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The second number is the focal length of the telescope. All other things being equal, an 80x600mm scope will have 50% more of a field of view than an 80x900mm scope, and 2/3 the magnification. However, it will require significantly less time to take a picture due to the "faster" optics in this example of f/7.5 vs f/11.25. Faster (lower) is better for a focal ratio.
Another thing to look for in a scope is a decent focuser if you have the budget. This can be picked up as an upgrade, I would recommend the 10:1 crayford style which will give you much more precise focus control.
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