[ATM] what causes coma?

[alaskawolfjoe at hotmail.com (lance clarke)]

I know coma is occurs when parabolic mirrors are used, and I know it gets 
worse the faster the mirror, but what I can't figure out is the reason for 
this.  Can anybody give an easy explanation for this effect?
Also, is this a problem in catadioptric systems where the primary is 
parabolic, ie a classical cass or gregorian?

Thanks
Lance Clarke
Toronto

_________________________________________________________________
MSN 8 with e-mail virus protection service: 2 months FREE*  
http://join.msn.com/?page=features/virus&pgmarket=en-ca&RU=http%3a%2f%2fjoin.msn.com%2f%3fpage%3dmisc%2fspecialoffers%26pgmarket%3den-ca