10-INCH DOB The Sky-Watcher Dobsonian Telescope 254mm/1200mm (10) have a paraboloidal primary mirror to eliminate spherical aberration and a four-arm, secondary- mirror bracket with fine supports (0.5mm thick), to reduce diffraction spikes and light loss. In addition, this model include a 2 focuser with a 1.25 adapter, allowing the use of either eyepiece size. This model also include the new feature-tension control handle. PYREX GLASS ON THE 10 The Sky-Watcher 10” Dobsonian features a primary mirror made of Pyrex glass. Because of its very low coefficient of thermal expansion, Pyrex glass is less affected by changing temperatures. This greatly reduces the cooling time of a telescope larger than 8” in dameter without using an extra fan. The great stability of this low expansion glass ensures high quality images. It is also substantially more resistant to scratching then plate glass. Available on 254mm/120mm (10) only WHAT is Dobsonian Telescope and WHY the Tension Control Handle? A simple, elegant form of an alt-azimuth mount made to carry a Newtonian reflector was popularized by John Dobson in the late 1970s. The Dobsonian mounted telescope is popular among amateur astronomers and telescope makers because of its simplicity. In its simplest form, the Dobsonian mount consists of a box which allows the optical tube assembly to pivot in altitude, while the box itself is swiveled on a base in azimuth. The Dobsonian mount usually relies on the friction between the side bearings on the optical tube of the telescope and a frictional material on the saddle to hold the optical tube in place. If there is too much friction, the telescope is difficult to move to center an object in the filed of view. If there is too little friction, the telescope will not sty where it is positioned. This makes stabilizing the optical tube of the telescope difficult when using a Dobsonian mount, es