Achromats correct for two colors, bringing the green and red to a common focus. ( Red and Green combine to make yellow, which is close to the yellow-green light at 550 nanometers in wavelength where our vision peaks in performance. ) A secondary spectrum is left along the principle axis of the lens.
Chromatic aberration can be further corrected by adding another element or two to the doublet.  One of these additional elements will be made from an extra low dispersion glass, or from Fluorite glass. Such a lens corrects for three or more colors and drastically reduces the secondary spectrum.  This type of lens is called an Apochromat.

Chromatic aberration does not affect reflectors, so a telescope that uses a mirror as its objective will almost be free from this type of aberration.  Almost?  Yes, because most telescopes use an eyepiece, and eyepieces are refractor type lenses subject to this type of aberration.