Your Home Observatory Help (page 4)
Your Home Observatory
Now that you have some knowledge of what goes on beyond the reaches of Earth’s atmosphere, you can more fully appreciate what you see when you look up after the Sun goes down. You don’t have to spend your life’s savings on hardware, but a few instruments can help you see a lot of interesting celestial objects.
Location, Location, Location!
As our cities swell, good places for astronomical viewing are becoming hard to find. We light up the darkness so that our streets are safe for driving powered vehicles, even as the exhaust from those machines thickens the veil between us and the Sun, Moon, planets, stars, nebulae, and galaxies. Tall buildings turn fields into canyons. Some children grow up without learning to recognize any celestial objects other than the Sun and Moon. It doesn’t have to be this way.
If you happen to live in a rural area, especially in one of the less populated parts of the country, consider yourself blessed.
Before You Venture Out
Read this chapter before you start to shop for astronomical viewing aids. Then check out several stores; hobby shops are excellent. If there is a local astronomy club in your area, find out where and when it meets, and get some input from experienced amateur astronomers before spending any money. Your needs will depend on what you want to see “up there” and how important amateur astronomy really is to you.
Sky and Telescope magazine online has information about astronomy clubs all over the world. Go to the following Web site:
Click on “Site Map,” and then click on “Astronomical Directory.” As you know if you have used a computer online lately, the Web is always changing, and by the time you read this, the links may be different. In that case, go to this site:
Click on “Advanced Search,” and input the words astronomy clubs in the “exact phrase” box. Then take it from there!
Wherever you live, you need not travel far to get to a place where latter-day contrivances don’t interfere with your view of the nighttime sky. There are plenty of places, even near Boston, London, or Sydney, where the stars twinkle and the planets stand out like beacons. In this respect, ironically, some of the world’s poorest people are well-to-do. Have you ever wondered what folks in remote Afghanistan or Tibet see when they look at the sky on a clear and moonless night?
The next time the weather is favorable for sky watching, get out in a rural area, out on a big country lake, or offshore in the ocean in a small boat. Find a quiet place, a safe place, where human and animal pests will not disturb you. Bring along some insect repellent unless it’s winter. If it is winter, wear plenty of warm clothing! You’re not going to be jogging around or doing aerobics. Don’t trespass or put yourself in danger. Put at least 75 miles between yourself and the nearest big town.
Don’t expect to find a spot entirely without any human-made lights in view, but if they’re few in number and more than a city block away, it should be good enough. Give your eyes at least 15 minutes to adjust to the darkness. Then gaze upward. Better yet, lie flat on your back with an unobstructed half-sphere of sky above you.
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.
Some amateur astronomers recommend that you obtain a pair of binoculars before you spend any money on a telescope. This is an individual choice. Binoculars are good for general star viewing at low magnification. Telescopes are a requirement for resolving detail in the planets, observing lunar terrain up close, or examining Sunspots.
Figure 20-1 is a simplified functional diagram of a pair of binoculars. You can think of the assembly as two identical telescopes placed beside each other. The eyepieces are spaced to match the distance between the pupils of the observer’s eyes. This spacing is adjustable. In most types of binoculars, when the eyepiece spacing is adjusted, the spacing between the objectives also varies.
The objectives are farther apart than the eyepieces. This exaggerates perspective for scenes within a few hundred meters but does not affect perspective for celestial objects, which are too far away for parallax to exist relative to any single observation point. The light enters the objectives, passes through a pair of prisms that bring the light beams closer together by means of internal reflection, and finally leaves the eyepieces to enter the observer’s eyes. The principle of operation of each half of a pair of binoculars is identical to that of a Keplerian refracting telescope. The prisms turn the image right-side up and also orient the view properly left to right.
Binoculars are rated in terms of the magnification (the number of times the apparent diameters of distant objects are increased), as well as in terms of the objective-lens diameter in millimeters (mm). You’ll see a pair of numbers separated by a multiplication symbol, for example 7 × 50, printed somewhere on the assembly. The first number is the magnification, and the second number is the objective-lens diameter.
In general, the light-gathering power of binoculars is proportional to the square of the objective-lens diameter. However, this holds true only when the binoculars are optimized for a particular observer. If you divide the objective-lens diameter by the magnification, you should get a number between approximately 4 and 8. This number is called the exit pupil of the instrument. For best viewing, the exit pupil of a pair of binoculars should be the same as the diameter of the pupils of the observer’s eyes (in millimeters) when adjusted to the darkness. In general, larger exit pupils (6 to 8 mm) are a good match for younger observers, and smaller exit pupils (4 to 6 mm) are better for older observers.
In terms of physical bulk and mass, binoculars range from tiny to huge. At least, this is the impression you’ll get. Some binoculars can fit in your pocket. (But always keep them in a carrying case when you’re not using them). Others are so large that you’ll want a tripod to support them. The most massive binoculars will make your arms tired if you have to hold them up for a long time. High-magnification binoculars, especially those greater than 8×, need the extrasteady support that a tripod can provide.
The biggest binoculars are more appropriately called binocular telescopes or stereoscopic telescopes . These are fabulous for viewing star clusters, galaxies, and nebulae. They also can deplete the average person’s bank account.
The prisms inside binoculars serve as mirrors to reflect the incoming light between the widely spaced objectives and the narrowly spaced eyepieces. Prisms provide better image resolution and contrast than mirrors. The best prisms are called porro prisms (Fig. 20-2 A ). They reflect the light entirely by total internal reflection, which you learned about in Chapter 17. Less effective but still superior to mirrors are roof prisms (see Fig. 20-2 B ), which have aluminized back surfaces that help reflect the light rays. Porro prisms are more expensive than roof prisms because a higher grade of glass must be used to get the highest amount of total internal reflection to occur without aluminized surfaces.
Another factor to consider in binoculars is whether or not the lenses are specially coated to minimize the amount of light they reflect. As you have seen if you’ve looked at the window of a darkened house from the outside during the daytime, all glass reflects light as well as transmitting it. Any light reflected is light that doesn’t pass through the glass. In binoculars or a telescope, you’ll want as much of the light as possible to reach your eyes and not be reflected back into space from the objective(s) or into the internal chamber of the instrument from the eyepiece(s). The best lenses have multiple coatings on both the inside surfaces and the outside surfaces. These binoculars will be specified as having fully multicoated optics .
Field Of View
When you look through a properly adjusted pair of binoculars, with the focus and barrel spacing optimized for your vision, you should see a single, large circular region. The absolute field of view is the angular diameter, in degrees of arc, of this region as measured against the background of distant stars. In some sets of binoculars, the absolute field of view is expressed in terms of feet at 1,000 yards instead of degrees.
The absolute field of view, which is defined in the same way as it is with telescopes (see Chapter 17), depends on the magnification and also on the apparent diameter of the circular region—the apparent field of view—as seen through the eyepiece. Binoculars with large apparent fields of view offer a more pleasant viewing experience than those with narrower apparent fields regardless of the absolute field of view.
Even if a pair of binoculars has a wide apparent field of view, the image quality won’t necessarily be good. How well do objects near the outer periphery of the field stay in focus compared with objects near the center? Are stars near the edge distorted or blurred? To what extent do “little rainbows” appear around stars, especially near the edge of the field? You can’t easily test these things in a hobby store when you are deciding which pair of binoculars to buy. Therefore, it’s a good idea to check out the return policy of any store with which you do business. If the binoculars prove unsatisfactory, you should be able to return them to exchange for a better pair or to get a full refund within a few days of purchase. Keep the sales receipt!
Practice problems of this concept can be found at: Your Home Observatory Practice Problems
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