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Striker you are asking the (almost) impossible, the most flexible lens is zoom, for astro you want 100-400mm and as fast as possible. So you are into the Canon L series lenses if you do not want either horrid coma and abberation or you must stop down so much the lens becomes too slow to use. Consensus out there is the F2.8 70-200L is a great option if you must go for zoom, the F4 version is as good but slower (and cheaper). You do not need IS for astrowork and other than that you should stick to L series primes.
OR
You could go for the Sigma 70-200 APO which is cheaper and good, I think John Drummond uses this and gets wonderful results?
OR
Use your ED80 and a flatener for you wide field shots.
You can play with the canon kit lenses (I do) but you will find yourself cropping a lot of the image and using 25% scaling on the remainder to reduce the distortions to acceptable levels which is a shame with such a superb camera.
As a guide you should probably spend about 30% on the glass (scope) and the rest on the body (mount), that should give you a clue as to the lens budget that matches your camera. BTW what does the 20Da come with as a kit or is it supplied body only?
Boy you really are going for broke aren't you...really jealous here....looks like a superb imaging set up you are putting together....please do let us know how you get on with the EQ6/Skysca/autoguiding I am wanting to go this way shortly (with a C9.25 rather than 11) so I am very interested in your experiances.
Clear skies!
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