A Coma Corrector is an additional lens system that reduces the coma at the edges of the field to provide you with images that are sharp across the entire eyepiece or photographic field, which means you will no longer have to continually move your telescope in order to keep objects centered for sharp viewing.
It's a fact of life that Coma is an inherent property of telescopes using parabolic mirrors. All parabolic mirrors used in Newtonian reflecting telescopes are limited in field sharpness as a result of coma.
Light coming from a point source in the center of the field, such as a star, is perfectly focused at the focal point of the mirror whereas light from a point source that is off center will not reach focus at the same point.
Very often the stars at the edge of the field will appear like a blurry or hazy comet. The faster the focal ratio of your telescope is the more apparent the effect. Even the most perfectly made newtonians are limited in field sharpness due to the coma effect.
For general viewing sharpness at the edge of field may not be a problem. However if you want the best views, and images if you're into astrophotography, you will want to check out what companies like Skywatcher, TeleVue and Baader have to offer.
Skywatcher Coma Corrector
The Skywatcher Coma Corrector is optimized for all Skywatcher fast-ratio f/5 Newtonian Reflectors /Dobsonians, with a 2” focuser.
It delivers stunning wide-field views with pinpoint stars right to the edge of the field. Magnification remains unaltered. It is supplied complete with all necessary adaptors for visual use.
For imaging use with a DSLR camera, a suitable M48x0.75 adaptor is required to match your camera. Skywatcher manufactured Canon, Sony, and Nikon M48x0.75 adaptors are available.
More often then not the stars at the edge of the field can look like a fuzzy or obscure comet. The faster the focal ratio of your scope the more obvious the effect. Even the most perfectly built newtonians are restricted in field sharpness as a result of the coma effect.
The Baader Planetarium Multi-Purpose Coma Corrector (MPCC) enables owners of fast Newtonian telescopes to use their instrument for taking stunning wide field images
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