Posted 09 February 2013 - 02:26 PM
The little oval train that has been sliding along below the Oval BA are pretty small and have pretty low contrast My estimate is tha they are only about one arc second or so. This makes them pretty difficult even in a C14, and I don't think I have ever seen ovals this small in my 6" APO.
Even when they are well positioned, they can be be difficult for me in the C14. Well positioned means on the meridian. If they are to far off to one side or the other, they get foreshortened and get even smaller and just loose to much contrast (the more oblique the are, the more contrast is lost because contrast transfer is a function of angular size... The narrower you make it the more contrast is lost).
In the C14, ovals generally have to be maybe 1.5 to 2 arc seconds to see well. A bit bigger for the 6" APO. 2 arc seconds or a bit bigger and they come into view.
But so much depends on the contrast.
Notice that an observer using a a fairly small scope can easily see the small dot from a shodow transit of Io even though it is only 1.1 arc seconds..
But the difference between Io's shadow and an 1.1 arc second oval is that the shadow starts with much closer to 100% contrast against the bright background of Jupiter's cloud tops than an most ovals start with. A small scope might loose 75% of the contrast from the shadow, but with 25% contrast difference, it will still stand out from the background.
But an oval that starts out the same size, but with only 20% contrast will loose the same 75%, and that will put its remaning contrast at 5%. This is below the contrast sensitivity threshold of the dark adapted eye for most observers.
The bigger the scope though, the less contrast is lost for small details, and the more likely it is to be seen.
But a 1.1 arc second oval with 20% contrast is a very difficult target visually even in a C14. Seeing has to be just about perfect, and the oval has to be well placed.
I have seen occasional large ovals in but a C11 and a 6" APO, but they had to be pretty big and pretty well placed.
Camera's (and maybe more importanly software) make it easy to detect things that are very hard visually.