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I have the Canon 70-200mm f/4 IS and it is a superlative lens. Truly awesome. It is well noted as being sharper than the 2.8, one of the sharpest lenses ever, from any company.
However, if you are after the lens for purely astro work, the 70-200's don't have a zoom lock. I'd probably be picking a prime like the 300mm f/4 and whacking on the IDAS LPS filter. As you probably know, Doug has been there, and his results speak for themselves!
Cheers,
Jason.
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