Not stuck at all

Prism rails and tube rings connect the telescope tube and mount. But what can you do if the rail no longer holds the tube correctly?

Some things suddenly become standard without anyone formally announcing that it is the case. This definitely applies to the 45mm wide Vixen-style prism rail. It’s the benchmark for telescopes up to 8 inches aperture – or sometimes a little more. Usually the rails are made of cast aluminium and hollow, except for stability-giving struts. But what can you do if you feel that the normal 18cm long rail is not holding the tube correctly?

A misconception is that a longer rail offers better stability due to the greater distance between the rings. But this merely changes the frequency of the vibrations that arise, and which can also occur lengthwise on a rail that is too weak. And long rails tend to be made of the most undisputed weakest material: aluminium extrusion.

To remedy this, it really makes sense to stabilise the tube ring structure by screwing a second rail or, better still, a handle on the opposite side to the rail in question. Many handles fit the Vixen rail lengthwise with M6 or photo-thread screw holes at 160mm intervals. But the connection between rail and ring should also be considered: some rings feature small lock screws that transmit the forces instead of the uneven surface of the typical hammer finish enamel.

Often the rail is also attached directly to the tube – not a good idea if the tube is too flexible. Reinforcement plates do not help if the tube simply flexes where the plate is attached. Tube rings give such tubes more stability, but can also deform them if the ring shape does not fit exactly or the fasteners are over-tight.

Author: Sven Wienstein / Licence: Oculum Verlag GmbH