingerprints, eyelash grease, cosmetics and dew.
Pollen can be particularly harsh: some pollens are sticky, extremely sharp-edged and capable of etching lens coatings.
Fingerprints, eyelash grease and some cosmetics can start to etch the lens surface if they are left for a few days.
Dew is inevitable in Britain. It’s a good idea to wipe it off the body of the binoculars with a clean cloth (you’ll take some dust off with it), but never wipe it off a lens; let it evaporate.
You can speed this up with a hairdryer (for field use, a 12V camping hairdryer is usually a fraction of the cost of a dedicated astronomical ‘dew-gun’ that does the same job).
Initially you should point the hairdryer away from the lenses so that any dust that’s settled inside the dryer isn’t blasted at them.
Otherwise just leave the binoculars, uncapped and horizontal, somewhere warm and dry for a few hours.
Moisture is ruinous to binoculars, so always keep a sachet of silica gel in their case so that any residual moisture is absorbed.
When you clean your lenses, the watchword is ‘care’.
Never be tempted to use a handkerchief or the hem of your tee-shirt, but carefully follow our cleaning advice, and your binoculars will continue to give you their best.
The dirtiest part of your binoculars will almost certainly be the body, so use a clean, barely damp cloth to wipe down as much of the body as you can, and use cotton buds for any difficult-to-reach areas.
Don’t neglect the insides of the lens caps!
A bright light will reveal any contaminants that have accumulated on the lens.
With the binoculars horizontal so that dust cannot fall onto the lens, use a puffer – with the lens brush if necessary – to remove loose dust and flakes of debris from the lens.
Scrunch up a sheet of lens tissue, then roll it loosely and tear the roll in half to make two paper ‘brushes’.
Spray lens-cleaning fluid on one of the torn ends and gently clean the lens, starting in the middle and carefully working your way outwards.
When you’ve cleaned the entire surface, use the second lens tissue ‘brush’ to gently polish the lens, again starting from the middle and working outwards.
This should remove any residual cleaning fluid. Blow off any dust that collects on the lens during this process.
When you think the lens is clean, breathe on it to reveal any remaining marks or smears, and remove the condensation with another lens tissue.
Then re-examine the lens under your bright light and repeat the cleaning process if it’s not yet properly clean.
A lens pen is ideal for ‘emergency’ cleaning if you accidentally get a fingerprint or other small mark on the lens while observing.
Use the retractable brush to remove any dust from the lens, then work from the centre outwards with the carbon pad.
Binocular astronomy specialist Steve Tonkin has written multiple books in the Patrick Moore Practical Astronomy series