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Telescopes · Binoculars · Microscopes


 
Meade LX200 Instruction Manual
7" Maksutov-Cassegrain, and 8", 10", and 12" Schmidt-Cassegrain Telescopes
 H. MAGNIFICATION AND FIELD OF VIEW
IMPORTANT NOTICE! Never use a telescope or spotting scope to look at the Sun! Observing the Sun, even for the shortest fraction of a second, will cause irreversible damage to your eye as well as physical damage to the telescope or spotting scope itself.
[ toc ] 1. Magnifications

The magnification, or power, of the telescope depends on two optional characteristics: the focal length of the main telescope and the focal length of the eyepiece used during a particular observation. For example, the focal length of the LX200 7" f/15 is fixed at 2670mm, teh focal length of the LX200 8" f/10 telescope is fixed at 2000mm; the focal length of the 10" f/10 telescope is fixed at 2500mm; and the focal length of the 12" f/10 telescope is fixed at 3048mm. To calculate the power in use with a particular eyepiece, divide the focal length of the eyepiece into the focal length of the main telescope. For example, using the SP 26mm eyepiece supplied with the 8" f/10, the power is calculated as follows:

Power = 2000mm/26mm = 77X

The type of eyepiece (whether "MA" Modified Achromatic, "OR" Orthoscopic, "SP" Super Plössl, etc.) has no bearing on magnifying power but does affect such optical characteristics as field of view, flatness of field and color correction.

The maximum practical magnification is determined by the nature of the object being observed and, most importantly, by the prevailing atmospheric conditions. Under very steady atmospheric "seeing," the 7" LX200 may be used at powers up to about 450X on astronomical objects, the 8" LX200 may be used at powers up to about 500X, the 10" LX200 up to about 600X and the 12" up to about 750X. Generally, however, lower powers of perhaps 250X to 350X will be the maximum permissible, consistent with high image resolution. When unsteady air conditions prevail (as witnessed by rapid "twinkling" of the stars), extremely high-power eyepieces result in "empty magnification," where the object detail observed is actually diminished by the excessive power.

When beginning observations on a particular object, start with a low power eyepiece; get the object well-centered in the field of view and sharply focused. Then try the next step up in magnification. If the image starts to become fuzzy as you work into higher magnifications, then back down to a lower power—the atmospheric steadiness is not sufficient to support high powers at the time you are observing. Keep in mind that a bright, clearly resolved but smaller image will show far more detail than a dimmer, poorly resolved larger image.

Because of certain characteristics of the human eye (in particular, eye pupil diameter) and because of optical considerations inherent in the design of a telescope, there exists minimum practical power levels also. Generally speaking, the lowest usable power is approximately 4X per inch of telescope aperture, or about 28X in the case of the 7" telescope, 32X in the case of the 8" telescope, about 40X in the case of the 10" telescope and about 48X in the case of the 12" telescope. During the daytime, when human eye pupil diameter is reduced, the minimum practical power with the 8" LX200 is increased to about 60X, to about 75X with the 10" LX200 and to about 90X with the 12" LX200; powers lower than this level should be avoided during daytime observations. A reasonable magnification range for daytime terrestrial observations through the 7" LX200 is from about 70X to 180X; 8" LX200 is from about 80X to 190X; through the 10" LX200 from about 100X to 200X, the 12" LX200 from 120X to 240X. It should be noted, however, that the higher magnifications may not be used due to severe air turbulence near the ground.

Accessories are available both to increase and decrease the operating eyepiece power of the telescope. See your Meade dealer and the latest Meade Telescope Systems and Accessories Catalog for information on accessories.

[ toc ] 2. Apparent Field and Actual Field

Two terms that are often confused and misunderstood are "Apparent Field" and "Actual Field". "Apparent Field" is a function of the eyepiece design and is built into the eyepiece. While not totally accurate (but a very good approximation), "Apparent Field" is usually thought of as the angle your eye sees when looking through an eyepiece. "Actual Field" is the amount of the sky that you actually see and is a function of the eyepiece being used and the telescope.

The "Actual Field" of a telescope with a given eyepiece is calculated by dividing the "Apparent Field" of the eyepiece by the power obtained using that eyepiece.

The following table lists the most common optional eyepieces available and the "Apparent Field" for each eyepiece. The power and "Actual Field" of view that each eyepiece yields is listed for each basic telescope optical design.

  7" f/15 8" f/6.3 10" f/6.3 8"f/l 0 10" f/10 12" f/10
Eyepiece/Apparent Field Power/ Power/ Power/ Power/ Power/ Power/
  Actual Fiield Actual Field Actual Field Actual Field Actual Field Actual Field
Super Plössl Eyepieces (5-elements; 1-1/4" O.D., except as noted)
6.4mm/52° 417/0.12° 200/0.26° 250/0.21 ° 313/0.17° 391/0.13° 476/0.11°
9.7mn/52° 275/0.19° 132/0.39° 165/0.32° 206/0.25° 258/0.20° 314/0.17°
12.4mn/52° 215/0.24° 103/0.50° 129/0.40° 161/0.32° 202/0.26° 246/0.21°
15mn/52° 178/0.29° 85/0.61° 107/0.49° 133/0.39° 167/0.31° 203/0.26°
20mm/52° 134/0.39° 64/0.81 ° 80/0.65° 100/0.52° 125/0.42° 152/0.34°
26mm/52° 103/0.50° 49/1.06° 62/0.84° 77/0.68° 96/0.54° 117/0.44°
32mm/52° 83/0.63° 40/1.30° 50/1.04° 63/0.83° 78/0.67° 95/0.55°
40mn/44° 67/0.66° 32/1.69° 40/1.35° 50/0.88° 63/0.70° 76/0.53°
56mm/52° (2"O.D.) 48/1.08° 23/2.27° 29/1.82° 36/1.46° 45/1.16° 54/1.04°
Super Wide Angle Eyepieces (6-elements; 1-1/4" O.D., except as noted)
13.8mm/67° 193/0.35° 93/0.72° 116/0.58° 145/0.46° 181/0.37° 221/0.30°
1 Smm/67° 148/0.45° 71/0.94° 89/0.75° 111/0.60° 139/0.48° 169/0.40°
24.5mm/67° 109/0.61° 52/1.28° 65/1.03° 82/0.82° 102/0.66° 124/0.54°
32mm/67° (2"O.D.) 83/0.81° 40/1.67° 50/1.34° 63/1.07° 78/0.86° 95/0.71°
40mm/67° (2"O.D.) 67/1.00° 32/2.09° 40/1.67° 50/1.34° 63/1.07° 76/0.88°
Ultra Wide Angle Eyepleces (8-elements; 1-1/4" O.D., except as noted)
4.7mm/84° 568/0.15° 272/0.31° 340/0.25° 426/0.20° 532/0.16° 649/0.13°
6.7mm/84° 399/.021° 191/0.44° 239/0.35° 299/0.28° 373/0.23° 455/0.18°
8.8mm/84° (1-114"-2" O.D.) 303/0.28° 145/0.58° 182/0.46° 227/0.37° 284/0.30° 346/0.24°
14mm/84° (1-1/4"-2"0.D.) 191/0.44° 91/0.92° 114/0.73° 143/0.59° 179/0.47° 218/0.39°

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