This device provides an acute focusing, as precise as with a rack-and-pinion or a capstan. But here, even if the observer moves, the image does not move.

With its current design, the hydraulic focusing system is devoted to telescopes devoted to visual use. Dobsons and 115/900s would greatly benefit from it.

Its principle is the one of the hydraulic press. The movement of the eyepiece is controlled by the small syringe. When liquid is injected in the big syringe, the eyepiece moves back. When liquid is drawn out of the big syringe, the eyepiece moves forward. Given the diameter difference between the 2 syringes, the translation of the eyepiece is 7 to 10 times as small as the one of the piston of the small syringe moved by the observer.
 
 
 

PVC tubes support the eyepiece. They slide loosely into each other which does not shatter the quality of the images.

The piece of rag is to adjust the diameter of the tube to the one of the eyepiece.

The yellow parts are pierced to hold the big syringe. The diameter of the holes are adjusted in order to hold the syringe quite tightly and to remove it manualy.

The hose can be found in model shops.

Water is an uncompressible liquid. Observer's movements are totaly transmited to the eyepiece, without any play, with a constant friction. The pressure of the eye on the eyepiece does not change the focusing. These characteristics are only valid if the liquid is carefully bleeded. The smallest bubble jeopardizes the interest of the system. Practicaly, one bleed before each night is enough.