There could be a couple of problems. But the one you probably have is that your focus plane is off. 6" f/5 is a 30" focal length. Did you make the scope per the San Fransisco Sidewalk plans? If so, those designs and measurements are for a "slide type" low profile focuser. If you didn't change any measurement and slapped a rack and pinion focuser on there instead, then you'll never focus it. Because your focus plane is well below your racked in point of the focuser. Measure from the face of the mirror to the center of the diagonal. Then measure from the center of the diagonal up through the fosuer. Rememeber your focal length is 30". If you add up the length and 30" comes below the rack-in point, then you cannot achieve focus. Rack in your focuser all the way down. measure the distance from the bottom of the focuser to the top rim of the focuser. Now you want to at least get the focus point .5" ABOVE the racked in point. There a few ways to repair this. Move your focuser hole back, use a low-profile (helical) focuser or scoot your mirror up. Also check collimation. I'm very sure that the focus plane is off. I have built 6 scopes. My first one I built I had the focus plane off by 1". I had to move my focuser hole back. Check the Side walk Astronomers web site. They have a part there to help you get the right focus. There could be more areas. Good Luck.