I have an 8" f'6 Dob I just put together with what is apparently an old (1966 signature) mirror with an excellent figure. I've already had superb views of Saturn on a night of really good seeing, including being able to resolve Titan's disc and pick out Enceladus fairly easily. I couldn't confirm Mimas though. I'll try again some time when it's more ideally placed.

So I was looking forward to seeing an eclipse of a Galilean moon after reading the December 02 Sky & Telescope article "Mutual Events of Jupiter's Moons". I figured I could resolve it well enough, and it would be visually interesting. I wanted to see the Dec. 20th event (4:48, 2e1A), but was clouded out. I hope I manage to catch one of the up-coming eclipses soon.

What I did see, however, was the Dec. 30th occultation (5:55, 2o3) of Ganymede by Europa. I wasn't expecting much. The seeing was only maybe a little better than average. I was using 440x, but the planet looked better at 240x. I thought I may be able to resolve the discs.

Well I could easily see their difference in size, and as they came together, their difference in surface brightness as well. It seemed to take forever to close the last arc-second since they were both moving in the same direction. Still, I was expecting to lose Europa fairly quickly, but I found that the division remained visible for several minutes. I did lose it halfway through. But since it wasn't an annular event, I could always see that it wasn't round. I thought the seeing had deteriorated slightly. I didn't see the dividing line on the way out.

It only occurred to me later WHY I had seen the line on the way in, but not on the way out. It was only about 5 weeks from opposition at the time, but it was the dark limb of Europa that provided that contrast. Now I've read that the Encke gap is only about 1/20th of an arc-second (correct me if I'm wrong) and it's visible in a scope this size under ideal conditions. But a 99+% illuminated Europa (just about 1") leaves less than 1/100 of an arc-sec. un-illuminated. And even if you add a little for limb-darkening near the terminator, that's still an EXTREMELY small feature to be able to see with an 8" telescope.

Granted, this is a very high contrast feature, probably higher than the Encke gap. But where does it end? Just how small a feature can be seen with an 8" scope? Obviously Dawes limit goes completely out the window here. But has anyone ever done any research or experiments on this subject?

If I can still see that terminator a week before or after opposition, I'm never selling this scope!

I submitted a copy of this to S&T and Roger Sinnott replied with a quote in an optics textbook stating that "The narrowest black line on a bright field that the eye can detect subtends an angle of from 1/2 to one second of arc."  This refers to the naked eye looking at a black line against a bright background, like a power line in the distance.

Of course, this says nothing about telescope capability, but on the human end at least, it means that at 440x, it would be theoretically possible to see a high-contrast line of only .002".

Keep in mind, the Dawes limit refers primarily to the ability to split double stars, and even marginal optics will do this. Somehow the rules for planetary details is somewhat different. I don't know why. So with really excellent optics, what might be possible? Don't know till you try, do you?

- Scott E.