Cleaning Telescope Lenses, Mirrors & Filters – Extensive Guide

Photo of author
Written By: Zane Landers
Category: Learn

Cleaning telescope lenses, especially your frequently-used eyepieces, is something that pretty much has to be done. Some people are so frightened of the idea that they’d rather pay half the cost of the gear itself to have it done by professionals. On the flip side, some overzealous users can cause damage by cleaning their optics too much or incorrectly. The good news is that if done right, cleaning your optics is an easy process that’s likely to incur little risk while saving you a lot of money compared to paying the pros for such a job.

Slightly Dirty Mirror
Slightly dirty mirror; pic by Zane Landers

When should I clean my lenses in the first place?

If your telescope’s objective lens or front corrector plate is obviously smudged with fingerprints or has stuff like pollen or dew residue on it, you should clean it right away. The oils and acids in all of these substances can eat the optical coatings on lenses or even chemically etch the glass itself. Similarly, fog from outgassing inside the telescope should also be cleaned. Otherwise, cleaning is usually not necessary unless a lot of dust has accumulated.

Eyepieces get regularly smudged with dew, dandruff, eyelash oil, makeup, eyeliner, and, of course, accidental fingerprints. Some of this gunk is not visible until it has built up for quite a while. In general, cleaning your eyepieces at least a few times a year, or every few nights, is probably wise.

A dirty eyepiece which is to be cleaned
An eyepiece with dust and fingerprints

Telescope filters and Barlow lenses generally don’t need cleaning unless they fall under the same circumstances as larger optics, where something potentially damaging has accumulated and needs to be removed right away.

Always remember: the only day your precious optic will look pristine is the day it arrived. Dust is only a concern if it builds up in large amounts, and it’s always better to let things be than risk pointlessly potentially dropping or scratching your optic if you can avoid it. However, cleaning the telescope mirrors, lenses, and eyepieces is a must when there is potentially damaging residue, as mentioned above, or when the optic is absolutely filthy from dust and dirt. Ordinary dust often has abrasive grit particles in it, which can scratch your lenses and mirrors if you’re not careful!

Removing lenses for cleaning

In general, you should not remove lenses from their holders and retaining rings unless absolutely necessary; this is a job that is rarely, if ever, required and is best left to professionals. It is far too easy to contaminate things between lenses or to put things back together incorrectly. The exception to this is Schmidt-Cassegrain corrector plates, which we will get to later. However, if the lens is hard to reach, it might be worth taking the lens/cell out or unscrewing the barrel of an eyepiece or accessory to get to its optics more easily.

Refractor and Maksutov-Cassegrain lenses are generally mounted in a cell that unscrews from the front of the telescope. In the case of refractors, the dew shield also generally unscrews or pops off. You probably don’t need to unscrew the lens from the front of the telescope unless the inside surface is somehow contaminated, and you should be careful to keep the inside of the telescope clean while the lens is off.

Eyepieces generally have barrels that can be unscrewed from the bottom to more easily access the inner lenses. However, if your eyepiece has lenses in the unscrewable barrel, you shouldn’t attempt to take it off. Even tiny dust particles getting in between lenses will permanently mar your views and are next to impossible to avoid or remove once they get stuck there. Barlow lenses generally have a lens cell that unscrews from the rest of the barrel for easy cleaning.

Schmidt-Cassegrain telescopes’ corrector lenses are cited by many as being practically impossible to remove and re-install correctly. As they are made uniquely for each telescope, they have small shims to keep them centered in the tube and must be rotated back to a particular orientation when re-installed. However, it’s actually fairly easy to do if you must clean the inside of the corrector (typically, this is needed for older telescopes that have a film of outgassed substance from internal adhesive). You just unscrew the retaining ring, carefully mark where all the shims go with a marker, and mark the edge of the corrector and a spot in the housing to keep the corrector’s rotation when you put it back in. 

While it’s not something you should do every day, there’s no need to send your SCT back to the manufacturer and spend hundreds of dollars as well as risk your precious telescope being destroyed in transit. In addition to removing any debris on the corrector itself, you may need to remove it anyway to clean out the optical tube as well as the primary and secondary mirrors in your Schmidt-Cassegrain.

Ways to clean your telescope lenses

Using air blowers

If your lens is just dusty, using a bulb blower is the easiest way to clean it since you don’t have to worry about touching it and risking fingerprints or scratches. A can of compressed air can deposit gunk on lenses, and the force is enough to shatter some thinner optics like Schmidt corrector plates; as such, stick with a rocket-type squeeze blower.

But a blower won’t get rid of fingerprints, pollen, or other residues. To clean these, you need a liquid solvent solution.

Using cleaning solutions & wipes

Microfiber cloths can be used to remove films and residues from eyepieces, but their tendency to pick up and drag dust particles over larger surfaces makes them an otherwise poor choice. Likewise, cotton balls can pick up and drag debris as well as deposit lint. The best way to clean your lenses without scratching them is to use disposable wipes and only clean small areas at a time, moving in circular motions and not dragging the wipe around too much.

Kimtech Science Dry Wipe, combined with virtually any lens cleaner solution, is one way to go. If you can’t find any trustworthy lens cleaner, making your own with distilled water and alcohol, or using pure alcohol, also suffice. The coatings are hard to damage with a typical cleaning solvent; the bigger concern is that it will leave a film or lint. Alternatively, pre-moistened Zeiss wipes are hands-down our favorite choice due to their convenience and trustworthiness. If you wear eyeglasses, you can use the Zeiss wipe while it’s still fresh to clean your glasses after you take care of your eyepiece or another lens (though it should not be used on another precision optic; eyeglasses won’t be affected by some small picked-up dust particles as they’re generally more resilient).

For cleaning small lenses where a wipe won’t reach all the way around, such as in eyepieces or Barlow lenses, a Q-tip with a small amount of lens cleaner solution works great, as does a LensPen, though the LensPens have issues with wearing out and we would still recommend a wipe for the surface if it’s big enough.

Other cleaning methods

If your telescope is extremely expensive or has extremely delicate coatings or lenses (such as a fluorite lens or a vintage/collectible instrument), using First Contact polymer might be worth the money. The First Contact polymer sticks to your lens or mirror and peels off much like a layer of glue, taking virtually every type of contaminant with it with zero risk of scratches, chemical damage, and so forth. However, it costs well over $100 USD for enough polymer to clean a typical telescope’s optics a few times at most, so it’s not the most cost-effective option for many.

Zane Landers

An amateur astronomer and telescope maker from Connecticut who has been featured on TIME magazineNational GeographicLa Vanguardia, and Clarin, The Guardian, The Arizona Daily Star, and Astronomy Technology Today and had won the Stellafane 1st and 3rd place Junior Awards in the 2018 Convention. Zane has owned over 425 telescopes, of which around 400 he has actually gotten to take out under the stars. These range from the stuff we review on TelescopicWatch to homemade or antique telescopes; the oldest he has owned or worked on so far was an Emil Busch refractor made shortly before the outbreak of World War I. Many of these are telescopes that he repaired or built.

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