An ordinary mirror will not reflect X-rays. Instead, they penetrate into the material of the mirror. Soft X-rays (comparatively long wave-lengths) can be reflected if they hit the mirror at a very small angle (grazing incidence). Because a single grazing incidence mirror intercepts only a small ring of radiation, grazing incidence mirrors can be nested to obtain more reflecting area. For harder X-rays (shorter wavelengths) and gamma rays, an entirely different type of telescope must be used. Each photon is detected individually. To form an image, grids may be rotated in front of the detector or a detector that shows the path of the photon is used.

To get any image resolution with radio waves, it is necessary to use a very large mirror but it does not need to be as accurately smooth as does an optical telescope. Often, to get the resolution of a larger telescope than can be built in the form of a single mirror, the images from two or more telescopes are combined. Sometimes these telescopes can be on opposite sides of the world. Recently, an even larger size "telescope" was formed by using a comparatively small telescope on a satellite as one instrument combined with several larger telescopes on the earth.

Most telescopes made in the 19th century used a lens to focus the light. Such telescopes are called refracting telescopes or refractors. However, when a lens is large it must also be thick. Thus it becomes too heavy to mount in such a way that it will maintain its figure. In addition, it absorbs too much light. The largest refracting telescope, built more than 100 years ago, has a diameter of forty inches. Although lens are still used for many small telescopes, particularly for amateur telescopes, most professional telescopes built in the 20th century have used mirrors to collect and focus the light. These are called reflecting telescopes or reflectors. Even the standard one-piece mirrors that were used up until 1970 cannot be made in very large sizes. Not only would they become too thick and too heavy but because they change shape slightly as they sag with changes of pointing position, they would not consistently form good images. The largest telescope of this type is the famous 200-inch telescope on Palomar Mountain. Today astronomers are building telescopes almost 400 inches in diameter. These are of two kinds. Both depend heavily on computers and both are continually adjusted to give a good image of a nearby star, either natural or artificial. . In one type, a very thin mirror is used. The other type, such as the Keck, is made of many segments whose relative positions are continually adjusted.