Most exploration of the Universe is carried out using . More recently exploration has been done with satellites and . However sending satellites into space is very expensive.

Ground based telescopes are mostly used, but observations can be made difficult by cloud, pollution, and distortion due to the . This is why stars twinkle. These problems can be reduced by putting the telescope on the of a high mountain.

Even better results can be obtained by putting a telescope on an , or sending it into orbit as a . The images are very clear.

Lens or telescopes are limited because their are small. Lenses also cause distortion. Reflecting telescopes can have a much wider aperture, so have better of detail. They have very accurately made mirrors.