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Based on what I have learned over the last few years, I will show you a quick and easy way to clean a telescope mirror. Before I get into that, here’s a quick answer to the question summarised for you.
You can clean a telescope mirror by immersing it in slightly soapy water or by spraying it with water and detergent and then wiping it very carefully with a lint-free cloth. The final step is to rinse the mirror with distilled water and let it dry in a vertical position.
However, if your telescope mirror has not been cleaned before or if it is very dirty, you should do the following:
To clean a telescope mirror, first make sure the mirror is cool and not exposed to sunlight or any other source of heat. Gently blow any loose dust off the mirror, then carefully moisten a soft, lint-free cloth with a mixture of warm water and a mild, alcohol-free detergent. Wipe the mirror in a circular motion, starting from the centre and working your way outward. Avoid pressing too hard or using too much liquid, as this can damage the mirror. Once you have finished cleaning the mirror, use a clean, dry cloth to carefully remove any excess moisture. Allow the mirror to air dry before using the telescope again.
There is more detailed information and guidance on this page at Sky and Telescope.1
If your telescope has a mirror that is not very dirty, I’ve got a much simpler method for you to try. It worked for me.
First, let’s check a couple of important points about your telescope’s lens, primary mirror, secondary mirror, and glass because you don’t want to do this unnecessarily nor do you want to damage your precious telescope.
Disclaimer: Please use caution when learning how to clean a telescope mirror. The method described worked for me, but it may not work for everyone, and there is no guarantee that it will be safe for use with your specific equipment. Attempting to clean your telescope mirror can potentially cause damage if not done properly. I take no responsibility for any damage that may occur as a result of following the method described. It is important to carefully read and follow all instructions provided by the manufacturer of your telescope and to use caution and good judgement when attempting any maintenance or cleaning tasks.
Is it safe to clean the telescope mirror myself, or should I take it to a professional?
Yes, it is absolutely safe to do it yourself (if you are careful not to drop anything and take precautions against any possibility of scratching the glass surface of your telescope mirror). How do I know? because I have done it a couple of times on my telescope, the Celestron 130slt. At first, I was worried, but it is easy to take out the mirror; it is just a matter of removing three screws and taking out the mirror housing.
Many videos I have seen and articles that are written on the topic of telescope mirrors advise you to remove the glass completely, but in my opinion, this is not necessary and could risk damaging either the mirror mountings, losing screws, or scratching the mirror of your reflector telescope.
My method for cleaning a reflector telescope is fast and easy and minimises any risks of damage to the mirror’s surface. First, determine if your reflector telescope mirror actually needs cleaning. Reflecting surfaces can continue to work well even though they are dusty.
Recently, I also cleaned my secondary mirror. This is not as exposed as the primary mirror, but it still sometimes needs attention. I removed the primary mirror very carefully, losing a screw in the centre of the reflector lens support. I made sure to stop the mirror from dropping out and falling into the scope by lightly holding it on the edges of the mirror.
Once it was out, it was easy to see the dirt and clean it. Replacing the mirror was easy and simply the reverse of the above. At this point, be sure not to touch the mirror surface, or you’ll cover it with greasy fingerprints. Once the mirror is back in place, you’ll need to do collimation to line everything up. I use a handy laser collimation tool like this, which is quite inexpensive to buy and easy to use.
Cleaning the secondary mirror is fiddly but shouldn’t take more than about ten minutes.
How often should a telescope mirror be cleaned?
I have looked at a number of videos and read many online articles that tell you your mirror may not need cleaning for years. One person I watched even stated that he hadn’t cleaned his telescope in ten years and that this is not a problem. Wrong!
Just yesterday, I read online that someone was referring to a popular saying in astrophotography (apparently), which is that a telescope mirror doesn’t normally require cleaning. I totally disagree!
Why do I say this is wrong? Based on my own experience, The best research anyone can do is to actually try different methods and learn from experience. So, here’s what I have found:
How often you need to clean your mirror depends on how often you use your telescope and where you use it. For example, I live in a place where I can use my telescope for astrophotography almost every night during the summer months, which covers a period of about 5–6 months of the year. What’s more, it is generally dry here and tends to be windy and a bit dusty as I am close to the sea.
How Often Do You Use Your Telescope?
Sometimes I image the whole night long and over the whole year (taking into account how often I find the time to do astrophotography), and I would say I can image about 2 times a week, which is more than 100 times a year. Maybe this might be an overestimate because the Moon is sometimes too bright for astrophotography, but it’s a good estimation.
I have found it necessary to clean the mirror twice in the last two years. Most recently, I cleaned it because I had started to see some light circular motes on my images. In my opinion, it is good practise to clean your equipment at least once a year.
Your Environment Affects Your Equipment
It doesn’t matter which telescope you have they are all susceptible to dirt over time.
Therefore, in a dusty environment, when you use your telescope often, 80–100 times a year, the lens and mirrors will need cleaning each year. It is better not to clean the lens and mirrors often and wait until you see a problem with your images or if you inspect the mirror and find patches of dirt on its surface.
In my case, where I live, it is hot and dry with strong winds. The dust is easily carried onto any surface and will build up over the course of weeks and months. This is why I consider cleaning at least every 4–6 months or so. Your case may be quite different, so you need to decide for yourself what is right for you.
Check out this page for more about the best brands of telescope if you’re looking to get one or upgrade.
Prevention is better than cure: how to protect your mirrors from getting dirty
A little bit of attention to detail can go a long way towards maintaining the cleanliness of your telescope and its optics, including the lens, primary mirror, and secondary mirror. Here are a few simple steps you can take to minimise the need for cleaning your telescope’s mirrors.
Keep everything capped when not in use, unless the optics have fogged over after a session (in which case, allow them to dry out before capping).
When swapping eyepieces or filters, be sure to put the caps back on the previous one.
Keep your equipment packed away when not in use, rather than leaving it out in the open.
Use a bulb blower before and after using all optics, this includes mirrors, and be sure to bulb blow eyepieces after every use (your eye or eyelashes contain gunk!).
Proper dew management is essential to keeping your optics clean. Invest in a dew heater strip and use it on nights when dew is a concern. After a session, allow your equipment to warm up slowly in a dry environment. If your optics have fogged over, keep the dew heaters on until all moisture has evaporated, and only cap when dry. If your scope has a front corrector plate, consider getting a portable hair dryer to help remove moisture.
Silica gel packets are a must-have for any astronomer. Use them liberally, and replace or replenish them regularly.
When selecting an observing site, be mindful of the potential for pollen and other debris to stick to your optics. Choose a site that is free of plants and trees that may produce pollen.
If there is wind, make sure you do not set up anywhere too close to a dusty area. Grass is best.
By following these simple steps, you can help keep your telescope lens, primary mirror, secondary mirror, and mirrors clean and in top condition.
Can I use household cleaning products to clean my telescope mirror?
It’s not really a good idea. You should avoid, at any cost, any abrasive cleaning products, as a scratched mirror is ruined. Often, detergent is recommended, but again, I don’t use it and don’t like the idea; I only use it for really dirty mirrors!
How do you clean a telescope mirror that has stubborn dirt or stains?
Stubborn dirt or stains should not be on your mirror, but in the exceptional cases where you have such dirt on your mirror, do not think of rubbing your mirror even lightly. In this case, my method should work, but you may need to leave the liquid on the mirror for a short while to remove these kinds of marks.
How to clean a telescope mirror
Now, my method for how to clean a telescope mirror surface is what I have done the last two times without any problems, and I managed to do this very quickly in 5–10 minutes maximum. Here’s what I do:
I finely spray the mirror’s surface with a cleaning fluid, such as the one shown in the image. The type of spray I use is the same as what I use for cleaning my laptop screen. It is a clear fluid that is quicker and easier to use, leaving no residue behind.
I noticed marks on the surface of my mirror a few months ago, and after spraying it with this water-based solution, I waited a few minutes and gently wiped it off with a lint-free cloth. Problem solved!
This has worked for me. The dust motes on the surface of my lens were gone, and my mirror was clean.
How to Clean a Telescope Mirror that is Very Dirty
If your mirror is extremely dirty, then a different method can be used, as suggested in other videos, such as this one by Small Optics2, but the above method works on any telescope that you have used since it was new and has been cleaned regularly.
Clean mirror, big rewards!
Now you know how to clean a telescope mirror. I hope this has helped you, and the views you’ll get from your telescope will be as amazing as possible!
References
[1] Steve Richards. BBC Sky at Night Magazine. How to Clean a Reflector Telescope. Retrieved from https://www.skyatnightmagazine.com/advice/how-to-clean-a-reflector/
[2] Small Optics. How To Clean a Telescope Mirror – YouTube. Retrieved from https://www.youtube.com/watch?v=FnZvCVjqj80&t=344s
