LIMITATIONS OF REFRACTORS

There are certain problems inherent to telescopes that use objective lenses. These are known as spherical aberration, chromatic aberration, and lens sag. Spherical aberration results from the fact that spherical convex lenses don’t bring parallel light rays to a perfect focus. A refracting telescope with a spherical objective focuses a ray passing through its edge a little differently than a ray passing closer to the center. The actual focus of the objective is not a point but a very short line along the lens axis. This effect causes slight blurring of images of objects that have relatively large angular diameters, such as nebulae and galaxies. The problem can be corrected by grinding the objective lens so that it has a paraboloidal rather than a spherical surface.
Chromatic aberration occurs because the glass in a simple lens refracts the shortest wavelengths of light slightly more than the longest wavelengths. The focal length of any given convex lens is shorter for violet light than for blue light, shorter for blue than for yellow, and shorter for yellow than for red. This produces rainbow-colored halos around star images and along sharply defined edges of objects with large angular diameters. Chromatic aberration can be almost, but not completely, corrected by the use of compound lenses. These lenses have two or more sections made of different types of glass; the sections are glued together with a special transparent adhesive. Such objectives are called achromatic lenses and are standard issue on refracting telescopes these days.
Lens sag occurs in the largest refracting telescopes. When an objective is made larger than approximately 1 m in diameter, it becomes so massive that its own weight distorts its shape. Glass is not perfectly rigid, as you have noticed if you have seen the reflection of the landscape in a large window on a windy day. There is no way to get rid of this problem except to take the telescope out of Earth’s gravitational field.