I would say that CA control is not the strong point of the BT81. If CA is the main issue, the BT-ED70 would be the much better choice, but it is double the price, and I find the image not as bright as I would have hoped, which means I don't use the ED70 much beyond 36x or so.
At 30x, I would say that the ARK is as good as the BT81 in terms of CA control, if not slightly better. However, the BT81 offers a much better off-axis sharpness and a brighter image. Observing e.g. the moon at 30x, which fills a large part of the fov of both instruments, in the ARK you can only see a fraction of the moon surface as a sharp image, the larger part is increasingly blurred, the further out you go from the center of the field. In the BT81, the larger part of the moon appears sharp and well defined at 30x. You are right, though, that the compactness of the ARK is a big plus, because you can just take it along and mount it on a tripod almost as easily as a 15x56 or so (which is what I sometimes do when I am not sure yet whether conditions are good enough to take out one of the larger binos).
I am not extremely sensitive to CA so I may be a bad reference point. But I find the BT81, considering its price, offers me good, bright and sharp images up to magnifications of well over 50x, with a little CA in the center and a bit more off-axis. I am even regularly using a 7mm DeLite, which gives 68x, and it has proven to show me further details not that easily visible with my 9mm Naglers.
I have to admit that I seem to have an extremely well collimated sample of the BT81, allowing me under ideal conditions to even use a 5mm DeLite = 96x (other BT81s I have seen have collimation issues from about 50x, and the same is unfortunately true for my BT-ED70 - I know, I know, Vixen does not recommend using eyepieces with less than 10mm.....).
Overall, I think the BT81 is well worth its money. But whether you can live with a certain amount of CA, which no doubt is there, depends much on the individual user (esp. if such user has a fine instrument such as the Conquest HD; although if you want to be pedantic, the HD also displays a little bit of off-axis CA). You for sure cannot compare the BT81 with same size premium binos such as Docter or Kowa (I happen to have both of those as well), but for its price, the BT81 in my view performs very well.
Edited by Pinac, 12 November 2016 - 09:59 AM.