Telescope shopping advice

Here are a few words of shopping advice—much of which applies to telescopes as well:

• Examine and feel the binoculars. They should strike you as a well-crafted precision instrument.
• Test the focusing mechanism. It should be smooth and offer steady resistance.
  
• Look for antireflection coating on all lenses. This thin coating will make the lenses appear blue, yellow, magenta, or purple when held at an angle to the light.
• Look through the binoculars. Try focusing on a point of light (a distant bulb, for example). It should be absolutely sharp, at least until the point of light gets very near the edge of the field of view.
• Focus on a vertical straight line such as the corner of a building. Even with very good binoculars, the straight line will bend at the extreme edges of your field of view. However, if the line remains bent a third of the way from the edge, the quality of the optics is poor.
• The twin barrels of binoculars must be perfectly parallel with one another. If they aren’t, you will see a double image. Your eyes will work hard to compensate and fuse that double image, but ideally, there should be no double image to fuse.