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The Advantages of a Large Refracting Telescope Over a Large Reflecting Telescope

The refracting telescope is what most people think of when they hear the word "telescope." It uses a large optical lens at one end to focus light to a much smaller point at the other end, magnifying the image and making dim objects appear brighter in the night sky. While the refracting model has some inherent size limitations at the observatory scale, a large refracting telescope has some advantages over a large reflecting telescope.

Ease of Use
- A refracting telescope is a supremely simple device, and using one to scan the night sky is straightforward and intuitive. The objective lens, the large lens at the end of the telescope, is fixed in place, requiring no adjustment. Any focusing takes place at the eyepiece, where the hand and the attention of the user is fixed in any case. The optics of a reflecting telescope, on the other hand, need to be regularly calibrated to prevent serious image degradation.
Durability
- Despite the fragile lenses involved, a refracting telescope is quite rugged. The objective lens of a large telescope is made of much heavier glass than the thin material of a reflecting telescope, making it more likely to survive minor accidents intact. The sealed tube also protects the interior optics from getting dirty, while reflecting telescopes often require complex cleaning of the interior optical mirrors. This can be especially difficult for large telescopes, as the optics may be extremely hard to reach from the outside.
Image Quality
- At the scale of a large hobbyist telescope, a refracting telescope's method of collecting light has a couple of important advantages over a reflecting telescope. The reflecting telescope will suffer from some light loss due to diagonal obstruction from the tilted secondary mirror. This means that dim objects are likely to appear more brightly in a refracting telescope. The sealed tube also eliminates some possible sources of image distortion, such as wayward air currents, that reflecting telescopes need to deal with.
DIY Telescopes
- In general, a ground lens is a more forgiving piece of optics than a homemade mirror. Small errors will be less visible in a lens than in a mirror, where tiny variations can result in great loss in image quality. Therefore, quite serviceable lenses for large reflecting telescopes can be ground from porthole glass, making this design a popular choice for hobbyists who wish to build their own telescope at home.

Astronomy