Jim Beam wrote in message ...

I read different numbers for how much you can milk out of a
specific aperture. I am considering a 7" APO. Some people say
30x per inch, thats it for detail. Well, thats only 210x which
seems conservative for a 7" APO. Many people say 50x per inch,
and thats 350x for a 7" APO, sounding better.

To my mind, this is utterly the wrong way of looking at the problem.
What magnification you choose to use depends on your eyes, your
tastes, and the target you're viewing. It is *not* a useful measure
of how much detail you can see through a telescope -- certainly not
when comparing among different observers.

For instance, I customarily view Jupiter at 120X in my 70mm refractor,
about 40X per inch. That works out well for me, and I have an eyepiece
that I like that delivers that magnification with that scope. Other
people might prefer 80X and yet others might prefer to use 150X. Yet
all of us would probably be seeing a similar amount of detail.

In any case, I certainly don't consider 40X per inch too high for that
little refractor; in fact, I have used twice that for viewing the Moon,
or for splitting tight doubles. 40X per inch is also a tad on the low
side (for me) when viewing Saturn's rings or details on Mars.

However, the inherent limitations of 70mm aperture are already *quite*
visible at 120X. On nights of mediocre seeing, I also customarily use
120X for viewing Jupiter in my 177mm reflector, so I end up using the
same magnification on both scopes despite the fact that one has 2.5x
the aperture of the other. But that doesn't mean that I see the same
amount of detail in both scopes. On the contrary! Except on truly
terrible nights, I always see more detail at 120X in the 177mm scope
than at 120X in the 70mm scope, despite the fact that this magnification
is still nowhere near the small scope's "limit" -- if such a concept
is meaningful.

- Tony Flanders