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Meade ETX-90EC Astro Telescope Instruction Manual
 Chapter 2: TELESCOPE FUNDAMENTALS
WARNING! Never use the Meade ETX-90EC 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.
[ toc ] INTRODUCTION

The ETX-90EC can be used right out of the box to start observing. However, becoming familiar with the fundamentals of telescopes makes the experience easier and more rewarding.

[ toc ] Choosing an Eyepiece

The function of a telescope's eyepiece is to magnify the image formed by the telescope's main optics. Each eyepiece has a focal length (expressed in millimeters, or "mm"). The smaller the focal length, the higher the magnification. Low power eyepieces offer a wide field of view, bright high-contrast images, and eye relief during long observing sessions. To find an object with a telescope it is always best to start with a low power eyepiece such as the SP 26mm supplied with the ETX-90EC. When the object is located and has been centered in the eyepiece, switch to a higher power eyepiece to enlarge the image as much as practical for prevailing seeing conditions.

Lower power eyepieces are recommended for terrestrial viewing. Haze, heat waves, and particulate matter in the air distort images when using higher powers.

For astronomical observing a selection of several different eyepieces is recommended. For general observing of the Moon and planets, low to medium powers are preferred. For deep-sky objects such as nebulae and galaxies, higher powers may be needed for the best view if conditions permit.

NOTE: Seeing conditions vary widely from night-to-night. Turbulence in the air, even on an apparently clear night, can distort images. If an image appears fuzzy and ill-defined, back off to a lower power eyepiece for a more well-resolved image (see Fig. 8).

[ toc ] Understanding Magnification

The magnification, or power, at which a telescope is operating is determined by two factors: the focal length of the telescope and the focal length of the eyepiece employed.

Telescope Focal Length is the distance that light travels inside the telescope before reaching a focus. In the mirror-lens design of the ETX-90EC, however, this focal length is, in effect, compressed by the telescope's secondary mirror, so that a long effective focal length is housed in the short ETX-90EC optical tube. The telescope's focal length is 1250mm, or about 49". This means that if the ETX-90EC were a classical refracting-type of telescope, its optical tube would be more than four feet long instead of the ETX-90EC's compact 11" tube length.

Eyepiece Focal Length is the distance light travels inside the eyepiece before reaching focus. Focal length is usually printed on the side of the eyepiece. The Meade ETX-90EC is supplied with one eyepiece as standard-equipment, a Super Plössl (SP) 26mm eyepiece. Thus, the focal length of the provided eyepiece is 26mm. "Super Plössl" refers to the optical design of the eyepiece, a design specifically intended for high-performance telescopes and one which yields a wide, comfortable field of view with extremely high image resolution.

Technical note to the advanced amateur astronomer: The SP 26mm eyepiece supplied with the Meade ETX-90EC is a special low-profile version of the standard Meade SP 26mm eyepiece which is about 1/4" (6mm) shorter than the standard eyepiece. This low-profile SP 26mm is designed to harmonize with the ultracompact scale of the ETX-90EC and utilizes the exact same optics as the standard SP 26mm eyepiece. The SP 26mm low-profile eyepiece is not parfocal with other eyepieces in the SP series (i.e., the eyepiece requires re-focusing when it is interchanged with other SP eyepieces).

Calculating Magnification: On a telescope, such as the ETX-90EC, different eyepiece focal lengths are used to achieve different magnifications, from low to high. The standard-equipment SP 26mm eyepiece yields 48X ("48-power"). Optional eyepieces and the #126 2x Barlow Lens are available for powers from 31X to over 300X (see OPTIONAL ACCESSORIES).

To calculate the magnification obtained with a given eyepiece, use this formula:

Power = Telescope Focal Length
Eyepiece Focal Length

Example: The power obtained with the ETX-90EC using the SP 26mm eyepiece is:

Power = 1250mm
26mm
= 48X


Fig. 8: Example of too much magnification (Galaxy M51).
Too Much Power: The most common mistake of the beginning observer is to "overpower" the telescope by using high magnifications which the telescope's aperture and typical atmospheric conditions can not reasonably support. Keep in mind that a smaller, but bright and well-resolved, image is far superior to one that is larger, but dim and poorly resolved (see Fig. 8). Powers above 300X should be employed with the ETX-90EC only under the steadiest atmospheric conditions.

Most observers should have three or four eyepieces and the #126 2x Barlow Lens to achieve the full range of reasonable magnifications possible with the ETX-90EC.

[ toc ] Telescope Mountings

The mechanical means which causes a telescope's optical tube to move in different directions is called the telescope mounting, or mount. Telescope mounts are of two basic types:


Fig. 9: Alt/Az mounting moves the telescope in vertical and horizontal directions.
Altazimuth (Alt//Az) mounts permit motion of the telescope tube in vertical (altitude) and horizontal (azimuth) directions. The ETX-90EC telescope as it is shown sitting on its drive base in Fig. 9 incorporates an altazimuth mount. For all terrestrial applications and for casual astronomical observing the telescope operates very well in the altazimuth configuration. The telescope may be placed on a rigid tabletop or the optional Meade #883 Deluxe Field Tripod may be employed to provide a secure, variable-height, altazimuth observing platform. To track objects, either terrestrial or astronomical, with the telescope in the altazimuth configuration, the observer pushes the arrow keys of the Electronic Controller).


Fig. 10: Equatorial mounting aligns the telescope with the Celestial Sphere.
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. 10) to point at the celestial pole (i.e., by pointing one axis of the telescope to the North Star, Polaris), astronomical objects may be followed, or tracked, by turning only one axis of the telescope, instead of the simultaneous motions of two axes required of the altazimuth mount. An equatorial mount which has one of its axes (the so-called polar axis) pointing to the celestial pole is said to be polar-aligned. The ETX-90EC can be polar-aligned either by adding the Meade #880 Table Tripod (as shown in Fig. 10) or the #883 Deluxe Field Tripod.

With the ETX-90EC polar-aligned (see Polar Alignment Procedure) the telescope's internal motor drive may be activated (see Modes of Operation) to enable fully automatic "hands off" tracking of celestial objects; in this configuration the observer does not need to push 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.

[ toc ] Terrestrial Observing

The ETX-90EC makes an excellent, high-resolution terrestrial (land), telescope. By setting the telescope on its drive base, as shown in Fig. 1, the telescope may be used for an extremely wide range of observations. Keep in mind, however, that 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 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).

Viewing terrestrial objects requires looking along the earth's surface through heat waves. These heat waves often cause degradation of image quality. Low power eyepieces, like the SP 26mm 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, where the heat waves do not have such an effect on image quality. Observing in early morning hours, before the ground has built up internal heat, produces better viewing conditions than during late-afternoon hours.

[ toc ] Astronomical Observing

Used as an astronomical instrument, the ETX-90EC allows full use of its many optical and electromechanical capabilities. It is in astronomical applications where the ETX-90EC's extremely high level of optical performance is readily visible. The range of observable astronomical objects is, with minor qualification, limited only by the observer's motivation.

Never use the ETX 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.

[ toc ] 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 to one of the optional tripods available for the ETX-90EC) the motor drive on the ETX-90EC 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.

[ toc ] 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. 5) 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 of 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 SP 26mm (48X).
Light 4: The slowest slew speed is for centering an object in the field of view of a high power (e.g., 200X) eyepiece.

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.

[ toc ] THE ELECTRONIC CONTROLLER

Briefly described, the Electronic Controller is the primary method used to move the ETX-90EC. Electronic Controller functions include: slewing the telescope (see Observing with the Electronic Controller); turning on the telescope motor drive to automatically track celestial objects (when the telescope is polar aligned); changing the hemisphere of operation, when required; and changing tracking speed (see APPENDIX A).

[ toc ] Modes of Operation

The Electronic Controller can be set to operate in either of two primary modes: the Alt/Az mode (used when the telescope is operated in the altazimuth configuration) and the polar mode (used when the telescope is polar-aligned.)

  • Alt/Az (altitude-azimuth, or vertical-horizontal) mode should be chosen for all terrestrial operations of the telescope. In the Alt/Az mode the arrow keys can be used to slew the telescope to terrestrial or astronomical objects and, once located, to follow these objects if they move. However, in this mode astronomical tracking is not automatic and requires continuous key pushes. As the telescope is removed from its original packing box, the Electronic Controller is factory pre-set to the Alt/Az mode.
  • Polar mode should be chosen in cases where the telescope is equipped with either the optional #880 Table Tripod or #883 Deluxe Field Tripod, permitting polar-alignment of the telescope for extensive astronomical observations. In this mode the arrow keys may be used to slew the telescope to objects, as in the Alt/Az mode above, but in addition the telescope's internal motor drive is turned on, enabling fully automatic tracking of celestial objects.

Two methods are available to change the Electronic Controller between the Alt/Az and polar modes:

  1. Physically remove one of the mode screws (see Using the Mode Screws).
  2. Use the MODE key on the Electronic Controller as described in Electronic Controller Modes.

[ toc ] Using the Mode Screws

The Electronic Controller has two screws on the bottom rear of the handbox that can be removed to change the "default" mode of the Electronic Controller to Alt/Az or polar and, if polar, which of the Earth's hemispheres the observer is located in.

NOTE: Mode screws A and B are the outer screws in the recess directly under the letters A and B on the rear of the Electronic Controller. Do not remove either of the inner two screws in the recess.


Fig. 11: Mode screws on rear of Electronic Controller. (1) Mode Screw A; (2) Mode Screw B.
Mode Screw A (1, Fig. 11): Remove Mode Screw A to automatically default the Electronic Controller to Northern Hemisphere Polar mode when power is applied (i.e., the motor drive is activated for operation in the Earth's northern hemisphere - the U.S.A., Europe, Japan, etc.).

Mode Screw B (2, Fig. 11): Remove Mode Screw B to automatically default the Electronic Controller to Southern Hemisphere Polar mode when power is applied (i.e., the motor drive is activated for operation in the Earth's southern hemisphere - Australia, South America, Africa, etc.).

NOTES: (1) Leaving both A and B screws in place (or removing both screws) keeps the telescope in its original Alt/Az mode. (2) Removing either A or B screw only affects the telescope's default mode; the observer may still make mode changes at will during telescope operation using the MODE key (see Electronic Controller Modes, below).

[ toc ] Electronic Controller Modes

The Electronic Controller is in the Alt/Az mode when activated, unless a mode screw has been removed. To use the Electronic Controller to change to northern or southern hemisphere polar mode, or to use Alt/Az mode with one of the mode screws removed, follow this procedure:

1. Complete the Polar Alignment Procedure.
2. Complete steps 2 through 8 in Observing with the Electronic Controller to initialize the Electronic Controller.
3. Press and hold the MODE key (5, Fig. 5) until Lights 1 and 2 are on steady and Lights 3 and 4 start blinking.
4. Press the SPEED key (3, Fig. 5) once. This changes the system to polar mode for the Earth's southern hemisphere with the motor drive set to operate at the sidereal rate.
5. Press the SPEED key a second time. This changes the system to polar mode for the Earth's northern hemisphere with the motor drive set to operate at the sidereal rate.
6. Press the SPEED key a third time and the system returns to the Alt/Az mode. In the Alt/Az mode the motor drive does not activate when exiting from the Mode function.
7. Use the SPEED key as described in the above steps to cycle between these three modes (i.e., Alt/Az, southern hemisphere polar, or northern hemisphere polar) until the desired mode is shown by the appropriate light configuration.
8. Press and hold the MODE key until only a single light is on. This exits the Mode function. If northern or southern hemisphere polar mode was chosen, the motor drive starts operating at the sidereal rate.
9. Use the four arrow keys (1, Fig. 5) to slew the telescope to the desired object. To change the slew speed, press the SPEED key.

NOTE: See APPENDIX A for advanced functions of the Electronic Controller.

Chapter 3: Polar Alignment

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