The following information on the Sky-Watcher Quattro Coma Corrector comes from Highpoint Scientific
The Skywatcher S20204 Coma Corrector is an optical correction device with 48mm threads and is made expressly for all f/4 Quattro Imaging Newtonians. Made of FPL51 and Schott glass, the Quattro Coma Corrector will improve the appearance of stars around the outer edges of the field of view.
Coma, which is an off-axis aberration common to all classic Newtonian telescopes, gives stars located along the edges of the field a comet-like, or cone-shaped appearance. Stars in the center of the field of view are not affected. While many amateur astronomers do not mind a little coma, if you will be taking wide-field images, you will appreciate the correction the Quattro Coma Corrector provides.
Please note that the addition of the coma corrector requires 55 mm of back focus. The Quattro Coma Corrector uses the larger M48 threads, and Sky-Watcher makes Canon EOS and Nikon DSLR adapters should you require one for your set-up. They are available separately.
This Sky-Watcher coma corrector is threaded to accept 2" filters
This coma corrector was purchased for use with a TPO 12-inch f/4 Imaging Newtonian Telescope.
Review of Sky-Watcher Quattro Coma Corrector from Cloudy Nights.