Celestron 8i manual Focusing, Calculating Magnification, Determining Field of View

Page 25

Focusing

The NexStar's focusing mechanism controls the primary mirror which is mounted on a ring that slides back and forth on the primary baffle tube. The focusing knob, which moves the primary mirror, is on the rear cell of the telescope just below the star diagonal and eyepiece. Turn the focusing knob until the image is sharp. If the knob will not turn, it has reached the end of its travel on the focusing mechanism. Turn the knob in the opposite direction until the image is sharp. Once an image is in focus, turn the knob clockwise to focus on a closer object and counterclockwise for a more distant object. A single turn of the focusing knob moves the primary mirror only slightly. Therefore, it will take many turns (about 30) to go from close focus (approximately 25 feet) to infinity.

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. Critical focusing is best accomplished when the focusing knob is turned in such a manner that the mirror moves against the pull of gravity. In doing so, any mirror shift is minimized. For astronomical observing, both visually and photographically, this is done by turning the focus knob counterclockwise.

Figure 3-3

The emblem on the end of the focus knob shows the correct rotational direction for focusing the NexStar.

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 Plossl eyepiece. To determine the magnification you simply divide the focal length of your telescope (the NexStar 8i has a focal length of about 2000mm) by the focal length of the eyepiece, 25mm. Dividing 2000 by 25 yields a magnification of 80 power.

Although the power is variable, each instrument under average 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 NexStar is 8" in diameter. Multiplying 8 by 60 gives a maximum useful magnification of 480 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 160 to 280 times for the NexStar 8 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 Plossl eyepiece has an apparent field of view of 50°. Divide the 50° by the magnification, which is 80 power. This yields an actual field of .63°, or a little more than a half of a degree.

To convert degrees to feet at 1,000 yards, which is more useful for terrestrial observing, simply multiply by 52.5. Continuing with our example, multiply the angular field .63° by 52.5. This produces a linear field width of 33 feet at a

26

Page 24 Page 26
Image 25
Page 24 Page 26
Contents Page Hand Control Operation Alignment Procedures Care and Cleaning of the Optics Collimation Page NexStar 8i SE StarPointer Finderscope Eyepiece Diagonal Visual Back Appendix C Longitude Latitude Mounting the Telescope to the Tripod Assembling the NexStarHand Control Powering the NexStarStar Diagonal Eyepiece Star Pointer InstallationStar Pointer Finderscope Star Pointer Operation NexStar Computerized Hand Control Double Stars Hand Control OperationAuto Two-Star Align Alignment ProceduresTwo Star Alignment EQ North / EQ South Alignment Object CatalogNexStar Re-Alignment Selecting an ObjectFinding Planets Tour ModeSlewing to an Object Direction Buttons Setup ProceduresRate Button Page Menu Scope Setup FeaturesUtility Features Helpful Hint NexStar Ready Image Orientation Calculating Magnification FocusingDetermining Field of View General Observing Hints Celestial Coordinate System Motion of the Stars Wedge Align Polar Alignment with optional WedgeFinding the North Celestial Pole Observing the Planets Observing the MoonObserving the Sun Lunar Observing HintsSeeing Conditions Observing Deep Sky ObjectsTransparency Sky IlluminationPage Short Exposure Prime Focus Photography Full Eyepiece ProjectionPlanet ISO Moon Mercury Venus Mars Jupiter Saturn Long Exposure Prime Focus PhotographyPage CCD Imaging Terrestrial PhotographyMetering Reducing VibrationCollimation Care and Cleaning of the OpticsCollimated telescope Should appear Symmetrical with Page Adapter, Car Battery #18769 Page Page Appendix a Technical Specifications Page Appendix B Glossary of Terms Trillion km 5.87 X 1 trillion mi Right Ascension RA Appendix C Longitudes Latitudes Georgia Minnesota Rhode Island Canada Communication Protocol Appendix D RS-232 ConnectionAppendix E Maps of Time Zones Page Page Page Page Page Page Page Celestron TWO Year Warranty

8i specifications

The Celestron 8i is a highly regarded telescope that has made a significant impact in the world of amateur astronomy, renowned for its performance, advanced features, and user-friendly design. Part of Celestron's Advanced Series, the 8i model boasts an impressive 8-inch Schmidt-Cassegrain optical system, which combines the advantages of a large aperture with a compact design. This allows enthusiasts to explore celestial objects with greater clarity, providing bright, sharp images of planets, star clusters, and nebulae.

One of the standout features of the Celestron 8i is its Fast Star capability, which allows for rapid imaging of celestial objects. Coupled with its advanced StarBright XLT coatings, this system enhances light transmission and increases contrast, resulting in a more vivid and detailed observation experience. This combination makes it an ideal choice for both novice and experienced astronomers who seek high-quality visuals of deep-sky objects.

The telescope's computer-controlled GoTo mount simplifies the process of locating and tracking celestial objects. With the push of a button, users can easily access a database containing over 40,000 astronomical objects, allowing for an effortless stargazing experience. The Celestron 8i also features an intuitive hand controller that enables users to customize their observations and seamlessly navigate the night sky.

The Celestron 8i is designed for portability, despite its robust optical performance. Weighing in at around 45 pounds, it is relatively easy to transport to different observing locations. The telescope's sturdy construction ensures stability while providing ease of setup, facilitating a hassle-free stargazing experience.

In terms of accessories, the 8i comes equipped with a 25mm eyepiece, offering a moderate magnification that is perfect for a variety of targets. Additionally, users can expand their observing capabilities with optional eyepieces, filters, and focal reducers, catering to both visual and astrophotography pursuits.

Overall, the Celestron 8i stands out as a versatile and powerful telescope that meets the needs of amateur astronomers at various skill levels. With its advanced features, superior optics, and impressive usability, the 8i offers a unique and rewarding experience under the night sky. Whether for planetary observation or deep-sky exploration, the Celestron 8i remains a top choice for astronomy enthusiasts.
Top Page Image Contents