To clean the primary mirror you will need:
A kitchen sink
A couple of small towels
Plain, clear dishwashing liquid, like Ivory
A large bag of cotton balls
90% or higher purity alcohol
To remove the primary mirror from the telescope:
Remove the tube from the base, and place upright on the ground. There should be screws positioned in a ring around the bottom of the tube. Unscrew them (make sure to put them somewhere safe!) then slowly lift the tube up and away from the mirror cell. (Be prepared for the sudden lightness and top-heaviness of the tube, and be careful not to bang it into the mirror!) Set the tube aside somewhere safe. If the inside of the tube needs to be cleaned after the flood, now is the time to do that.
The mirror should be held into the mirror cell with three clips on the front of the mirror. My XT8’s mirror clips are tightened and loosened with two screws on the top of each mirror clip; I can’t guarantee it will be the same for every scope, but I also can’t imagine it being too much different. Remove the mirror clips, and take out the mirror. It should now just be a disk of glass, with one coated surface, no elements of the mirror cell still attached.
To clean the primary mirror:
First, if you can do this in absolute, total silence, do. Why? Because scratches and sleeks make a very tiny noise when they are being created. If you are in total silence you should be able to hear the noise, thus letting you know if you are doing something wrong. If you can’t work in total silence, that’s ok, don’t worry about it- it’s not a requirement, just a nice thing to have.
Place the mirror in a clean sink, resting on its edge on a small towel. Rinse it off using a sprayer, if there is one. Otherwise, just pour lots of water over it to get rid of most of the dust. (If there was some kind of gunk or debris on there that you didn’t feel comfortable lifting off of the mirror, knock it off with the water.)
The mirror should now have no remaining large particles.
Mix a couple of drops of plain, clear dishwashing liquid like Ivory in a small bowl with water. Dip 100% cotton balls into the soapy water.
Using ONLY the weight of the cotton balls against the mirror, gently run the soapy cotton ball across the mirror. (That is to say, let the gravity of the wet cotton ball be the only force holding it against the mirror. Do not apply pressure!)
Start in the center of the mirror with a cotton ball, and run straight out to the edge. Turn the cotton ball as you do, so that any grit picked up is carried up and away from the mirror surface, and not dragged against it. Try to rotate the cotton ball so as to minimize bringing the same spot against the mirror surface repeatedly. Change the cotton balls often. I would recommend every 2 or 3 strokes, but you should be able to figure out fairly quickly what the best interval is.
Once the mirror looks clean, run lots of water over it to get rid of all traces of soap. The final rinse should be a mixture of 1 part at least 90% alcohol and 3 parts distilled water.
Place the mirror on a dry towel, standing on its edge so that water can run off. Use a hair dryer to press a paper towel onto the surface of the mirror- do not apply any pressure yourself! The gentle pressure of the air from the hair dryer should just lightly touch the paper towel to the surface to blot off the water. If there are some particularly stubborn droplets, touch them lightly with the dry corner of a paper towel to soak them up.
Your primary mirror is now clean!
If the mirror cell was dirty (and it likely was if the primary was flooded) you should wipe it down before you put the primary back in. Once both the primary mirror and mirror cell are clean and dry, you can put the primary back in. Set the mirror in the mirror cell and replace the mirror clips. But before you go any further, here’s a good opportunity to possibly improve your optical quality a bit!
Telescopes are often shipped from the factory with the mirror clips severely overtightened. Overtight mirror clips pinch the optics, and actually hold the mirror out of the correct shape, causing astigmatism. This makes images appear asymmetrical and blurred. When I cleaned my XT8, I discovered that the mirror clips were severely overtightened- so much so that the mirror clips themselves were bending.
When you tighten your mirror clips, leave a tiny sliver of space in between the clips and the surface of the mirror- about the thickness of your fingernail, or a business card. This will prevent pinched optics. The purpose of mirror clips is only to make sure the mirror does not fall out of the mirror cell, so a tiny gap like that is fine.
After the primary is secured in the mirror cell (and the inside of the tube is cleaned, if need be), set the mirror cell on the floor, lower the tube over it, and reattach the screws holding the mirror cell to the tube.
Congratulations! Your scope is now functional once again!
Cleaning the secondary mirror:
You may also want to clean the secondary mirror. Even if it didn’t get affected by the floodwater, it is much easier to clean than the primary, and it can be worth it to have a truly clean optical system.
To clean the secondary, lower the tube to horizontal (so that the secondary does not drip on the primary). Put a couple of paper towels in the tube under the secondary, to catch drips of water. Wet some cotton balls in distilled water, and using the same gentle, gravity-assisted technique that you used on the primary, clean the secondary. It will not need as much time or as many cotton balls, due to the smaller size. Once the secondary is clean, use a blow dryer aimed through the focuser to dry it off.
THE BOTTOM LINE:
Any kind of movement across the surface of a mirror *with pressure applied* is bad. Even the tiniest particles of dust and grit will drag along underneath whatever is rubbing the mirror, and score the coating as they do. A few tiny scratches are okay. A lot of scratches, such as the amount acquired over a few careless cleanings, cause problems.
HOWEVER: as long as you are careful in the way that you clean, you will be fine! It is easy to get intimidated with all the warnings about “BE CAREFUL WITH THIS!” or “DON’T DO THIS!”. But truthfully, it is not as hard as you’d think- all those warnings are just to make sure you follow the cleaning process carefully, and don’t just start scrubbing at it with the cotton balls, or decide that using a power washer is quicker or something.
The reason scratches are bad is because they scatter light inside the telescope, and reduce contrast. That is the exact same reason dust is bad. It is better to have a mirror with a few tiny scratches than a mirror coated in dust and cat hair. (I would know!)
I will post some photos shortly that I took when I cleaned my XT8’s mirror, as well as instructions on collimation.
Good luck, and Clear Skies!