How to Clean the Optics or Lenses of a Telescope

Every telescope or binocular used for astronomy, no matter how cheap, deserves the best care, including cleaning the optics. Many times you will use it to the limit of its ability and when you are trying to see very weak objects or faint details, a good cleaning can make the difference.

The first tactic against dirt is the preventive one. Always keep the caps when the tool is not in use, and if a cap is missing, create it yourself, with cardboard or even with a plastic bag. The eyepieces should be covered on both ends or stored in plastic bags or small food containers.

Never touch the surface of a lens or mirror with your fingers. Acids in skin oil can attack optical coatings over time. So if you leave a fingerprint, for example, on a binocular lens, clean it using the method described in the next few paragraphs.

Every telescope gets dirty! Dirt on lenses or mirrors, make dark skies less dark and less obvious luminous objects – but not as much as we probably believe.

Harold Richard Suiter analyzed the effects of dirt on optics, using mathematical methods. His conclusion? “The maximum tolerable amount of dirt on a lens should be about 1/1000 of the surface, or a 1/30 of the diameter to be appreciable”.

If you are in doubt or not whether to clean the optics, then this is the time to stop. In fact, the lenses should be cleaned only when you believe it must be done!

A dirty lens or mirror can always be cleaned in the future, but a scratched optic is forever. The cleaning causes little scratches if not done properly, so it is advisable to clean it rarely, but if the lenses become really dirty, here’s how to clean them.

Cleaning the Lenses, Eyepieces and Filters

The quickest and easiest way to remove dust from the eyepieces is to blow strongly on the lenses, but be careful not to leave traces of saliva.

If this does not work, the traditional method for cleaning the lens is to brush lightly with a camel hair brush. Sold in camera shops, these brushes have soft bristles with the slightest tendency to scrape the grit against the lens. Brush lightly and store the brush in a place protected from dust.

Camera shops also sell cans of compressed air to wipe away the dust, but be careful with the types that use liquid propellants. These have the reputation of leaving residue on the lenses if the can is shaken during use, so be careful.

For more dirty or stained lenses, different cleaning solutions are available. Good ones are pure isopropyl alcohol or methyl alcohol (methanol), available in all hardware stores or household. Standard isopropyl alcohol works well and is easier to find, but avoid alcoholic preparations with other components that may leave stains. Camera shops sell liquid for cleaning lenses such as Crystal Clear, which is pure methanol, but it is possible to obtain much cheaper methanol in a hardware store. Also available are “pens” with a soft solvent-impregnated cleaning mat.

Generally, for a good cleaning, you will need a soft cotton cloth, moisten it with the fluid and pass it gently on the lens or filter, without exerting too much pressure. If necessary, give another coat with a dry cloth to prevent any residue from remaining on the mirrors.

Pay particular attention to fingerprints that can permanently discolor coatings if they remain long enough on the surface. Even with the formation of these spots, the performance should not be affected, but better to be avoided.

Cleaning the Mirrors

Images taken from Rob Hawley blog, take a look at this article here:

Newton mirror before the cleaning

The mirror during the cleaning with neutral soap

Use a cotton swab dipped in alcohol or water for more persistent dirt

To clean the mirrors of a reflecting telescope, you will need to be able to disassemble the telescope and then reassemble it, then re-collimate it again.

How do you know if your mirrors need cleaning? It’s simple: if you’re asking the question, do not do it! And do the cleaning only in case you have no doubts.

If cleaning is necessary here is how to proceed:

Remove the screws that secure the primary mirror cell to the rear end of the tube. Reach at the back and gently pull out the cell with the mirror inserted. Unscrew the hooks that hold the mirror and push the mirror from the back without touching the surface.

Also, you may need to remove the small secondary mirror holder inside the front end of the tube, then pull the secondary mirror out of the holder, or if you can clean it without removing it, better!

The first thing to do is to remove all the dirt correctly. In fact, the ordinary dust that we have at home contains particles of rock dust that rubbed on the reflecting surface, causes micro-scratches.

You will need a sink or receptacle, two towels, a neutral liquid detergent, a bottle of distilled or demineralized (“de-ionized”) water available in pharmacies and sterile cotton. Wash the sink, rinse it well and place a folded napkin on the bottom. Remove all the jewels from your hands and wrists. Put the mirror face up on the napkin and open the tap. This will remove most of the dust.

Close the tap and give the mirror a final rinse with a layer of distilled or demineralized water. This will not leave any mineral deposits when it will dry out. Place the mirror on the edge (on a folded sheet to prevent slipping) and let it dry. It is possible to carefully wipe resistant water droplets with the corner of a paper towel. If the mirror looks clean enough you are already well under way and you can leave it like this.

If it is still dirty, and on the surface there is not only dust, then you have to opt for the plan B. Put the mirror in the sink, put the mirror on the napkin and fill the sink in half with warm water. Add some liquid detergent and leave to rest for 5 or 10 minutes. Then, holding it underwater, take a cotton ball and gently wipe it over the mirror.

Disconnect the sink and let the warm water pass over the mirror for a minute. Finish with a rinse of distilled water and tilt the mirror to the edge to dry. Repeat the process with the small secondary mirror.

The right attitude is to be vigilant in preventing dirt on lenses or lenses. Perfectionists will never be happy, but astronomy should be fun. After all, what matters is not what you see on the telescope, but what you see through it.

Filippo Pavone

Ciao, sono Filippo! Giovane astrofilo con la passione per le riprese di oggetti del sistema solare. Nel tempo libero sviluppo Emout e mi occupo della sua manutenzione!

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