These are great telescopes for everyone who wants to enjoy observing skies at an affordable price. Although achromatic telescopes are not completely chromatic aberration-free, they still provide good images of the moon and planets.
Usually, these telescopes are quite small in size, which makes them very portable.
Achromat is a special lens used in a refracting telescope to correct chromatic and spherical aberration. An achromatic lens has a unique design that helps two colors (red and blue) reach the same focus.
The problem occurs because the green color focuses on a different point. This is the reason why refracting telescope using the achromat lens is not utterly chromatic aberration-free.
With a refracting telescope, light passes through a lens. Visible light consists of different colors with a different wavelength. When passing through a lens, each color focuses at a different point. This defect is seen as a fringe of color around bright objects. To fight this defect, an achromatic lens can be used.
An achromat lens is usually made of two components. Each component is made of different glass:
There is an air gap between them, or they are cemented together. The concave glass has a high dispersion, while convex glass has low dispersion. Together they bring red and blue light to the same focus.
Achromatic lens correcting two 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/Achromatic-lens-correcting-two-wavelengths_fig3_321017277 [accessed 28 Oct, 2019])
A short presentation is available here.
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