Glossary
Coma corrector
An accessory lens that removes the star-shaped distortion, called coma, that fast reflector telescopes produce toward the edge of the field.
A coma corrector is an accessory lens assembly placed in the focuser before the eyepiece or camera sensor that corrects coma, an optical distortion common in fast Newtonian reflectors where stars near the edge of the field appear stretched into small comet-like or wing-shaped smears instead of sharp points. The faster the focal ratio, meaning the shorter the focal length relative to aperture, the more pronounced this edge distortion tends to be.
In casual visual observing at moderate magnification, mild coma toward the field edge often goes unnoticed, but it becomes obvious in wide-field eyepieces and is especially visible in astrophotography, where a camera sensor captures the entire field at once and elongated stars near the corners are hard to miss in the final image. A coma corrector reshapes the light path just enough to bring those edge stars back to sharp points.
Coma correctors are matched to a telescope's focal ratio and are most commonly used with fast Newtonian reflectors used for imaging, while slower focal ratios and other telescope designs are less affected and may not need one at all. When buying, check that the corrector is designed for the specific focal ratio range of the telescope, since a mismatched corrector can under-correct or even introduce its own aberrations.