An apochromatic telescope features an apochromatic lens that has a better correction for chromatic aberration than achromat lens.
This type of telescope provides a better quality image than the achromat telescope. This becomes very noticeable when observing very bright celestial objects, such as Moon and Venus.
Apochromatic telescopes are more expensive than achromatic telescopes. The apochromatic telescope can become very pricey if the user wishes to have a larger aperture.
The achromat lens brings only blue and red light to the center on the same plane. The apochromat lens, on the other hand, corrects all wavelengths to focus on the same point (red, blue, and green).
An apochromat lens consists of three different types of glass:
The glasses are cemented together or have an air-gap between them, or a combination of both. Because of a higher number of glass elements, other defects such as coma and spherical aberration also have better correction.
Apochromatic lens correcting three wavelengths. (Source: Synthesis of high refractive index materials for manufacturing apochromatic lens by 3D printing - Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/Apochromatic-lens-correcting-three-wavelengths_fig4_321017277 [accessed 28 Oct, 2019])