Dobson telescope N 254/1200 Pyrex Skyliner Classic DOB

Dobsonian telescopes are among the most practical instruments in astronomy. They are of simple construction and yet a brilliant idea. They essentially consist of two parts: firstly the optics, which in most cases is a solid tube or truss tube design; and secondly the mount, a wooden box (also called a 'rocker box') which sits on the ground and into which the telescope is inserted. This design allows you to begin observing sessions immediately without first having to spend time setting up and aligning a complicated mount. The basic idea of the Dobsonian design came from the wish to have a big telescope at a low price, a wish that has succeeded brilliantly. A true fan base has developed since the invention of the Dobsonian telescope which is dedicated to these instruments and their fascinating simplicity and to the idea of 'high performance at low price'.

The N 254/1200 optics:

This Dobsonian has a diameter of 254 mm (10") and a focal length of 1200 mm. This gives it an aperture ratio of f/4.8. Such optics offer very high light gathering power (1275x). In a reasonably dark sky, you can observe stars down to a magnitude of 14. With an aperture of 254 mm, you can see deep sky objects that were not possible with smaller instruments. The secondary mirror struts are very thin and therefore low in diffraction. Due to the diameter of the secondary mirror (58 mm), the obstruction of the optics is 23%.

This telescope has a Pyrex mirror. This type of glass exhibits much smaller thermal expansion than, for example, the commonly used BK7 glass - which is about 2X more than Pyrex. During the cooling phase, the mirror does not warp very much and so keeps its geometric shape much better than a BK7 mirror. So, as a result, the mirror offers better image quality during the cooling phase. Also, Pyrex is easier to figure into a parabola. However, it does not cool down any faster than other mirror materials. The mirror meets at least 75nm p.t.v. or better. So the well-known precision condition Lambda/8 is more than satisfied. The telescopes have a Strehl Ratio of 0.8 or higher. This value expresses something of the optical performance and therefore the quality of the figured surfaces. When theoretically 80% of the light is incorporated in the Airy disk of the telescope image, the optics are described as 'diffraction limited'. That means that the resolution of a telescope is only limited by the diffraction of light. As it is not possible for this to be 100%, the definition of the diffraction limit has been set at 0.8. This always means that you have excellent optics (other systemic losses are not included here).

The 2" focuser is smooth and comes provided with a 1.25" reducer. The finder scope has a fine crosshair, allowing easy location of even the somewhat fainter objects.

The Skywatcher rocker box is very easy to use. The telescope can be pointed in azimuth and altitude easily and smoothly by using only a couple of fingers. The telescope moves on slide and pivot bearings. It remains stable in any position, allowing highly enjoyable observing. And observing still remains a joy even at higher magnifications. The Tension Control System also lets you easily adjust the friction setting and so the amount of resistance when moving the optics on the rocker box. This allows you to always have the appropriate resistance set.

It not only matters which telescope you buy but also where you buy it. Our additional services:

• We are a leading telescope dealer and know the devices we sell. Our customer service team will gladly help you after your purchase if you have problems with assembly or operation.
• We provide a copy of the 80-page Telescope ABC beginners handbook with every telescope.