Achromat Telescopes

These are great telescopes for everyone who wants to enjoy observing skies at an affordable price. Although achromat 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 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. 

Achromat lens:

An achromat lens is usually made of two components. Each component is made of different glass:

• one is concave,
• and the other is convex.

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])