[astro744]

See http://stargazerslounge.com/topic/16...g-reduce-coma/

Also there is a coma correcting Barlow at http://www.apm-telescopes.co.uk/apm-coma-barlow.html

also http://www.astronomyalive.com.au/apm...rlow-2-7x.html

and http://www.astronomyalive.com.au/apm...rlow-2-7x.html

and some images at http://www.astrobin.com/gear/1451/ba...ma-2x-apo-apl/

Note the Paracorr will eliminate coma and give you a 1.15x increase in focal length and ratio.