Observing Tips

When you get a new telescope or dust off an old one, a fantastic world of observations opens for you. At the Francis Marion University Planetarium and Observatory we are always trying to help the amateur astronomer get the most from his or her telescope. Below you will find some good hints from one of our staff of amateur astronomers, Francis Parnell. We hope this will help you enjoy your new hobby. For further viewing information we recommend Sky and Telescope Magazine and Astronomy Magazine for clear star maps and monthly viewing columns.

Atmospheric Conditions

Whenever we look at objects in the sky, we are looking through a thick layer of gases, the earth's atmosphere. This atmosphere is always in motion, some nights more so than others. The term "seeing," as used by astronomers, is used to describe the steadiness of the atmosphere during observing sessions. If the atmosphere is calm, the seeing is good, and the stars and planets will shine with a steady light. A little turbulence in the air will create average seeing, with some twinkling of stars. Planets usually don't twinkle as much as stars, so they may be steady. Bad seeing is when the twinkling effect is very noticeable for stars and planets.

The seeing is a problem particularly when viewing planets. Dust and turbulence in the atmosphere will keep you from seeing clearly an object that is near the horizon. Let it rise to at least 25 degrees altitude for a better view.

To see detail on planets, you want to use a high magnification. But this not only magnifies the planet's image, it also magnifies the instability of the atmosphere. The more you magnify the image, the more it dances around and becomes blurred. Strange as it seems, though, the hazy skies of summer mean calm air and good planet seeing.

The Telescope

The primary purpose of the telescope is to gather light from faint objects. The greater the diameter of the primary lens or mirror, the more light it will gather and the better image you will see. So you need a telescope with a diameter of at least 3 inches for effective and enjoyable viewing. The eyepiece, on the other hand, simply magnifies the image. You should choose the eyepiece that gives you the best image.

Choosing an Eyepiece

Your telescope may have come with several eyepieces, each with a different magnification. Eyepieces are labeled with a focal length, usually in millimeters (mm). The larger the focal length, the smaller the magnification. To calculate the actual magnification, divide the eyepiece focal length into the focal length of the telescope. For example, a 40 mm eyepiece on a telescope with a focal length of 1200 mm will produce a magnification of 1200 / 40 = 30x, or 30 power.

Because every night is different, when you begin your viewing you should always start with the eyepiece of lowest magnification (largest focal length). If the view is good, try the next higher magnification. At some point, you will notice the image actually getting worse with increased magnification. This is due partly to magnified turbulence, and partly because you are taking the same amount of light and spreading it out over a larger image, creating a dimmer view.

The larger the diameter of the main lens or mirror (depending on the type of telescope you have), the greater magnification you can use. Theoretically the maximum useful magnification for a telescope is 50x for each inch of diameter of the telescope lens or mirror. A three-inch refractor then would have a maximum useful magnification of 3 times 50, or 150-power (150x).

Because of typical atmospheric conditions, you'll seldom use anywhere near this high a power. The usual rule of thumb is, "Don't use more power than the atmosphere allows." Amateur astronomers know that some nights you'll be able to use your medium power eyepiece (25x to30x), but most nights you can only use your low power eyepiece (10x to 20x per inch). Those nights that allow high powers, like 40x to 50x, don't come often, so enjoy them when you can.

Eyepiece Filters

For more advanced observers, a set of colored eyepiece filters is very useful. They reduce glare and bring out subtle detail when viewing planets. For larger telescopes, a basic set might include #12 yellow, #23A red, #58 green, and #80A blue. For telescopes smaller than 8 inches, a set of lighter shades will keep the image bright. Appropriate filters would be #8 yellow, #21 orange, #56 light green, and #82A pale blue. Specific uses for these filters are explained below.

Viewing the Moon

The brightest nighttime object in the sky is the moon. Any size telescope will reveal an impressive and rugged landscape of large and small craters, mountain ranges, deep valleys, lunar "seas," and much more. Larger telescopes with higher power capability can be used to explore craters more fully. By observing night after night, as the moon goes through its phases, you can watch the slowly advancing sunrise reach the interiors of these craters. Watch for mountaintops to appear in the darkness as sunlight illuminates their peaks. Study the "seas," which are huge lava flows created 3 ½ billion years ago.

The worst time to view the moon is at full moon. At that phase the sun is shining directly down on the moon and there are few shadows. Without shadows, little detail can be seen. The best times for viewing the moon in the early evening are from shortly after new moon until about two days after first quarter, a period of about a week. In the early morning sky, view from about two days before last quarter to almost new moon, again about a week. During these times the shadows are longer and features stand out in sharp relief, especially along the terminator, the line that separates the lighted part from the dark part. During the crescent phase, you'll also want to observe "earthshine" on the unlit portion. This is sunlight reflecting off the earth and on to the moon.

The moon is so bright that the brightness may be a problem. Use a higher power eyepiece so a smaller portion of the moon is visible or use a #12 yellow, #80A blue, or a neutral density filter to reduce glare and enhance detail.

Viewing the Planets

Mercury never gets far away from the sun in our sky, so it can be very difficult to find and observe. It is best seen in the spring at sunset or in the fall at sunrise when it is at maximum elongation. This means it is as far from the sun as it can get in our sky. At best this is only 28 degrees, so you need to look for it before the sun gets too far below the horizon. Don't expect to see any detail at this low angle because of atmospheric distortion. With a three-inch or larger telescope, a #21 orange filter can be helpful in seeing the planet's phases.

Venus is the brightest of all the planets and shows phases just like our moon and Mercury. Because of its dazzling white color it is sometimes better to view it just after sunset or just before sunrise when the sky is still fairly light. A #80 blue filter can be used to enhance the appearance.

Mars, the red planet, is very small, only about half the size of the earth, and so it shows little detail in the telescope. The best time to view it is at opposition, when the planet is closet to earth and therefore largest in the telescope. At this point some detail is visible. Use a #23A red filter to enhance contrast between the lighter plains and the darker areas. The frozen polar ice caps stand out with a #58 green or #80A blue filter. The disk of Mars is small, so to observe the features of interest a six-inch reflector, or a good three-inch refractor, is necessary. With above average seeing, try 150 power on a three-inch, 200 power with a six-inch, and 250 power with an eight-inch telescope.

Jupiter has plenty to keep the curious observer busy. Several dark cloud belts and lighter zones parallel the equator. The Great Red Spot, a hurricane larger than the earth, can be seen under ideal conditions. The four large moons of Jupiter, Callisto, Ganymede, Europa, and Io appear as tiny star-like points lined up on either side of Jupiter. You can readily follow their motion around the planet from night to night. Occasionally you can see the shadow of one of these moons cross the disk of Jupiter. (This would be a solar eclipse as seen from Jupiter!) Recommended filters are #80A blue and #58 green. With average seeing conditions, use a 120 to 150 power eyepiece with a three-inch, 150 to 200 power with a six-inch, and 200 to 250 power with an eight-inch telescope.

Saturn seems to be everyone's favorite planet because of its bright ring system, visible even in small telescopes. Features on the disk of Saturn are subtler than those on Jupiter. The darker polar regions, the equatorial band, and the pale equatorial zones are evident in telescopes of three-inches or more. The view is improved with a #12 yellow filter. If the atmosphere is calm, the dark Cassini's division in the rings is easily seen. Look for the shadow of Saturn on the rings, too. Even small telescopes will show Titan, Saturn's largest moon, but with a six-inch or eight-inch telescope, four or five more are visible. With average seeing use the same powers as listed for Jupiter above.

Uranus shines at 6th magnitude, just beyond the limit of visibility with the naked eye. Even at high magnifications, no features can be seen on its tiny, greenish disk.

Neptune is smaller and even dimmer at magnitude 8. High powers are necessary to see this bluish disk

Pluto has a star-like appearance at 14th magnitude. A ten-inch or larger telescope is usually needed to even glimpse this elusive planet. You'll need a star chart showing stars down to 13th and 14th magnitude to find this tiny point of light.