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Don't Stop Down Your Scope

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Posted by Daniel Johnson on October 26, 2002 04:23:31 UTC

It is a myth that stopping down the scope will improve resolution--Sky and Telescope covered this recently. You mentioned that Jupiter was closer to the horizon, and I'll bet that was the problem--probably the ONLY problem--if you saw Saturn crisply.
I have a 10-inch LX200 GPS. What I can see on Jupiter varies tremendously from night to night. One recent night I was able to see the shadow of one of the moons, Ganymede, as distinctly round on Jupiter, with detail visible within the main belts, as well as momentary glimpses of other minor belts. It makes a big difference how steady the air is, and closer to the horizon you're looking through a much longer column of air, with more chance for turbulence.
On exceptionally good nights you'll see two or three of the satellites as disks, not just points, but that's not often.
Sometimes you just can't see much. In unsteady air, lower magnification helps the signal-to-noise ratio by making some of the turbulent fuzziness too small to perceive. (This same thing makes small scopes seem to outperform big scopes on bad nights--the small ones just aren't good enough to let you see the noise, so they SEEM to give a steadier view. It's an illusion.) Sometimes the red spot is clear as can be. Other times, even though I know it's there from reading timetables of its appearance, I can't see the red spot at all.
My magnifications vary from 50x to over 500x. For planets I love my 10mm Radian (250x). I have shorter focal length Radians, but often the air isn't steady enough. I also have an 18mm Radian and 25mm Plossl, and use each of them when the seeing isn't as good. What you can see depends upon the night. There's no "best" magnification.
If Saturn is crisp, then the problem isn't your scope. Fiddle with the magnification and be content with what the night gives you.

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