Telescope Basics
A telescope is an instrument that collects and focuses light. The nature of the optical design determines how the light is focused. Some telescopes, known as refractors, use lenses. Other telescopes, known as reflectors, use mirrors. The StarSeeker 70mm and 80mm telescopes are refrac- tor telescopes that use an objective lens to collect light. The StarSeeker 114mm and 130mm are reflecting telescopes with a primary and secondary mirror to gather and focus light.
Focusing
Once you have found an object in the telescope, turn the focusing knob until the image is sharp. To achieve a truly sharp focus, never look through glass windows or across objects that produce heat waves, such as asphalt parking lots.
For astronomical viewing, out of focus star images are very diffuse, making them difficult to see. If you turn the focus knob too quickly, you can go right through focus without seeing the image. To avoid this problem, your first astronomical target should be a bright object (like the Moon or a planet) so that the image is visible even when out of focus.
Image Orientation
The image orientation of any telescope changes depending on how the eyepiece is inserted into the telescope. When observing through the StarSeeker 70mm or 80mm using a diagonal, the image will be right side up, but reversed from left to right. When observing straight through, with the eye- piece inserted directly into the telescope, the image will be inverted.
When observing through the StarSeeker 114mm or 130mm,
areflecting telescope, the image will appear
Reversed from left to right, as | Inverted image as viewed |
viewed with a star diagonal | with the eyepiece directly in |
| telescope |
Calculating Magnification
You can change the power of your telescope just by changing the eyepiece (ocular). To determine the magnification of your telescope, simply divide the focal length of the telescope by the focal length of the eyepiece used. In equation format, the formula looks like this:
Focal Length of Telescope (mm)
Magnification =
Focal Length of Eyepiece (mm)
Let’s say, for example, you are using the 25mm eyepiece. To determine the magnification you simply divide the focal length of your telescope (for example, the StarSeeker 130mm has a focal length of 650mm) by the focal length of the eyepiece, 25mm. Dividing 650 by 25 yields a magnification of 26 power.
Although the power is variable, each instrument under aver- age skies has a limit to the highest useful magnification. The general rule is that 60 power can be used for every inch of aperture. For example, the StarSeeker 80mm is 3.2" (80mm) in diameter. Multiplying 3.2 by 60 gives a maximum useful magnification of 192 power. Although this is the maximum useful magnification, most observing is done in the range of 20 to 35 power for every inch of aperture which is 64 to 112 times for the StarSeeker 80mm telescope.
Determining Field of View
Determining the field of view is important if you want to get an idea of the angular size of the object you are observing. To calculate the actual field of view, divide the apparent field of the eyepiece (supplied by the eyepiece manufacturer) by the magnification. In equation format, the formula looks like this:
Apparent Field of Eyepiece
True Field =
Magnification
As you can see, before determining the field of view, you must calculate the magnification. Using the example in the previous section, we can determine the field of view using the same 25mm eyepiece. The 25mm eyepiece has an apparent field of view of 56°. Divide the 56° by the magnification, which is 26 power. This yields an actual field of view of 2.2°.
To convert degrees to feet at 1,000 yards, which is more use- ful for terrestrial observing, simply multiply by 52.5. Continuing with our example, multiply the angular field 2.2° by 52.5. This produces a linear field width of 115.5 feet at a distance of one thousand yards. The apparent field of each eyepiece that Celestron manufactures is found in the Celestron Accessory Catalog (#93685).
General Observing Hints
When working with any optical instrument, there are a few things to remember to ensure you get the best possible image:
•Never look through window glass. Glass found in house- hold windows is optically imperfect, and as a result, may vary in thickness from one part of a window to the next. This inconsistency can and will affect the ability to focus your telescope. In most cases you will not be able to achieve a truly sharp image, while in some cases, you may actually see a double image.
•Never look across or over objects that are producing heat waves. This includes asphalt parking lots on hot sum- mer days or building rooftops.
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