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As high up as possible!
The reason is because when a planet (or anything, for that matter) is low on the horizon, you're looking through more atmosphere than when it's up high. This has 2 effects:
1) The atmosphere causes atmoshperic dispertion, where the colour channels come through misaligned and each channel can focus at a slightly different point. This will play havoc on your images, especially when captured with a 1-shot colour camera like the LPI, ToUcam, NexImage etc.
2) It's the atmosphere which produces unsteady seeing, the jetstream in particular is high altitude winds which basically blur your images as the light is distorted on its way to you. The more atmosphere you're looking through, the more affected by atmospheric turbulance you're going to be.
Also when it's low on the horizon, you're looking through smoke, fog, heat haze from houses and other things which will all distort the light as it comes into your telescope.
So either side of straight up is best.
However with Joop having transit events, it's usually best to time your run when there's a GRS transit, or a moon/shadow transit. Sometimes this doesn't co-incide with Jupiter being overhead, so at times you still have to image in less than ideal conditions, and hope for the best with regards to the seeing!
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