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An Eyepiece PrimerLow or High Power?
For viewing planets and close double stars, keep the image at the center of the field where most eyepieces perform best. I suggest using medium powers (20x to 30x per inch), since the contrast is better than at higher powers. Only when atmospheric conditions allow should you use a higher power.
Unlike a star, an extended source such as the Moon, a planet, galaxy, and even the sky background will grow dimmer as the magnification is increased. Such an object's surface brightness is proportional to the area of the exit pupil. Thus, an object viewed with an exit pupil 1 mm in diameter has only 2 percent of the surface brightness it has with a 7-mm exit pupil. Faint stars are best seen at high magnifications since the star's image remains constant while the background grows dimmer, improving contrast.
However, even for nebulae and galaxies higher magnifications aren't bad. As magnification is increased, the sky background grows dimmer at the same rate that the object does, so the contrast remains the same. But with higher magnifications, delicate structure is larger and hence more visible.
I suggest an eyepiece collection that yields exit pupils of ½, 1, 2, 4, and 7 mm. Buy the highest-quality, widest-angle eyepieces you can afford, especially if you use an f/4 to f/6 telescope. Eyepieces, like stereo equipment, improve your whole "record" collection when you upgrade. Every eyepiece and telescope has its own personality. There are guidelines to their use, but no absolutes. Explore the sky with as much variety of equipment as you can. You will soon find that every object exhibits special delights at different magnifications.
