How much magnification can I use, and how much is too much?

Telescope magnification is actually a relationship between two independent optical systems – the telescope itself and the eyepiece you are using. To determine power, divide the focal length of the telescope (in mm) by the focal length of the eyepiece (in mm). By exchanging an eyepiece of one focal length for another, you can increase or decrease the power of the telescope. For example, a 25 mm eyepiece used on a 1250 mm focal-length telescope would yield a power of 50x (1250/25 = 50) and a 10 mm eyepiece used on the same instrument would yield a power of 125x (1250/10 = 125). Since eyepieces are interchangeable, a telescope can be used at a variety of powers.

There are practical limits of magnification for telescopes. These are determined by the laws of optics and the nature of the human eye. As a rule of thumb, the maximum usable power is equal to 50-60 times the aperture of the telescope (in inches) under ideal conditions. Powers higher than this usually shows a dim, lower contrast image. For example, the maximum power on a 127 mm telescope (5” aperture) is in the range 250x - 300x. As power increases, the sharpness and detail seen will be diminished. The higher powers are mainly used for lunar, planetary, and binary star observations.

Most of your observations will be done with lower powers (6 to 25 times the aperture of the telescope in inches). With these powers, the images will be much brighter and crisper, providing more enjoyment and satisfaction with the wider fields of view.

A good way to increase magnification is to use a Barlow lens. A Barlow lens rated at 2x can be used with your existing eyepiece and it will double the magnification of any existing eyepiece.

I have a refractor telescope with 900mm focal length and 60mm objective lens. Why can't I get a clear image when I try to use a 2x Barlow lens with a 6mm eyepiece?

This is because you’ve exceeded the practical limits of magnification for your telescope. The practical limits of magnification of a telescope are determined by the laws of optics and the nature of the human eye. 

As a rule of thumb, the maximum usable power is equal to 50x-60x the aperture of the telescope (in inches) under ideal conditions. Powers higher than this usually give you a dim, lower contrast image. For example, the maximum power on a 60 mm telescope (2.4 in aperture) is in the range 120x-142x. The higher powers are mainly used for lunar, planetary, and binary star observations. As power increases, the sharpness and detail seen will be diminished.

The combination of the 6 mm eyepiece, 2x Barlow, and 900 mm focal length gives 300x. This is well beyond the maximum usable power of the scope.

The Earth's atmosphere also plays an important part in limiting the maximum magnification you can use. Instabilities in the atmosphere such as heat radiating from the ground and surrounding buildings, high altitude winds, and other weather conditions can cause the image to blur. This "bad seeing" can drastically distort your image. This also explains why bright stars appear to twinkle. The best time to use high magnification is on nights when the stars do not appear to twinkle very much.