The reflecting telescope was first used by Issac Newton in 1672, consequently, they are commonly referred to as Newtonian telescopes. This telescope uses a curved mirror as an objective, and since mirrors are easier to make than large lenses, these are considerably cheaper than refractors. In fact, they have the lowest cost per inch of objective diameter of any telescope. This is one reason that they have been very popular with amateur astronomers. These telescopes do not suffer the chromatic aberration of the refractors, and gather light more effectively. This is because objective mirrors are much larger than lenses, and because they can be ground to much lower F ratios. Newtonian telescopes with F ratios between 3 and 5 are good for photography and observation of deep sky objects. On the other hand, F ratios of 8 to 12 are suitable for planetary observation. The Newtonian is a good all around design and can be had at a relatively cheap price. However, this design too has its problems. For various technical reasons, the image quality is not as good as for a refractor of the same diameter. But remember, it is possible to buy a much larger Newtonian. Another problem is the sheer size of these telescopes, which for larger models, can result in transportation problems. Also, they do not have a sealed tube so that they require some maintenance and adjustment. Fortunately, they are easy to adjust. Finally, they suffer from an inherent optical defect called coma which is a distortion of the image near the edge of your field of view. This effect is apparent only in Newtonians of low F number and even then it is hardly noticeable
