I own a Celestron Ultima 2k and have cleaned the plate once in the 4 years I
have owned this scope. The reason..some annoying smudges (fingerprints). I
used only distilled water with a squirt bottle and a hair dryer on low. A
few flushings and all was right again. I read that alcohol will leave a
residue and needs to be rinsed clean.

Just my 2 cents.