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One big consideration is your observing style. Do you typically have short viewing sessions or longer? Do you do all your observing from your house or do you plan on heading out to a dark site?
The C8 will take a lot longer than the 100ED to cool down so if you’re like me and have a lot of shorter viewing sessions (especially during the warmer months) then a 4” may be better.
If you don’t have to go to bed early and can just leave it sitting outside to cool down for a few hours and do your observing later in the evening, a C8 could be better.
If you don’t plan on heading to a dark site then the extra aperture will help resolve some globular clusters but not much more than that.
Open star clusters and quick planetary/Luna views are where the 4” excels as aperture doesn’t help much with open clusters. A lot of them are quite large too!
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