I have a 200 f/2.8L (same aperture as the 100-400L fully zoomed in) and has razor sharp images and pinpoint stars to the edge at full aperture. The same applies to my 70-300L at full aperture (fully zoomed in) f/5.6.
Even better than my ED110 + flattener.
A good telephoto lens has a flat field as well as they are designed for photography, and terrestial photography where they are primarily designed for also requires a flat field.
The ED110 is just a good ED scope but it still does have some small color errors, more visible in photos than visually, but OK for me. Visually it is an excellent scope even for planets at high power. Real APO's of 100mm cost at least A$3000, twice the ED110. And the best apo's (Takahashi FSQ-106 or TV NP-101) cost A$6000 or more, almost as much as the Canon 300 f/2.8L, but they are completely color error free and coma free, just like my small telephoto lenses, but with much larger aperture. Moreover they weigh also a lot less: the 300mm f/2.8L weight only 3kg and the 600 f/4L 4kg. No 150mm APO is that lightweight. So if you have the money (A$16000) you have a nice 150mm travel APO.
If you want an 'affordable' 75mm f/4 APO, go for the 300mm f/4L prime, costs about A$1800.
The 100-400L will do the job at 70mm.
Here a review:
http://www.dpreview.com/reviews/upgr...mark-ii-review