Leaving my dust cap on in my new Skywatcher 305mm Dobsonian. I took the little 40mm cap off but the rest of the dust cap was on
......... Instead of observing in a 305mm Dob I was observing with 40mm highly magnified el cheapo dob
..... After 2 months I finally found out what was wrong, just in time for dark skies
Learn from my stupidity.......