Meade ETX-90EC instruction manual Telescope Fundamentals

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	TELESCOPE FUNDAMENTALS

CHAPTER

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

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.

Fig. 11: Equatorial mounting aligns the telescope with the celestial sphere.

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.

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 dama ge 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.

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.