If you have a look here Dan
http://www.iceinspace.com.au/forum/s...d.php?t=103750 You can see the exact difference between 2X & 3X. The camera was dragged out of the barlow as far as it could go hence the F22 rather than straight F20 & the same goes when I used my extension tube to add the further F10 ratio. In theory the extension needs to be twice the length to get to F40-42 however I wouldn't like to be working with that length extension!! Sag/miscollimation of the system would start to factor in..