Asi, if you want to get huge image scale then there are alternatives to using a barlow and extension tubes. Using Positive Eyepiece Projection you can get any f ratio you want by varying the fl of the eyepiece the the distance the ccd chip is from the eyepiece. I put a calculator I put together up on the site the other day. It is based on using the Meade vari-extender. If you are interested there is a zip file containing the excel spread sheet
here