All Newtonian reflectors exhibit coma, an optical defect that turns normally point-like stars into annoying comet-shaped flares the further you get from the center of the field. The faster the scope’s focal ratio, the closer to the center of the field the coma starts to become apparent.
The photo/visual Astro-Tech Coma Corrector is a three-part 2” auxiliary lens system that drastically reduces the coma and the curvature of field in a fast focal ratio Newtonian reflector during visual observing and astrophotography. It essentially makes an f/4 telescope perform like an f/8 in terms of coma. While the Coma Corrector has been optimized for use with an f/4.5 reflector, it will reduce the coma of all carefully-collimated f/3 to f/6 reflectors.
The Astro-Tech Newtonian Coma Corrector was designed for Astro-Tech by Roger Ceragioli, the well-known optician and optics designer who was formerly the Senior Optician of the Small Optics Group at the world-class University of Arizona Steward Observatory Mirror Lab in Tucson, AZ. The "small" in Small Optics Group may be somewhat of a misnomer, as Roger’s "small" group specialized in mirrors and lenses with diameters up to a meter in size. The majority of backyard astronomers would probably consider a one meter scope to be a step up from "small," but it's small only in relation to the 6 to 9 meter spun mirrors the Mirror Lab is famous for producing for professional observatories.
Stars at the edge of the field change from comet-shaped flares to points of light with the Astro-Tech Coma Corrector, so that stars look virtually the same anywhere in the field photographically or with a good wide-field eyepiece. Faint stars at the edge of the field do not fuzz out to invisibility due to coma and field curvature, but show as tightly focused points as much as six times smaller than their size without the Coma Corrector. This not only is pleasing visually, but reduces significantly the need to constantly nudge a Dobsonian’s optical tube to keep an object centered in the scope’s “sweet spot." In effect, the entire field of a fast focal ratio Dob becomes the sweet spot with the Astro-Tech Coma Corrector in use.
The photo/visual Astro-Tech Coma Corrector does an exceptional job photographically with fast focal ratio imaging Newtonians, like the 8” f/4 Astro-Tech AT8IN system. It will increase the focal ratio by a small amount, about 10%, which is less than the 15% typical of premium coma correctors, some of which cannot be used both visually and photographically.
The economical Astro-Tech Coma Corrector has three parts. One is the 2” barrel that contains the fully-multicoated four lens/two group optical system. This 2” barrel slips directly into your scope’s 2” focuser drawtube. A groove machined into the Coma Corrector barrel engages the thumbscrew or compression ring of your focuser to act as a safety stop. This prevents the Coma Corrector from falling to the ground should the focuser’s thumbscrew or compression ring accidentally loosen during use, a wise precaution when multi-hundred dollar eyepieces and multi-thousand dollar cameras can be involved. The bottom of the 2” optical system barrel is threaded to accept standard 2” filters.
The second part is a removable 2” compression ring eyepiece holder that threads onto the optical system barrel for visual use. For extra security and a very solid accessory connection, two thumbscrews are provided to tighten this compression ring.
The 2” eyepiece holder unscrews from the Coma Corrector’s lens assembly to allow a supplied male T-thread adapter to be installed directly onto the lens assembly. This allows the Coma Corrector to be connected directly to the female T-thread mounting on the front plate of most CCD cameras that is revealed when the CCD nosepiece is unthreaded.
The back focus from the last Coma Corrector lens surface to the image plane is 75mm, although this spacing is not a tight tolerance. If it is kept within 2-3mm (+/-0.1") of the 80mm point, there should be no notable change in visual or imaging performance. This assumes that the system is refocused as a whole for the best focus once the Coma Corrector is in place. That refocusing might shift the Coma Corrector inward or outward from the nominal position by as much as ~5-10mm (~0.25-0.5").
Optional third part spacers may be needed to place your DSLR or CCD at the optimum position behind the Coma Corrector, depending on the length of your imaging equipment accessory train. Some experimentation will be needed to optimize the position for your particular combination of camera, filter wheel, etc. Exact spacer thicknesses are not as important as precise collimation of the telescope and refocusing for best focus with the Coma Corrector in place.