Your Home Observatory Help (page 2)

By — McGraw-Hill Professional
Updated on Sep 19, 2011

Night Vision

Astronomers have always had a problem with night vision. Now you’ll find out first hand how they deal with it. On one side of the visibility equation, your eyes must adjust to the darkness, especially when the Moon is not above the horizon. On the other side of the equation, you’ll want to read star maps or consult other reference materials from time to time. You might have to check eyepiece specifications, make adjustments to a telescope, or otherwise fiddle around with “stuff.” You’ll need some sort of lamp to do this.

Get a flashlight and some red cloth or thin red tissue paper. This will serve as a color filter. Cover the business end of the flashlight with the filter. Secure it with a rubber band. The resulting light should be dim; you’ll have to experiment with various coverings to find out what works best. Use a flashlight with size D cells or, better yet, a lantern with one of those bulky 6-volt batteries. Be sure the cells or batteries are fresh, and carry a spare bulb. The light from the lamp should be bright enough so that you can read your star charts, eyepiece numbers, and other information after your eyes have fully adjusted to the darkness. But it shouldn’t be any brighter than that.

Red light has some special properties. It does not desensitize your eyes to the extent white light does. If you keep the filtered light source just bright enough so that you can read by it (but no brighter), it won’t interfere with your stargazing. Another plus: Red light attracts fewer insects than white light.

Getting Your Bearings

Once your eyes have adjusted to the darkness, it’s time to locate some stars, constellations, or planets. These vary depending on the time of year, the hour of the evening, and the latitude on the Earth at which you happen to live. You can refer back to Chapters 1, 2, and 3 to locate some of the major constellations and to figure out what point(s) of reference to use. The positions of the Moon and the planets, as you know, vary among the background of stars.

Current maps of the heavens can be viewed by going to Weather Underground at the following Web site:


Click on the “Astronomy” link. You can input your location anywhere in the world, as well as the hour of the evening or night, and get a complete map of the sky. If this link isn’t available for some reason, Sky and Telescope online has excellent printable star maps. Go to:


If you can afford it, bring a notebook computer along on your stargazing expedition and have it equipped with wireless Internet access. In this way, you can check out the star maps on the fly. Turn down the display brightness to a low level so that it won’t degrade your night vision.

The circumpolar constellations are the best reference to begin with. This is so because they’re always above the horizon regardless of the time of year, unless you happen to live in the tropics (between approximately 20°N lat and 20°S latitude).

Your eyes alone can see a lot of interesting things in the sky once you know where to look. Mysterious fuzzy spots appear. Certain dim objects seem to pop out when you look slightly away from them, only to maddeningly vanish when you look straight at them. This is normal; it is a result of the anatomy of human eyes. The center of your field of vision is known as the fovea , representing the point on your retina where your gaze is directed precisely. This is where your eyes’ image resolution , also called resolving power , is greatest. However, this comes at the expense of sensitivity , which is better slightly off-center in your field of vision. Sensitivity and resolving power both can be improved dramatically, of course, with binoculars and telescopes. However, then you sacrifice absolute field of view.

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