I purchased a Stellarvue focal reducer field flattener (SFFR102-2). I have it to my Baader VariLock 29 T2 Variable Extension Tube - 20 mm - 29 mm, for use with my ES102mm APO Triplet refractor. In reviewing my Ha images of the Pelican from Saturday night I noticed that, for the most part, all the stars in the corners are elongated and pointing towards the center of the image. Stars in the center are pin point. I'm guessing my back focus is off a bit. My current back focus on the Varilock is set to 28.5mm.
In an effort to save trail and error time in adjusting I'm hoping the CN community can help me. To correct this issue would I set the Varilock less than, or greater than, the current 28.5mm setting?