The design is very simple: a large concave mirror (called the primary mirror), which sits at the back of the telescope tube, reflects incoming starlight back along the tube and brings it to a focus. A second small, flat mirror (the secondary mirror, or flat), which is angled at 45 degrees to the primary, then intercepts the focused light and directs it into the eyepiece located on the side of the telescope tube. The eyepiece magnifies the image thereby making distant objects appear closer.
The Reflecting Telescope has several advantages over the Refracting Telescope:
- it is simpler and cheaper to produce, which means for the same price Reflecting Telescopes will have a larger aperture
- larger apertures means larger light gathering capacity, which in turn means brighter images of faint, deep-sky objects
- Refracting Telescopes suffer from chromatic aberration (false colour around images) due to the fact that they use lenses to focus light; Reflecting Telescopes use a mirror to focus the incoming light and therefore do not have this problem.
- the mirrors tend to go out of alignment relatively quickly and will need to be periodically realigned, however this is a fairly simple process (compare to Refracting Telescopes which can go for many years without the need for realignment)
- all Reflecting Telescopes suffer from light loss due to the presense of the secondary mirror in the light path, and the fact that the mirrors only reflect around 90% of the incoming light
- the reflective coating on the mirrors will fade with time, which means the mirrors will need to be resurfaced every few years to maintain good reflectivity
- small errors in the grinding of the mirrors can result in distorted images
- larger telescopes are less portable, an important factor if you intend to transport your telescope to a dark-sky site by car
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