Hi Manoocher,
The simple answer is that it takes less time to capture the same amount of data. There is far more to it and some physics to understand why but that’s the crux of it. Becareful though thinking that you should just buy the fastest scope you can afford because there are caveats. Fast scopes are much more sensitive to collimation issues, they are more sensitive to focus, they require much better curvature correction and depending on fast you go some filters may not be suitable. As the old saying goes..... nothing comes for free, there’s always a price to pay.
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