With the help of the front lens and an eyepiece the focal length of the telescope is determined. The Beroflex lens used does not have a focal length of 500mm but less. In my case I determined a focal length of about 335mm.

With both lenses in your hand and a folding rule the required distance is determined based on a distant object. The eyepiece extension is turned out halfway to be able to vary in both directions later on.

Alternatively, you can take a direct look at the stars with the eyepiece extension turned in completely to determine the minimum length.

With my Beroflex a length of 220mm for the tube resulted. After cutting a suitable piece of tube and blackening it the assembly can start.

The diameter of the parts is such that they hold together without any further fixing. Gluing or other measures are therefore not necessary. Alternatively you can use a strip of tape to secure the parts.

In order to mount the telescope, I have provisionally fixed a rail with cable ties.

Later on I will build some pipe clamps for the telescope.