Coma is a fundamental abberation and a good way to avoid it is - (as the Irishman said) - not to start here ! Your focal ratio is a bit short for confort. However,since you already have the problem it is possible to buy a coma corrector in the commercial market - look in S&T mag. Or you might get a friendly mirror maker to make you one. A home designed coma corrector should help a bit but the field will always be a little rough in the edges. Let me know if you get stuck. I might be able to design you a suitable corrector. regards David. My e.mail is soc@onyxnet.co.uk