Posted 17 February 2013 - 11:02 PM
That is a common problem with Dobs. Using a Barlow is a form of eyepiece projection. Just about any Barlow will work, but it shrinks the FOV considerably. With barlows you don't have to worry too much about a specific length behind the Barlow, however the further back the greater the magnification. That is not true with coma correctors as you need to get the correct distance as specified by the manufacturer. I was able to achieve true prime focus or more correctly Newtonian focus by moving my primary mirror up about 1.5 inches. This was done by simply replacing the collimation bolts. I replaced the 1in bolts with 2 in. After doing this I had to get a tube extender to use my eyepieces and had to do a serious collimation.