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Hi Jowel,
The size of ANY object is determined by the magnification. Typically, the more you magnify something, the bigger it gets, BUT the dimmer it gets also.
What this means is that you can use say a 5x powermate like mine on an 8" scope to BLOW up Jupiter to a larger size, but the limiting factor to how bright it will be is the aperture of the scope. BIGGER opening means MORE light and therefore brighter and more detailed image. If I use my 5x barlow on my 130mm scope the planet is too dark to see anything.
In general terms, magnification limit is approximately 2 x the aperture in mm of your scope. Therefore, with my 130mm reflector, the max is 260x magnification, my 200mm reflector is 400x and my 300mm reflector is 600x. To work out the magnification of an eyepiece, you divide the focal length of the telescope by the focal length of the eyepiece. So, for example, on my 8" (FL=1000mm) a 10mm eyepiece gives: 1000 / 10 = 100X magnification. A barlow (magnifying lens) MULTIPLIES this by whatever factor it is. eg. a 10mm EP with a 5x barlow gives 500X magnification!!! This is pushing it a bit for my 8" scope. Therefore I also have a 2.5x barlow which gives me a more usable magnification on the 10mm of 250X. (my 300mm scope has a FL of 1500mm so the 10mm EP gives 150x, 375x (2.5x barlow) and 750x (5x powermate).
However, the maximum you can view is also dependant on the seeing conditions and the transparency of the sky. Bad seeing shows up as wobbly planet (looks like the planet is underwater and is like jelly in the eyepiece - sort of like heat haze) and this makes things like details, difficult to see. Bad transparency means that the planet looks stable and round but its hard to make anything out as its almost like there is a mist obscuring things.
If you are looking at a 8 inch scope - solid dobsonian (which you can use the tube to mount to an EQ platform later) then you will have either 1000mm focal length or even 1200mm, depending on the brand.
What this means is that you normally get a 10mm and a 20 or 25mm eyepiece with the telescope. Assuming its a dob with 1200mm FL, then the 10mm will give you a 120x mag, the 25mm will give you 48x mag. Rather than buying other eyepieces, you can simply get a 2 x barlow (get an APO one, they are better than ED in my opinion) for about $60 and then you will also be able to double the mag on the 10mm to 240x and 96x on the 25mm.
This will give you FOUR magnification levels; 48x, 96x, 120x and 240x which is PLENTY to get you started. and gives you a good range to work with. As I said, the conditions generally dont let you use REALLY high magnification often, so anything too high is a bit of a waste of money.
Looking at say Jupiter at 48x will show you some banding on the planet and perhaps the GRS but also be wide enough to easily make out probably all of the Galilean moons in the one view (depending on where they are). Switch to say the 10mm with the 2x barlow and you will be at 240x and then you should be MUCH closer and hopefully see more details in the cloud belts and make out more of the GRS and some of the closer moons will also be bigger.
By the way, the easy way I remember the four closest Galilean moons for Jupiter is "I Eat Green Cheese" which refers to (in order from closer to farther away) Io, Europa, Ganymede and Callisto.
When you do end up buying a scope, keep in mind that 2 eyepieces and an 2 x APO barlow will probably suffice for at LEAST 12 months and will enable you to see most of the planets as well as some really cool galaxies, nebulae, open and globular clusters as there are literally thousands of things within reach of an 8" scope and the primary reason most people on IIS say to start with an 8", its a great place to start.
Whew!~ another essay from me....hope your eyes arent too tired...
Cheers
Chris
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