Hi,
The 70-200 f4L + Canon 1.4x extender is a combo I used to use for astro work a couple of years ago. It worked well in my opinion, but other combinations weren't as forgiving on the accompanying aberrations which are introduced. My 300mm f4L + extender never gave results as good as the 70-200 extender combo.
link to an old post with an Eta pic...
http://www.iceinspace.com.au/forum/s...ad.php?t=42671
Doug