Choosing Eyepieces for Planetary Viewing
For planetary viewing, the most useful eyepieces are those that provide about 100X magnification and either a second eyepiece or a Barlow lens that will work with your eyepiece to achieve, on average, somewhere between 100X to 250X magnification. It's always preferable to have two or three magnification choices. This allows you to adjust your max magnification to the evening's seeing conditions and besides, you'll be looking at other objects with these eyepieces besides the planets.
In order to figure out the magnification that your eyepieces will provide for your telescope, divide the focal length of your telescope by the focal length of the eyepiece. For instance, if you have a telescope with a focal length of 1000mm and a 10mm eyepiece, it will produce 100X magnification. Add a 2X Barlow to that 10mm eyepiece to achieve 200X, or a 2.5X Barlow to hit a magnification of 250X. If you take a moment to write down the focal length and resulting magnification of any eyepieces or Barlows you already have, you will easily see where the biggest magnification "gaps" are and be able to choose the proper equipment to fill those gaps.
The planets do best under as much magnification as your telescope and the seeing conditions allow, but the views must be as sharp as possible. Experience with your telescope, eyepieces and viewing site will help you understand the limitations better as time goes on, but a general rule of thumb for a sharp view is to push the magnification to no more than 50X - 60X per inch of your telescope's aperture. There are always exceptions to the rule but I mention this because it is important to remember that a 60mm (2.36") telescope will usually experience degradation of the image if you go above about 125X, depending on the conditions at the time. If you push the magnification too far with any telescope, regardless of aperture, the view will be blurry, and nobody likes a blurry planet.
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