I cleaned a mirror of my 16” aperture dobsonian telescope today according to procedures found on the internet, using distilled water. After doing the cleaning procedure twice all the hairs, large dust, and similar things were gone. Under normal lighting conditions viewed from a distance of about a meter it looks perfectly clean however when I shined a powerful flashlight at the mirror and took a close look I found a ton of very fine particles covering the mirror, the layer is uneven and seems to resemble the paths I took when using cottonballs to clean the mirror. I used the highest grade distilled water, used plastic gloves, everything clean. Im not sure if this is some residue of the cotton or simply dust that cleaning was not able to remove. Do I leave it like this or should I do something about it?
Cleaning dobsonian primary mirror question
The flashlight test is VERY unforgiving. If it were me I’d push on and not worry about it. You could go back and give it a 2nd cleaning but with every cleaning there’s risk. That’s your call….
Tony Gondola · Jul 3, 2026, 07:41 PM
The flashlight test is VERY unforgiving. If it were me I’d push on and not worry about it. You could go back and give it a 2nd cleaning but with every cleaning there’s risk. That’s your call….
Well if you say so Ill leave it like this for now unless ill notice any issues with the view through the telescope, its still far better than what I had before the cleaning so it should be alright, thanks a lot
Hvboy738 · Jul 3, 2026, 07:32 PM
I cleaned a mirror of my 16” aperture dobsonian telescope today according to procedures found on the internet, using distilled water. After doing the cleaning procedure twice all the hairs, large dust, and similar things were gone. Under normal lighting conditions viewed from a distance of about a meter it looks perfectly clean however when I shined a powerful flashlight at the mirror and took a close look I found a ton of very fine particles covering the mirror, the layer is uneven and seems to resemble the paths I took when using cottonballs to clean the mirror. I used the highest grade distilled water, used plastic gloves, everything clean. Im not sure if this is some residue of the cotton or simply dust that cleaning was not able to remove. Do I leave it like this or should I do something about it?
You’ll need to add mild soap into the warm water (any potable water really) and leave the mirror to stew a short while. That dissolves most of the grime. You’ll then wash with running cold water and THEN with sprayed distilled water A LOT to make sure that no tap water is left at all. Then you blow dry with high power cold air blower of some sort (I use my daughter’s hair dryer set to no heat). The point here is to remove the water droplets by physically displace them from the mirror surface, not by letting them evaporate. You’ll then have a mirror as pristine as it’ll ever get. Mine have no fluff attached to it nor streaks, like at all but eventually dust will settle on it. Live with it.
Even a brand new mirror with fresh coatings can show some stuff with the bright flashlight test. If it looks clean under normal conditions, it’s clean enough.
If you want to get over-particular with how clean you can get it, I find a wipe using kimwipes with a mix of acetone and methanol does the trick, followed by some more distilled water. Methanol is not to be treated lightly though so read up on necessary precautions.
The solvent wipe is always preceded by a regular bath of warm water and a drop of dawn dish soap, cleaned using the fingertip method. No real need to do it more then once a year and even that may be unnecessary, all depends on your local conditions.
As far as the bright flashlight test goes… just don’t.
Beware of all the different ways people clean their reflective surfaces. They gladly report how their method works, because they have had success with it. Yet even here on this short thread, none have told you what that surface is. In fact you also did not specify the surface of your mirror. If it is a bare aluminum vs. aluminum overcoated with dielectric vs. just dielectric vs. silver uncoated, etc. etc. Yes you can get away with cleaning a hardened reflective surface with wipes, at least a few times, but at some point the odds will catch up. Unless you are in a clean room, wiping with anything could carry along an inadvertent sand grain, or the like. In any case, if you are following the advice from someone who has a hardened surface mirror and you don’t you’re likely be sorry.
The best method for all surfaces is basically the one that @andrea tasselli described. No contact with any wipes needed. He left out, if need be, spraying or pouring over with a solvent for any stuff resistent to water/detergent. Solvent meaning something like alcohol after which the water rinse, then copious dH2O rinses. The fault you made with particles following the cottom ball path likely was the result of the solvent (dH2O?) you used having dissolved any soluble materials in the cotton, which when deposited on the mirror and dried, leaving non-volatile residue where a tiny droplet was left behind. If you did not use distilled water, then the stuff was probably soluble salts in the regular water. His suggestion to remove the last distilled water from the surface with forced air is very important. Yes you would expect distilled water to dry without residue. Reasons such as, distilled water never being perfectly pure H2O, or the dH2O actually dissolved something on or near the cleaning area, only to redeposit it when droplets dry. Removing the droplets completely avoids that problem. I basically raise and support the primary on its side in a tub and do just about everything with a squirt bottle. No need to touch the surface with anything at all.
And yes, the flashlight, especially an LED flashlight, is a very tough test. So as others have said, you should not fret too much. Also, avoid cleaning too often. I have heard that professional observatories do not clean their large surfaces until the dust is thick enough to cause several percent of the light as lost to dust. Even up to and beyond 5%.