Limitations Of Refractors:

There are some problems inherent to telescopes which use objective lenses. These are termed as chromatic aberration, spherical aberration, and lens sag.

The spherical aberration outcomes from the fact that spherical convex lenses do not bring parallel light rays to an ideal focus. The refracting telescope with a spherical objective focuses a ray passing via its edge a slight differently than a ray passing closer to the center. The real focus of the objective is not a point though a very short line all along the lens axis. This effect causes small blurring of images of objects which have relatively large angular diameters, like galaxies and nebulae. The trouble can be corrected by grinding the objective lens therefore it has a paraboloidal instead of a spherical surface.

The chromatic aberration takes place since the glass in a simple lens refracts the shortest wavelengths of light a little more than the longest wave-lengths. The focal length of any particular convex lens is shorter for violet light than for the blue light, shorter for blue than for yellow, and shorter for yellow than for red. This generates rainbow-colored halos around star images and all along sharply defined edges of objects with big angular diameters. The chromatic aberration can be nearly, though not completely, corrected by the use of compound lenses. Such lenses have two or more portions made up of various types of glass; the portions are glued altogether with a special transparent adhesive. These objectives are termed as achromatic lenses and are standard issue on refracting telescopes nowadays.

Lens sag takes place in the largest refracting telescopes. Whenever an objective is made larger than around 1 m in diameter, it becomes so huge that its own weight deforms its shape. The glass is not perfectly stiff, as you have observed if you have seen the reflection of the landscape in a large window on a windy day. There is no manner to get rid of this trouble except to take the telescope out of the Earth's gravitational field.