If you want to achieve the greatest possible focus extension, the GSO 5x Barlow lens is a particularly good choice. The three-membered lens provides excellent focus elongation, as well as first-rate sharpness and contrast. Both the lens edges and inner wall surfaces are carefully painted black to eliminate unwanted reflections.
The mount is also first class: there is a copper clamping ring for the eyepieces, and the focuser is also equipped with an anti-fall-out solution.
Attention! It is recommended primarily for planetary photographers, as it is too high magnification for visual use in most cases.