The SkyWatcher aplanatic coma corrector is specifically designed for the SkyWatcher Newton Quattro. It is aplanatic (free from spherical aberration) and it corrects the coma on Newton telescopes with a focal ratio of f/4.
Newton telescopes, with such a short focal ratio, are free from field distortion and the astigmatism is not particularly strong. However, the coma aberration makes this telescope unusable with large sensor cameras.
The SkyWatcher aplanatic coma corrector corrects the coma, converting the SkyWatcher Newton Quattro in superb astrographs.