[renormalised (Carl)]

You can't barlow out CA in a refractor, so any CA present at it's native focal length will simply be magnified as you increase focal length through barlows and powermates. Since you are going to need long focal lengths for planetary imaging this is not a good thing. The amount of CA present is dependant just as much on aperture as focal length, so you can't accurately state any focal length range where CA is non existant in a refractor.

CA is entirely due to the degree to which the lenses of the scope bring the various wavelengths of light to focus at the focal point. It has nothing to do with aperture and everything to do with how the lenses are figured and the refractive indices of the elements of the lens assembly. The reason why they use fluorite is that it is optically more transparent than normal Schott glass and has a low RI which means there is little or no subsequent deviation of the light path through the lenses and hence little or no misalignment in the focusing the various wavelengths of light...they are brought into focus as close to the desired focal point as you can get.

True, but I would add that it's not just light gathering advantage for the reflector, but also resolution advantage with the larger aperture. Also contrast between a planet and skyglow is a non consideration with planetary imaging. As long as it is night time you can image a planet from the middle of a bright city and still get great results.

I forgot the resolution (oops!!), but you are correct there. Yes it is...any amount of light which effects the contrast of the features on a surface will effect the quality of the image. What looks brighter, a white circle against a dark blue background or a white circle against a nearly black background. Of course you'll get a good image with a CCD or even an DSLR against skyglow for the simple fact that you can post process out a lot of skyglow. Plus you have a bright target.

Simply not true. All of the worls best planetary imagers use reflectors or CATS. A quick browse of the images section of this forum will confirm this.

All of the world's best planetary images are taken with reflectors or cats because they cheaper than an equivalent sized, long FL refractor (or even a short FL one, at that) and they're using their large light gathering capacity to overcome the difficulties I mentioned earlier. Just because those piccies taken here and elsewhere by amatures of planets, are done with reflectors and/or cats is neither here nor there. Would anyone here be able to afford the $90000 to $200000 (or more) it would cost to buy an 8-10" f10-f15 APO??. Or even shell out $8000-$10000 to buy a very good 10-12" F12-15 Achro??. Most wouldn't consider the achro on the basis of how smaller and faster ones perform and the apo would be way out of range of most people. If you've ever looked at a planet, or double star or the moon through a good, long FL apo or achro of a reasonable size, you'll know what I mean when I say that there's no comparison. The refractor blows the reflectors away for clarity, crispness and resolution of a target.

Tolerance to seeing is a function of aperture, not design of the scope. Typical atmospheric cells that affect seeing are around 6" - 12" in size, so going above this aperture range you will need to be more considerate of seeing conditions when imaging.

Tolerance to seeing has a hell of a lot to do with the design of a scope. By their nature alone, mirror based scopes, such as newts and SCT's are plagued by tube currents, cooling difficulties etc etc. Sure, refractors do suffer from tube currents to an extent but the optics are far less prevalent to image distortion etc, caused by tube current variables and the tubes of refractors are essentially closed systems. Only an SCT is closed like a refractor, but they have a greater thermal mass due to the greater volume of air inside the tubes that's closer to the mirror cf to a refractor, therefore are far more prone to image distortion via tube currents. Aperture is also a consideration where external factors of the weather are concerned...and in any case a misty/cloudy/windy or whatever sky will seriously degrade any type of scope's performance. But with SCT's or newts, the smaller the scope the better the performance w.r.t. tube current effects, as you have mentioned.