I have been asked this question a couple of times
now so I thought that i would make a post on it, I the guide I am giving
will be based on my skywatcher
explorer 130. There are allot of things to take into consideration when
choosing what magnification to use for planets, one of them is your
scopes highest practical power. To figure this out you can use the rough
calculation 50x per inch of aperture so for my skywatcher explorer 130 that is about 250x but for a 8 inch scope that would be 400x.
Another thing to consider is the viewing conditions, a common mistake
that people make when they first get the scope is that they thing the
more magnification the better the view. this is totally wrong because
yes the magnification can show more detail, but often it make the image
worse when going over about 200x as it shows more of the atmospheric turbulence, light pollution, haze e.t.c
also the scopes optics and the quality of the eyepieces can make a
difference to how high the actual practical power is.
For Venus i would
probably use as high a magnification as the conditions allow to make it
easier to see its phases, for mars I would use about 155x
magnification, but be warned unless its at opposition or you have a big
scope you wont see much surface detail. For the Jupiter you can see its
moons at low magnification and above about 100x
you can start to see banding and surface details, for Jupiter I usually
use as much magnification as possible before the seeing conditions
start ruining the image which for me is about 200x.
For Saturn the rings are visible at low magnifications but just like
Jupiter if you move up to the higher powers for me at about 175x mag I can see the ring separation and if i'm
lucky one of the bands on the planet, one thing to be careful with is
that if Saturn is near the horizon it can bounce around allot in your
eyepiece. And finally for the moon I prefer using about 100x
and just letting the moon move across my field of view, but if the
seeing conditions allow you to you might want to pile on the
magnification a bit. To conclude the best view for planets are normally
found between about 100x and depending on the seeing conditions up to 250x. If you have any questions feel free to contact me, I hope my guide on what magnification to use for the planets was useful.