| | | 1.9 |
4.9 | | | |
2.4 | 1.7 | 3.4 | 2.4 |
|
1.9 | | | 2.5 |
| | |
Figure 7. Megrez connects the Big Dipper’s handle to it’s “pan”. It is a good guide to how conditions are. If you can not see Megrez (a 3.4 mag star) then conditions are poor.
In conditions of good seeing, star twinkling is minimal and images appear steady in the eyepiece. Seeing is best over- head, worst at the horizon. Also, seeing generally gets better after midnight, when much of the heat absorbed by the Earth during the day has radiated off into space.
Especially important for observing faint objects is good “trans- parency”— air free of moisture, smoke, and dust. All tend to scatter light, which reduces an object’s brightness. Transparency is judged by the magnitude of the faintest stars you can see with the unaided eye (6th magnitude or fainter is desirable).
One good way to tell if conditions are good is by how many stars you can see with your naked eye. If you cannot see stars of magnitude 3.5 or dimmer then conditions are poor. Magnitude is a measure of how bright a star is, the brighter a star is, the lower its magnitude will be. A good star to remem- ber for this is Megrez (mag. 3.4), which is the star in the “Big Dipper” connecting the handle to the “dipper”. If you cannot see Megrez, then you have fog, haze, clouds, smog, or other conditions that are hindering your viewing. (See Figure 7)
Cooling the Telescope
All optical instruments need time to reach “thermal equilibri- um.” The bigger the instrument and the larger the temperature change, the more time is needed. Allow at least 30 minutes for your telescope to cool to the temperature outdoors.
Let Your Eyes Dark-Adapt
Don’t expect to go from a lighted house into the darkness of the outdoors at night and immediately see faint nebulas, galaxies, and star clusters— or even very many stars, for that matter. Your eyes take about 30 minutes to reach perhaps 80% of their full dark-adapted sensitivity. As your eyes become dark-adapted, more stars will glimmer into view and you’ll be able to see fainter details in objects you view in your telescope.
To see what you’re doing in the darkness, use a red-filtered flashlight rather than a white light. Red light does not spoil your eyes’ dark adaptation like white light does. A flashlight with a red LED light is ideal, or you can cover the front of a
regular incandescent flashlight with red cellophane or paper. Beware, too, that nearby porch and streetlights and car head- lights will ruin your night vision.
Eyepiece Selection
By using eyepieces of varying focal lengths, it is possible to attain many magnifications with the SpaceProbe 3 EQ. The SpaceProbe 3 EQ comes with two Explorer II eyepieces, a 25mm and a 10mm. These give magnifications of 28x and 70x respectively. Other eyepieces can be used to achieve higher or lower powers. It is quite common for an observer to own five or more eyepieces to access a wide range of magnifications. This allows the observer to choose the best eyepiece to use depending on the object being viewed.
To calculate the magnification, or power, of a telescope and eyepiece combination, simply divide the focal length of the tel- escope by the focal length of the eyepiece:
Telescope Focal Length (mm)
Magnification =
Eyepiece Focal Length (mm)
For example, the SpaceProbe 3 EQ, which has a focal length of 700mm, used in combination with the 25mm eyepiece, yields a power of:
700mm ÷ 25mm = 28x
Every telescope has a useful limit of power of about 2x per
mmof aperture (about 152x for the SpaceProbe 3 EQ). Claims of higher power by some telescope manufacturers are a misleading advertising gimmick and should be dismissed. Keep in mind that at higher powers, an image will always be dimmer and less sharp (this is a fundamental law of optics). The steadiness of the air (the “seeing”) will also limit how much magnification an image can tolerate.
Whatever you choose to view, always start by inserting your lowest-power (longest focal length) eyepiece to locate and center the object. Low magnification yields a wide field of view, which shows a larger area of sky in the eyepiece. This makes acquiring and centering an object much easier. If you try to find and center objects with high power (narrow field of view), it’s like trying to find a needle in a haystack!
Once you’ve centered the object in the eyepiece, you can switch to higher magnification (shorter focal length eyepiece), if you wish. This is especially recommended for small and bright objects, like planets and double stars. The Moon also takes higher magnifications well.
The best rule of thumb with eyepiece selection is to start with a low power, wide-field eyepiece, and then work your way up in magnification. If the object looks better, try an even higher magnification. If the object looks worse, then back off the magnification a little by using a lower-power eyepiece.
What to Expect
So what will you see with your telescope? You should be able to see bands on Jupiter, the rings of Saturn, craters on the Moon, the waxing and waning of Venus, and many bright deep-sky objects. Do not expect to see any color as you do in NASA photos, since those are taken with long-exposure cam-
