Now that the primary mirror cell is mounted in the tube, look to see if you have extra tube length on the bottom of the telescope you don't need (the front of the telescope, which was laid out from our the NEWT design program, should be just right). The goal is to make the tube as short as possible, but not so short that that nuts and bolts from the mirror cell project out beyond the end of the tube.
As the photo at right shows, our scope has 3-5/8 of tube below the mirror cell. The mirror cell nuts and bolts project about ½ inch behind the cell, so about 1 inch of tube is a good amount. Therefore, we will remove 2-5/8 inches of tube from this end. The final length of our tube will therefore be 45-3/8 inches.
Why not just leave it? If this is going on a Dobsonian mount, the rocker box will have to be that much taller (and heavier) and will be less stiff. Also, the shorter tube will be more transportable. Overall, the main reason it keep the rocker box shorter and more stable.
Empty the Tube: Remove everything from the tube to prepare for cutting (we need to do this now anyway to be able to paint the tube in the next step).
Marking the Tube: Once you have determined how much tube to cut off, make several marks around the outside of the tube, measuring down from the rear of the tube.
Any easy way to draw a line around the tube is to use a sheet of newspaper as a straight edge: wrap it around the tube with the longest edge on the marks, pull it tight and tape it in place. You can now draw a pencil line along the edge of the paper to show you where to cut.
Cutting the Tube: Using a cross-cut hand saw, carefully cut along the line as shown in the picture. After cutting, use 100 grit sandpaper to clean up the edge, and smooth out any rough or uneven parts.
With the tube cut to length, we are now ready to paint both the inside and outside of the tube.