[raymo]

I forgot your second question. The length of sub you can get is
directly related to the length of a piece of string. Firstly, the longer
the focal length of your lens/scope the shorter the subs will be. Secondly,
the accuracy of your polar alignment is also factored into the equation,
and other factors include the wind shaking the scope, and the tracking
qualities of your particular mount. Lastly, the nearer your target is to the celestial equator the shorter the subs will be. You can get much longer
subs near the celestial poles.
As a rough guide, from my own experience, with very good polar alignment, using a 1000mm focal length scope, aimed near the celestial equator, you should get a very high percentage of usable subs at 30-45 secs, a good percentage at around 60 secs, and a low percentage at 75-90secs. I have had occasional round star subs at 120, and even one or two at 150.
Obviously, if using a shorter scope you will get longer subs, roughly twice as long with a 500mm lens/scope.
raymo