12.5" Discovery and Jupiters moons

I'm having a minor but bothersome problem with a 12.5" Discovery when observing Jupiter. I can't get the moons to pinpoint focus. Ths scope is collimated using a cheshire and star test ( I find that star test results vary greatly dependent on seeing conditions, but the secondary shadow is always dead center). Nonetheless, the moons twinkle. I've set a 7" Mak along side the dob, and at the same mag, I get pinpoint images with the LX200. I've squared the focuser, center spotted the secondary and primary, aligned everything numerous times with the same results. The dob provides crisp, clear and simply wonderful views of saturn, one evening with excellent conditions, I was even able to crank it up to 400x without image breakdown. DSO's are great...much more detail and brightness than with the Mak....but Jupiter's moons still twinkle(almost imperceptibly) at even 53x with the dob. Eyepiece selection doesn't seem to change the situation (Leica 20mm w/barlow, Meade 26mm plossl, Celestron 30mm Ultima,) A friend suggested trying a smaller secondary than the 2.60"....don't know what else to try...any suggestions would be most helpful....thanks...John

The more important consideration, when you crank up the power to atleast 200 under good seeing, can you see that the moons are in fact small disks? Never mind the spurious light surrounding the moons, those are artifacts of the secondary obstructions and the spider vanes. The jovian moons are pretty bright and will cause these effects even under the best of conditions. I use this as a test of the seeing. When the disks are very "hard" and round, I've even seen darkening on Ganymede. A couple nights ago Ganymede was eclipsed by Europa's shadow. It was a central eclipse and although I couldn't see the shadow, I could see that Ganymede got darker for a few minures.

Walt