Head to head comparison of the 3 long focal lengths. Make sure that "original" below right is active so that you see the images at 1:1 scale on your monitor. The field curvature leads to unacceptable degradation of the edge detail for the C8 at both 2030 and at 1250mm focal lengths. The TeleVue does quite a lot better on the sides but the slight degradation leads me to be sceptical about its ability in the corners of a full 35mm format DSLR like a 1Ds. One would expect a Canon 600mm lens to cover that large sensor significantly better than any of the above-tested optics.

Overall conclusions? Astronomical telescopes are impractically bulky for shooting birds at the beach. For astronomical use the conclusions are very subjective and may raise emotions amongst APO lovers and the rest of the astro community ;-) but mine are as follows:

For Visual Use:
Frankly I do not see much use for an APO. We often have the TV140 next to a C11 or an Obsession 20" at our club outings. Aperture rules! For DSOs it's obvious why the larger scopes would win, just from their light-bucket properties. For lunar/planetary the larger scopes again win when they are in good collimation (critical point!). There's often more contrast visible in an APO on the planets, but this is often at quite low magnifications, say, under 200x. For visual use on large scopes magnification is often limited to 300x because of seeing conditions. On a 20" that's only 15x per inch and the contrast is very satisfactory. There really is no comparison, IMHO, as to which is the better view. The combination of higher resolution and low power per inch makes the larger scopes win even on planetary views.

For Imaging:
Field curvature and coma on SCTs and Newts make them quite unsatisfactory on the larger imaging chips, eg on the 1.6x crop DSLRs and even more unsatisfactory on full format 35mm chips. This is definitely APO and camera-lens territory. The TV140 is a Petzval design (just learnt that in 2007! five years after the above testing) and at prime focus looks quite satisfactory on a 1.6x crop DSLR but much less satisfactory on a full 35mm format DSLR. Perhaps there are supplementary correctors available, but for myself I decided to go with a Canon 600mm/4.0L IS lens as my primary DSO imaging OTA, rather than purchase a 6" APO. I find it superbly compact and satisfactory on full 35mm format, even if imperfect (some gull wings become visible on bright stars in the corners of the frame after severe contrast stretching). Would there be smaller or worse gull wings with an APO+field flattener? Sorry, I cannot answer that since I have not shot them side-by-side. Nevertheless at my stage of expertise, my star images are still tracking-limited rather than lens limited. So I have ended up with a Canon 600mm lens plus 1.4x and 2x tele-extenders for DSO imaging (just being able to use autofocus on bright stars with or without an extender is unbelievably convenient), and a C14 for planetary imaging and all visual observing.

A few years after the above was written, I pitted a couple of serious Televue premium APOs against a couple of Canon lenses for astro-imaging, and also discovered how, amazingly, it is possible to attach a Hyperstar to a C14 and get a very flat image field, with pinpoint stars corner to corner on a 1.6x crop DSLR. That's 675mm focal length at f1.9! Makes the Canon 600mm/4L IS lens redundant for astro, except for full 35mm format chips. Please peruse my primary website for these and other reviews: http://www.samirkharusi.net/