How to Safely Clean and Maintain Your Telescope Mirrors

Keeping your astronomy equipment clean and in good working order is as important as a cloudless sky. Dirty mirrors can cause dim and blurry images taking away from your enjoyment of the night sky, and we can’t have that, now can we?

The Importance of Cleaning Telescope Mirrors

Telescope mirrors, especially the primary mirror in a Newtonian reflector, are prone to accumulating dust and other particles over time. While a layer of dust might not have an immediate detrimental effect on your view, it can gradually degrade image quality and clarity. Additionally, factors such as pollen, foliage, dead insects, and moisture from dew or rain droplets can further compromise the mirror’s surface. Therefore, periodic cleaning is crucial to maintain optimal performance and ensure a breathtaking stargazing experience.

This is particularly true if your equipment has been stored for a long period of time without use. It is amazing how stuff can get in there, even when the covers are on it.

Tools and Materials You’ll Need

Before diving into the cleaning process, gather the following tools and materials:

Clean towels: These will serve as a resting place for your mirrors while they dry.
Distilled water: It is best to use distilled water for the final rinse to prevent mineral deposits.
Cotton balls: You should get high-grade cotton balls, not cheap ones.
Screwdrivers, Allen keys, and pen: You’ll need these tools to disassemble and reassemble your telescope, as well as mark alignment positions. You may or may not need other tools as specified in your owner’s manual.
Tepid water and cleaner: Warm water and a mild detergent such as dishwashing soap will suffice for the cleaning process.
Instructions: If you have a manual, or can find one online, that details how to take the mirrors out of your particular telescope that will help considerably.

Be Careful!

The most important thing to remember is that these mirrors are not like regular household mirrors in that they are not meant to be robustly cleaned or even handled. In fact, if you are careful and store your telescope properly, you may never need to clean your mirror and so manufacturers do not go to extremes in protecting them from scratches and chemicals.

What this means is that you should not clean your mirror any more than necessary, never apply more force than is required, and never use harsh chemicals. One good example of a bad idea is using a glass cleaner on the mirror. Yes, the mirror is glass, but the coatings are so light and thin (so that they do not absorb too much light) that a good ammonia-based glass cleaner and a normal household rag can strip the coatings in a matter of seconds.

You should know that virtually all telescope mirrors have dust on them, and that is fine. You should only clean your mirrors when they are so dirty that they substantially affect the image you see through them. If looking through them, you can not tell that they are filthy, then they are fine!

Step 1: Remove the Primary Mirror

To begin, carefully remove the primary mirror from your telescope. This process may vary depending on the telescope model but typically involves removing the screws around the rim of the telescope base and taking out the primary mirror cell. The mirror is held in place by mirror clips, which can be removed by unscrewing small bolts. Before removing the clips, make sure to mark the vertical edge of the mirror and the cell to ensure accurate reassembly.

Note that when you go to take the primary mirror out, it is very heavy. In most cases, it is far heavier than the rest of the telescope so when you go to pull it out you should be prepared for that imbalance of weight. I recommend you remove the mirror assembly with the telescope on a bed or couch so that if anything comes loose rapidly and falls, it falls onto something nice and soft.

Step 2: Remove the Secondary Mirror

With the telescope set horizontally, you can now remove the secondary mirror. Hold the edge of the mirror gently and undo the center screw on the spider vane. Slowly take out the mirror and set it aside for cleaning. It is essential to handle the mirror with care to avoid any accidental damage.

Do not try this with the telescope sitting vertically as this mirror too is fairly heavy for its size and easy to drop. If you drop it two inches to the side of the telescope, you might not hurt it. Drop that same mirror three feet out the bottom and you probably will damage it.

Step 3: Cleaning the Mirrors

Before proceeding with the cleaning process, prepare a freshly washed plastic bowl to ensure a clean and safe environment for the mirrors. Each mirror should be cleaned separately to prevent contamination.

Blow off loose debris: If you have a can of compressed air, an air compressor, or a small hand blower, use that to blow away any dust and cobwebs that may be loosely attached.
Soak the mirrors: Place the mirrors in their own plastic bowl filled with tepid water and a small amount of detergent. Let them soak for approximately five minutes to loosen dust and particles.
Rinse the mirrors: After soaking, carefully rinse each mirror in tepid water. Place them on a soft, clean towel with the mirror surface at an angle to allow excess water to drain off. Refill the plastic bowl with fresh water after each step of the cleaning process.
Cleaning the mirrors: Take a piece of well-soaked high-grade cotton wool and gently drag it across the mirror surface using the weight of the cotton wool itself. Avoid applying excessive pressure or scrubbing the surface. A stuck piece of debris is far better than a scratched mirror. Each stroke should be followed by discarding the used cotton wool and using a fresh piece for the next stroke. Repeat this process until the entire mirror surface has been treated.
Rinse with distilled water: After completing the cleaning process, generously rinse the mirror surfaces with distilled water. This step ensures the removal of any residue or deposits that might have resulted from tap water. Rinse thoroughly to avoid leaving behind any traces of impurities. I like to rinse twice, just to make sure.
Drying the mirrors: Place the mirrors, supported at a steep angle, on clean towels to dry. Ensure that the mirrors are in a dust-free environment during the drying process. Avoid touching the mirror surfaces or using any abrasive materials that could scratch the delicate coatings.

Pro-tip: It is crucial to maintain a dust-free environment during the cleaning process to prevent any additional particles from adhering to the mirrors. Consider performing the cleaning process in a room with minimal air circulation or use an air purifier to reduce dust particles. You can cover the mirrors while they are drying with a large bowl or plastic tub, just make sure the bowl or tub is clean and dry before using for this.

Step 4: Reassembling and Collimating Your Telescope

Once the mirrors have dried completely, it’s time to reassemble your telescope and ensure proper alignment, known as collimation. Using the previously marked positions and the appropriate tools, carefully reattach the secondary mirror and the primary mirror cell. Take your time to align the mirrors accurately to optimize the telescope’s performance and make sure to not scratch or damage anything.

If you need assistance with collimation, refer to our guide on how to collimate your telescope. Proper collimation is essential since you have removed both mirrors. Without collimation, you may not be able to see anything at all.

Additional Tips and Considerations

Frequency of cleaning: In general, telescope mirrors should be cleaned once every 5 to 10 years. However, the frequency may vary depending on factors such as environmental conditions and telescope usage. If you notice a significant decrease in image quality or noticeable dirt accumulation, it may be time for cleaning.
Preventive measures: To minimize the need for frequent cleaning, store your telescope in a clean, dust-free environment. Use dust caps on the tube and eyepiece holder when not in use. Additionally, consider investing in a telescope case or protective covers to shield your instrument from dust and debris.
Avoid scratching the mirrors: Scratches on the mirror surface can have a more detrimental impact than dust. Exercise caution during the cleaning process, ensuring no abrasive materials or excessive force are applied.
Professional cleaning: If you are uncertain about cleaning your telescope mirrors or encounter significant issues, it is advisable to seek professional assistance. Telescope manufacturers or specialized telescope shops can provide expert cleaning services when needed or you may be able to get assistance from your local astronomy club.

Remember, a clean telescope mirror is just one aspect of maintaining your telescope. Regularly check and clean other components, such as eyepieces, lenses, and the telescope’s overall structure, to keep it in optimal condition for your astronomical adventures.