Magnification And Field Of View; Magnification; Apparent Field And Actual Field - Meade LX200 Instruction Manual

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:
2000mm
Power = ________ = 77X
26mm
The type of eyepiece (whether MA "Modified Achromatic," PL
"Plössl," 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" 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
Eyepiece/Apparent Field
Super Plössl Eyepieces (5-elements; 1.25" O.D., except as noted)
6.4mm/52°
9.7mm/52°
12.4mm/52°
15mm/52°
20mm/52°
26mm/52°
32mm/52°
40mm/44°
56mm/52° (2" O.D.)
Super Wide Angle Eyepieces (6-elements; 1.25" O.D., except as noted)
13.8mm/67°
18mm/67°
24.5mm/67°
32mm/67° (2" O.D.)
40mm/67° (2" O.D.)
Ultra Wide Angle Eyepieces (8-elements; 1.25" O.D., except as noted)
4.7mm/84°
6.7mm/84°
8.8mm/84° (1.25" - 2" O.D.)
14mm/84° (1.25" - 2" O.D.)
- 24 -
7" f/15 8" f/6.3
Power/Actual Field Power/Actual Field
417/0.12° 200/0.26°
275/0.19° 132/0.39°
215/0.24° 103/0.50°
178/0.29° 85/0.61°
134/0.39° 64/0.81°
103/0.50° 49/1.06°
83/0.63° 40/1.30°
67/0.66° 32/1.69°
48/1.08° 23/2.27°
193/0.35° 93/0.72°
148/0.45° 71/0.94°
109/0.61° 52/1.28°
83/0.81° 40/1.67°
67/1.00° 32/2.09°
568/0.15° 272/0.31°
399/0.21° 191/0.44°
303/0.28° 145/0.58°
199/0.44° 91/0.92°
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
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 particulate 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.
10" f/6.3 8" f/10
Power/Actual Field Power/Actual Field
250/0.21° 313/0.17°
165/0.32° 206/0.25°
129/0.40° 161/0.32°
107/0.49° 133/0.39°
80/0.65° 100/0.52°
62/0.84° 77/0.68°
50/1.04° 63/0.83°
40/1.35° 50/0.88°
29/1.82° 36/1.46°
116/0.58° 145/0.46°
89/0.75° 111/0.60°
65/103° 82/0.82°
50/1.34° 63/1.07°
40/1.67° 50/1.34°
340/0.25° 426/0.20°
239/0.35° 299/0.28°
182/0.46° 227/0.37°
114/0.73° 143/0.59°
observations. A reasonable
10" f/10 12" f/10
Power/Actual Field Power/Actual Field
391/0.13° 476/0.11°
258/0.20° 314/0.17°
202/0.26° 246/0.21°
167/0.31° 203/0.26°
125/0.42° 152/0.34°
96/0.54° 117/0.44°
78/0.67° 95/0.55°
63/0.70° 76/0.53°
45/1.16° 54/1.04°
181/0.37° 221/0.30°
139/0.48° 169/0.40°
102/0.66° 124/0.54°
78/0.86° 95/0.71°
63/1.07° 76/0.88°
532/0.16° 649/0.13°
373/0.23° 455/0.18°
284/0.30° 346/0.24°
179/0.47° 218/0.39°