[linkpng (Alfred)]

As others have said, think twice (3x) before cutting bits off your tube. Please consider all other options first, like fitting a lower focuser, using a slight Barlow such as 1.3x. The same Barlow lens has different effects depending on where it is placed. Have you looked around for a very low helical focuser? Or can you mount part of the focuser inside the tube? There is always about 15 mm free space inside the tube that would not cause vignetting.

If you really need to cut metal, in principle another option is moving the focuser and spider down and to the opposite side of the tube. All the old holes can then be covered nicely with eight screws and one plate (even thin black plastic will do). The scope can then be fully restored at any time to its original configuration for visual use. It will be very easy to swap between visual and CCD use.

Keep in mind that bringing the (heavy) primary up will also drastically alter the balance, so that the alt bearings have to be moved. Moving the lighter focuser, spider and eyepiece assembly will have less effect. Note that once you cut off the 50 mm it will be difficult to put it back on nicely for when you like to go back to visual use or sell the scope later. Cutting that large diameter of thin metal smoothly without bending the tube is also not a trivial task.