Achromatic telescope

Early refracting telescopes with non-achromatic objectives were constructed with very long focal lengths to mask the chromatic aberration.

An achromatic lens is a compound lenses made with two types of glass with different dispersion.

The crown lens is usually placed at the front due to the higher susceptibility of flint glass to atmospheric attack (exception: Steinheil doublet).

Together they form a weak positive lens that will bring two different wavelengths of light to a common focus.

R3 is set slightly shorter than R2 to create a focus mismatch between R2 and R3, thereby reducing ghosting between the crown and flint.

For an achromatic doublet, visible wavelengths have approximately the same focal length.
Achromatic telescope by Hooke , 18th century.