Posted By Andrew Riehl
etx 90 problem is due to the mak's extremely curved focal plane. without a field flattener, any type of camera will show images this way. What you need for your dob is to move the primary forward towards the secondary or vice versa, or use a relay lens set. Many barlows function as relays as well, slipping your camera adapter into a barlow then into the focuser will really slow down your f/ratio, but it should come to focus at least.