Second, due to the structural limitation, the aperture of refraction telescope is generally not very large, generally within 100mm, which is suitable for observing planets, such as Saturn's rings, nebula belts, Venus' profit and loss, etc. However, due to the chromatic aberration caused by refraction, the common achromatic refractor is not suitable for observing deep space objects (nebulae and galaxies), while ed (ultra-low dispersion) and APO (apochromatic) refract.

Thirdly, the biggest advantage of reflective telescope is that it has no chromatic aberration, and its aperture is easy to be made large, which is more suitable for observing deep space objects. But the disadvantage is that the main lens barrel is open, and the main lens is directly exposed to the air, which requires high dust and moisture resistance. Moreover, the reflective film of the main lens will become dim after several years of use, and it needs to be returned to the factory for coating.

4. The representatives of the catadioptric telescope are Zeca and Marca. Its chromatic aberration and aberration are well controlled, and the length of the mirror can be greatly reduced while keeping the focal length of the main mirror unchanged. It can see planets and deep space, and it is an ideal astronomical telescope, but the disadvantage is that the price is generally high;

As for how to choose, it depends on the needs of the landlord.