Leisure Time LX20 - Manual (Page 24)

MAGNIFICATION AND FIELD OF VIEW
1. Magnification
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
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 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
Eyepiece/Apparent Field Power/Actual Field 8"f/6.3 10"f/6.3
Power/Actual Field Power/Actual Field 8" f/10
Power/Actual Field 10" f/10
Power/Actual Field 12" f/10
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°
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