Meade ETX-125PE Instruction Manual download pdf (Page 10)

CHAPTER

TELESCOPE FUNDAMENTALS

Equatorial mounts are highly desirable in the operation of any

telescope used for extensive astronomical applications, because

celestial objects do not move in vertical or horizontal directions

but in a combination of these directions. By tilting one of the

telescope’s mechanical axes (see Fig. 11) to point at the

celestial pole (i.e., by pointing one axis of the telescope to the

North Star, Polaris), you may follow (track) astronomical objects.

You need to turn in only one axis of the telescope instead of

simultaneously turning in two axes, as required of the altazimuth

mount. An equatorial mount which has one of its axes (the polar

axis) pointing to the celestial pole is polar aligned. Both ETX

models can be polar aligned either by using the optional #883

Deluxe Field Tripod or the table tripod specific to your model of

ETX telescope (see

OPTIONAL ACCESSORIES

, page 17).

With the ETX polar aligned (see page 12) the telescope’s

internal motor drive may be activated (see Modes of Operation,

page 11) to enable fully automatic hands off tracking of celestial

objects. In this configuration the observer does not need to press

the arrow keys of the Electronic Controller in order to track

celestial objects. Notwithstanding this automatic tracking, the

arrow keys of the Electronic Controller are useful in this

configuration to enable the centering of objects within the

telescopic field or, for example, to rove the telescope over the

surface of the Moon or through a large star field.

Terrestrial Observing

Both ETX models make excellent, high-resolution terrestrial

(land) telescopes. When you set the telescope on its drive base,

as shown in Fig. 1, you may use it for an extremely wide range

of observations. However, terrestrial images are right-side-up,

but reversed left-for-right when viewed through the eyepiece.

Normally, such an image orientation is not bothersome, unless

you are trying to read a distant sign, for example. If the telescope

is to be used for extensive terrestrial observations, a fully

correctly oriented image is provided with the #932 45° Erecting

Prism (see

OPTIONAL ACCESSORIES

, page 17).

Viewing terrestrial objects requires looking along the Earth's

surface through heat waves. These heat waves often degrade

image quality. Low-power eyepieces, like the SP26mm eyepiece,

magnify these heat waves less than higher-power eyepieces.

Therefore, low-power eyepieces provide a steadier, higher-

quality image. If the image is fuzzy or ill-defined, reduce to a

lower power. Observing in early morning hours, before the

ground has built up internal heat, produces better viewing

conditions than during late-afternoon hours.

Astronomical Observing

Used as an astronomical instrument, either ETX model has

many optical and electromechanical capabilities. In astronomical

applications, the extremely high level of optical performance of

both ETX telescopes is readily visible. The range of observable

astronomical objects is, with minor qualification, limited only by

the observer’s motivation.

Sidereal Rate

As the Earth rotates beneath the night sky, the stars appear to

move from East to West. The speed at which the stars move is

called the sidereal rate.

If the telescope is polar aligned (enabled by mounting the

telescope to one of the optional tripods), the motor drive in each

ETX model is designed to rotate the telescope at the sidereal

rate so that it automatically tracks the stars. This tracking makes

it easy to locate objects and keep them centered in the

telescope’s eyepiece.

Slew Speeds

The Electronic Controller has four slew speeds that are directly

proportional to the sidereal rate. These speeds are signified by

the Electronic Controller indicator lights (2, Fig. 6) and have

been calculated to accomplish specific functions.

Light 1: The fastest slew speed moves the telescope quickly

from one point in the sky to another.

Light 2: The next fastest speed is best used for centering the

object in the viewfinder.

Light 3: The third speed is set to enable centering the object in

the field of a low-to-moderate power eyepiece, such as the

standard SP26mm.

Light 4: The slowest slew speed is for centering an object in the

field of view of a high-power eyepiece (e.g., 200X to 300X for the

ETX-90EC or 400X to 500X for the ETX-125EC).

The four available speeds are:

Light 1 = 1200 x sidereal (300 arc-min/sec or 5°/sec)

Light 2 = 180 x sidereal (45 arc-min/sec or 0.75°/sec)

Light 3 = 32 x sidereal (8 arc-min/sec or 0.13°/sec)

Light 4 = 8 x sidereal (2 arc-min/sec or 0.034°/sec)

The two slowest speeds (8x and 32x sidereal) should be used

for pushbutton tracking of astronomical objects while observing

through the eyepiece.

Never use a Meade ETX Astro Telescope to look at the

Sun! Looking at or near the Sun will cause instant and

irreversible damage to your eye. Eye damage is often

painless, so there is no warning to the observer that

damage has occurred until it is too late. Do not point

the telescope or its viewfinder at or near the Sun. Do

not look through the telescope or its viewfinder as it is

moving.Children should always have adult supervision

while observing.

Fig. 11: Equatorial mounting aligns the telescope with the

celestial sphere.

Fig. 10: Alt/Az mounting moves the telescope in vertical and

horizontal directions.

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Meade ETX-125PE Instruction Manual Page 10

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