Leisure Time LX20 Apparent Field and Actual Field, Magnification and Field of View

MAGNIFICATION AND FIELD OF VIEW

1. Magnification

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bright, clearly resolved but smaller image will show far more detail than a dimmer, poorly resolved larger image.

The magnification (power) of the telescope depends on two 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 telescope is fixed at 2670mm; the focal length of the 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.

Example: The power obtained with the 8" LX200 with the SP 26mm eyepiece is:

The type of eyepiece (whether MA "Modified Achromatic," PL "Plossl," SP "Super Plossl," 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" LX200 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

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 powers. Generally speaking, the lowest usable power is approximately 4X per inch of telescope aperture, or about 28X in the case of the 7" 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, and the 12" LX200 from 120X to 240X. It should be noted, however, that the higher magnifications may not be used due to atmospheric distortion caused by heat, moisture, and paniculate matter suspended in the air. Accessories are available both to increase and decrease the operating eyepiece power of the telescope. See your Meade dealer and the latest Meade Catalog for information on accessories.

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 table below lists the most common eyepieces available and the "Apparent Field" for each. 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/10	10" f/10	12" f/10
Eyepiece/Apparent Field	Power/Actual Field		Power/Actual Field Power/Actual Field		Power/Actual Field	Power/Actual Field	Power/Actual Field
Super Plossl Eyepieces (5-elements			; 1.25" 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°
9.7mm/52°	275/0.19°		132/0.39°	165/0.32°	206/0.25°	258/0.20°	314/0.17°
12.4mm/52°	215/0.24°		103/0.50°	129/0.40°	161/0.32°	202/0.26°	246/0.21°
15mm/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°
40mm/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; 1.25" 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 8mm/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°	65/103°	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 Eyepieces (8-elements; 1.25" 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°	399/0.21°		191/0.44°	239/0.35°	299/0.28°	373/0.23°	455/0.18°
8.8mm/84° (1.25" - 2 '	O.D.)	303/0.28°	145/0.58°	182/0.46°	227/0.37°	284/0.30°	346/0.24°
14mm/84° (1.25" - 2 "	O.D.)	199/0.44°	91/0.92°	114/0.73°	143/0.59°	1 79/0.47°	218/0.39°