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Meade ETX-90EC Astro Telescope Instruction Manual
 Chapter 4: OBSERVING
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.

Listed below are some of the many astronomical objects that can be seen with the ETX-90EC.

[ toc ] The Moon

The Moon is, on average, a distance of 239,000 miles (380,000km) from Earth and is best observed during its crescent or half phase, when sunlight strikes its surface at an angle, casting shadows and adding a sense of depth to the view (Fig. 17). No shadows are seen during a full Moon, causing the overly bright Moon to appear flat and rather uninteresting through the telescope. Using the ETX-90EC, brilliant detail can be observed on the Moon, including hundreds of lunar craters and maria, described below.

Fig. 17: Photo of the Moon shows rich detail afforded by shadows.
Craters are round meteor impact sites covering most of the Moon's surface. With no atmosphere on the Moon, no weather conditions exist, so the only erosive force is meteor strikes. Under these conditions, lunar craters can last for millions of years.

Maria (plural for mare) are smooth, dark areas scattered across the lunar surface. These dark areas are large ancient impact basins that were filled with lava from the interior of the Moon by the depth and force of a meteor or comet impact.

12 Apollo astronauts left their bootprints on the Moon in the late 1960's and early 1970's. However, no telescope on Earth is able to see these footprints or any other artifacts. In fact, the smallest lunar features that may be seen with the largest telescope on Earth are about one-half mile across.

NOTE: Except during its early or late crescent phases, the Moon can be an exceptionally bright object to view through the telescope. To reduce the brightness and glare, use the #905 Variable Polarizing Filter (see OPTIONAL ACCESSORIES).

[ toc ] Planets

Planets change positions in the sky as they orbit around the Sun. To locate the planets on a given day or month, consult a monthly astronomy magazine, such as Sky and Telescope or Astronomy. Listed below are the best planets for viewing through the ETX-90EC.

Venus is about nine-tenths the diameter of Earth. As Venus orbits the Sun, observers can see it go through phases (crescent, half, and full) much like those of the Moon. The disk of Venus appears white as sunlight is reflected off the thick cloud cover that completely obscures any surface detail.

Mars is about half the diameter of Earth, and appears through the telescope as a tiny reddish-orange disk. It may be possible to see a hint of white at one of the planet's polar ice caps. Approximately every two years, when Mars is closest to Earth in its orbit, additional detail and coloring on the planet's surface may be visible.

Jupiter is the largest planet in our solar system and is 11 times the diameter of Earth. The planet appears as a disk with dark lines stretching across the surface. These lines are cloud bands in the atmosphere. Four of Jupiter's 16 moons (Io, Europa, Ganymede, and Callisto) can be seen as "star-like" points of light when using even the lowest magnification. These moons orbit Jupiter so that the number of moons visible on any given night changes as they circle around the giant planet.

Saturn is nine times the diameter of Earth and appears as a small, round disk with rings extending out from either side. In 1610, Galileo, the first person to observe Saturn through a telescope, did not understand that what he was seeing were rings. Instead, he believed that Saturn had "ears." Saturn's rings are composed of billions of ice particles ranging in size from a speck of dust to the size of a house. The major division in Saturn's rings, called the Cassini Division, is generally visible through the ETX-90EC. Titan, the largest of Saturn's 18 moons can also be seen as a bright, star-like object near the planet.

[ toc ] Deep-Sky Objects

Star charts can be used to locate constellations, individual stars and deep-sky objects. Examples of various deep-sky objects are given below:

Stars are large gaseous objects that are self-illuminated by nuclear fusion in their core. Because of their vast distances from our solar system, all stars appear as pinpoints of light, irrespective of the size of the telescope used.

Nebulae are vast interstellar clouds of gas and dust where stars are formed. Most impressive of these is the Great Nebula in Orion (M42), a diffuse nebula that appears as a faint wispy gray cloud. M42 is 1600 light years from Earth.

Fig. 18: The Pleiades Star Cluster (M45) in the constellation Taurus.
Open Clusters are loose groupings of young stars, all recently formed from the same diffuse nebula. The Pleiades (Fig. 18) is an open cluster 410 light years away. Through the ETX-90EC several hundred stars are visible.

Constellations are large, imaginary patterns of stars believed by ancient civilizations to be the celestial equivalent of objects, animals, people, or gods. These patterns are too large to be seen through a telescope. To learn the constellations, start with an easy grouping of stars, such as the Big Dipper in Ursa Major. Then, use a star chart to explore across the sky.

Galaxies are large assemblies of stars, nebulae, and star clusters that are bound by gravity. The most common shape is spiral (such as our own Milky Way), but galaxies can also be elliptical, or even irregular blobs. The Andromeda Galaxy (M31) is the closest spiral-type galaxy to our own. This galaxy appears fuzzy and cigar-shaped. It is 2.2 million light years away in the constellation Andromeda, located between the large "W" of Cassiopeia and the great square of Pegasus. Under clear, dark conditions, M31 can be seen with the naked eye and is a fascinating object through the ETX-90EC.

[ toc ]


Photography through the ETX-90EC requires the addition of the optional #64 T-Adapter (see OPTIONAL ACCESSORIES). With the #64 T-Adapter attached to the telescope (Fig. 19), through-the-telescope photography is possible with any 35mm camera body with a removable lens. In this way the telescope effectively becomes the lens of the camera.

For through-the-telescope photography, turn the flip-mirror control, (5, Fig. 19) to the "down" position, allowing light to pass straight through the telescope and out the photo port (17, Fig. 1). With the flip-mirror control in the "down" position and the photo port's dust cover removed, the front lens of the telescope can be seen when looking through the photo port. The #64 T-Adapter threads on to the photo port, followed by a T-Mount for the particular brand of 35mm camera being used, followed in turn by the camera body itself (with camera lens removed).

Fig. 19: Using the #64 T-Adapter. (1) Short Section of #64 T-Adapter; (2) Extension Section of #64 T-Adapter; (3) Knurled Attachment Ring; (4) T-Mount; (5) Flip-Mirror Control in "down" position; (6) 35mm Camera Body; (7) Inset shows Format 1.

Note that the #64 T-Adapter consists of two sections (1 and 2, Fig. 19) which are threaded together in shipment. Either of the following photographic mounting formats may be used to couple the camera body to the telescope's photo port thread.

  • Format 1 (1250mm focal length): Camera Body + T-Mount + Section (1) of the #64 T-Adapter (7, Fig. 19). Format 1 utilizes the short section only of the #64 T-Adapter to permit close-coupling of a camera body to the telescope at an effective photographic speed of f/14 and a transmission value (the so-called "T"-value) of 18. In this format vignetting will occur: the photographic image will appear on film with a slight darkening at the corners of the 35mm frame (see Fig. 20).
  • Format 2 (1450mm focal length): Camera Body + T-Mount + Sections (1) and (2) of the #64 T-Adapter (see Fig. 19). Format 2 utilizes both sections of the #64 T-Adapter threaded together to form a rigid unit as shown in Fig. 19, the telescope is operating at a photographic speed of f/16 and T-value of 23, but without any field vignetting: images are illuminated to the edges of a standard 35mm frame (see Fig. 21).
To frame an object in the viewfinder of the 35mm camera body, loosen slightly the knurled ring (3, Fig. 19) which threads the #64 T-Adapter to the telescope's photo port; rotate the camera body to achieve proper framing of the object; then re-tighten the knurled ring.

Photography through a long lens such as the ETX-90EC requires special technique for good results, and the photographer should probably expect to waste a roll or two of film in acquiring this technique. Long-lens photography has its own rewards, however; rewards that short-focus lenses can not duplicate.

A few tips on photography through the ETX-90EC:

  1. Use a rigid tabletop or the optional #883 Deluxe Field Tripod as a platform for the telescope. At effective focal lengths of 1250mm (Format 1) and 1450mm (Format 2), even small external vibrations can easily ruin an otherwise good photo.

    CAUTION: With the #64 T-Adapter and a camera body mounted to the ETX-90EC photo port, the telescope can only be rotated vertically 45°. Moving past this point may damage the telescope and camera.

  2. Use a cable-operated shutter release. Touching the camera body to initiate shutter operation will almost certainly introduce undesirable vibrations.
  3. Focus the image with extreme care. While observing the subject through the camera's reflex viewfinder, turn the ETX-90EC's focus knob (9, Fig. 1) to achieve the sharpest possible focus. Note that some 35mm cameras may have an optional focusing screen (available from the manufacturer) for use with a long telephoto lens. This screen provides a brighter and clearer image to focus and is highly recommended.
  4. Correct shutter speeds vary widely, depending on lighting conditions and film used. Trial-and-error is the best way to determine proper shutter speed in any given application.

    NOTE: The camera used with the ETX-90EC may have a built-in exposure meter that is still active when the standard lens is removed and the body is connected to the telescope with the T-Mount. If used for terrestrial photography, the camera meter should be acceptable. If used for astrophotography, the meter probably will not provide good results since camera meters are not made to compensate for a dark sky.

  5. Terrestrial photography through the ETX-90EC is sensitive to heat waves rising from the earth's surface. Long distance photography is best accomplished in the early morning hours before the earth has had time to build up heat.
  6. Photography of the Moon and planets through the ETX-90EC can be especially gratifying, but points 1 through 4 should be particularly noted in this case. Lunar or planetary photography requires that the telescope be polar aligned (see  Polar Alignment Procedure), and that the telescope's motor drive be in operation (see Electronic Controller Modes).

    NOTE: Long exposure photography of deep-sky objects is not practical with the ETX-90EC, since this type of photography requires special electronic and optical guiding devices not available for this telescope.

Fig. 20: Example of Format 1 Photo (1250mm).

Fig. 21: Example of Format 2 Photo (1450mm).

Chapter 5: Optional Accessories

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