I had the same problem you have now. I found that my spider was(secondary) was not completly aligned at 45 degree. It looked like it was, but I found out later it wasn't! It seems that collimating is good, but if the diagonal is slightly off, you don't get a good focus. My planets were blurry and stars wouldn't focus the way they ought to. I double checked my focal length from the face of the mirror to the focus point (1" above the rack in spot) and it was good. Then I used the method of "spotting" to check alignment of diagonal relative to the objective mirror. I put a 8" template with a hole in the middle and lay it on the mirror (I have a 8" dobs..homebuilt)then I use a black marker and make a dot in the middle of the mirror. I use that to collimate. It works well. I install the mirror and use a small round plastic cap with a hole dead center of of it and place it on the focuser (no eyepiece) I look through it to see if my diagonal is showing a symetrical view of the objective. In other words, when looking straight through the peep hole, I can tell if by looking at the diagonal if I see an equal amount of space around the mirror. If not, I move the diagonal. If this is the case, light is not hitting your focus point straight on. You get parts of the image and this does cause blurryness. Brighter objects are the worst because of the uneven distribution of the light. Also check your eyepieces. If you are using Plossels, Make sure the elements aren't too tight. This causes "warping". I always loosen my elements in the eyepiece, until..if I shake it, I will hear it rattle, this is OK. As for the mirror, Robert May will probably answer that one for you. Check your secondary alignment, make sude it is exactly 45 degrees. You have to have that focal hitting your focus point right on. I hope this works. Then again I could be totally wrong and will be banished from here for ever...:)