Thanks Jim, John, Sonny and 2star! Haven't checked in for a while and saw your responses just today. Yes, it was a simple issue of dust/debris on the sensor glass. Used a new Pro Master Clean Air Blower..has a one way bellows with a replaceable air filter..to gently clean the sensor. Worked perfectly. Since then, I've been careful to do things that should minimize other debris being deposited on the glass such as gently sliding extensions, focal reducers, etc. in and out of the image train and carefully setting the tripod on the ground when repositioning the scope. So far, so good! Thanks!