Celestron AstroMaster 90EQ Telescope

What the AstroMaster 90EQ Is

The AstroMaster 90EQ pairs a 90mm (3.54") f/11 refractor with a manual CG-3 equatorial mount. A refractor uses a lens to focus the light, and with the included erect image diagonal it also works for daytime land viewing.

Put simply, you can discover the Solar System, ready to observe in minutes thanks to a quick and easy no-tool setup. The tripod and mount come preassembled, and the equatorial mount teaches you how the sky moves: once it is polar aligned, one knob follows an object as the Earth turns.

It is a manual telescope with no computer, so you find objects yourself with the finderscope, a star map or an app.

Refractor Optics

The AstroMaster 90EQ is a refractor with fully coated optics. The 90mm (3.54") aperture and 1000mm (39.37") focal length (f/11) gather 165x more light than the human eye. The limiting stellar magnitude is 12.3 and the resolution 1.55 arc seconds (Rayleigh) or 1.29 arc seconds (Dawes). Highest useful magnification is 213x, and the lowest is 13x.

The aluminum tube is 770mm (30.3") long and weighs 4.4 lbs (2 kg).

The CG-3 Equatorial Mount

The equatorial mount lets you tilt the telescope's axis of rotation so you can follow objects as they move across the sky. It moves in two axes based on the celestial coordinate system: right ascension (RA) moves the telescope east and west, declination (DEC) north and south. Two slow motion control knobs give fine pointing adjustments in both axes.

For large moves, loosen the RA and DEC clutch knobs counterclockwise, swing the telescope by hand close to your target and tighten them again. Then use the slow motion knobs to center the object; they only work with both clutches locked. The two knobs are identical, and the RA knob can go on either side of the mount.

Balancing the Telescope

Balance the telescope to spare the mount and gears undue stress. For RA, release the RA clutch, swing the tube to one side with the counterweight bar level, let go slowly and slide the counterweight toward or away from the tube until it stays put. For DEC, lock the RA clutch, hold the tube, release the DEC clutch with the tube level and slide the tube forward or back until it no longer tips.

Polar Alignment

Polar alignment points the RA axis parallel to the Earth's axis so you can follow objects by turning one knob. First level the tripod by eye, then set your latitude: loosen the latitude locking knob, turn the latitude adjustment screw at the back until the pointer reads your latitude, and lock it again. The manual notes this usually needs doing only once, unless you travel far enough north or south to change a degree of latitude, about 70 miles.

Then point the polar axis north. With the tube directly above the mount and pointing over the counterweight bar, clutches locked, loosen the knob that holds the mount to the tripod and turn the mount until the tube points north. A compass is quickest but magnetic north can be off by as much as 15 degrees; for better accuracy, sight Polaris, about 0.8 degrees from the celestial pole, found from the end stars of the Big Dipper's bowl. Tighten the knob again. In the Southern Hemisphere you point south instead, toward the star Sigma Octantis.

Finding Objects: Setting Circles and Star Hopping

The mount carries two setting circles, 3-inch graduated disks on the RA and DEC axes. Polar alignment sets the DEC circle; the RA circle must be set on a bright star of known coordinates each time you use it, since it does not turn with the telescope. The manual is candid that, with the accuracy of this mount, your target will usually not be in the eyepiece at first, so you finish with a slow search using the slow motion knobs, and it recommends starting with star hopping.

For finding your way, the manual recommends a red flashlight, a planisphere (a rotating star map set to your date and time), a star atlas, or an astronomy app or computer program. Its star hopping method uses the StarPointer's field as a ruler on the atlas to step from bright stars to fainter targets.

Tripod, Assembly and Weights

The tripod has 1.25" steel legs and an accessory tray on its center brace. The mount and tripod weigh 9.3 lbs (4.24 kg) together, and the complete kit 13.7 lbs (6.21 kg).

The manual suggests a first assembly indoors. Extend the legs with the knobs at their base, stand the tripod up and spread the legs until the center brace snaps into place, then twist the accessory tray onto the brace until its tabs lock. Set the mount into the tripod head and secure it with the large knob underneath, without overtightening, and thread in the latitude adjustment screw a short way. Screw the counterweight bar into the bottom of the declination axis, remove the orange safety screw, slide a counterweight about half way up, lock it and refit the safety screw, then slide the slow motion knobs onto the RA and DEC gear shafts and tighten their set screws.

The tube attaches with a quick release dovetail fixed directly to the tube: back out the two mounting knobs on the mount's dovetail bracket, slide the dovetail fully in and tighten both knobs.

Eyepieces and Finderscope

Two 1.25" eyepieces are included: the 20mm gives 50x for finding objects and wider views, and the 10mm gives 100x for a closer look. Magnification is the telescope's focal length divided by the eyepiece's focal length. The manual suggests finding and centering an object with the 20mm eyepiece first, then changing to the 10mm to zoom in, and notes that more eyepieces can be bought to change the magnification.

The erect image diagonal goes into the back of the focuser and uses a small prism to turn the light at a right angle, for a more comfortable viewing position. It gives a 100% correctly oriented image, making the telescope suited to daytime terrestrial observing. You can turn the diagonal to any angle before tightening its set screws.

Celestron's own Telescope Basics manual adds that what looks focused to you may not be for someone else, so remind others to refocus when you share a view.

Align the StarPointer red dot finderscope once, in daylight: pick an easily recognizable object such as a streetlight, a car license plate or a sign, as far away as possible and at least a quarter mile off. Center it in the 20mm eyepiece, turn the finder's power dial all the way up, and without moving the telescope use its two adjustment knobs until the red dot sits on the same object. It should not need realigning unless it is bumped, dropped or removed. Before first use, pull out the small plastic tab under the battery cover, and turn the red dot off when you are not using it to save the battery.

What You Can Observe

It provides bright, clear images of the Moon, planets, star clusters and more for great nighttime viewing. The telescope can show the five brightest planets; since they move against the background stars, a web source or a planetarium app helps you locate them.

Try the Moon for your first night: the best time is from two days after New Moon to a few days before Full Moon, when the craters and lunar mountain ranges show the most detail. Put the red dot on the Moon, focus in the 20mm eyepiece, then swap to the 10mm for a much larger view; you may need to refocus when you change eyepieces.

The manual is frank about deep-sky objects: nebulae and galaxies can be large but are very faint, and the vivid reds and blues in photographs come from long exposures; they are not visible in the eyepiece, because the eye is most sensitive to green. Darker skies, away from city lights, make the biggest difference.

Getting the Best Views

Sky conditions matter as much as the telescope. When stars twinkle or flash colours the air is unsteady, so use low power and look for brighter objects; when they shine steadily, the Moon and planets take higher magnification well. Humidity, smoke or haze dim faint objects, and city sky glow can hide them, while the Moon and planets are bright enough to be little affected.

For the best views, the manual suggests setting up on dirt or grass rather than a deck or concrete, which pass vibrations to the telescope; staying out of the direct path of streetlights and house lights; avoiding objects within 5 to 10 degrees over a roof, which radiates the day's heat; and never observing through a window, open or closed. Objects look best near the meridian, where they are highest, and the air is often steadier later in the night than just after sunset.

The manual says a telescope needs at least 10 minutes to cool to the outside air temperature, longer if the difference is large, to minimize heat wave distortion inside the tube. Your eyes take about 30 minutes to adapt fully to the dark, so use a red LED flashlight on its lowest setting and avoid white lights. Observe with both eyes open to avoid fatigue, and for a faint object look slightly to the side of it rather than straight at it.

Photographing the Moon and Planets

The manual says the telescope can image the Moon and planets, while deep-sky objects need a more advanced telescope and a mount with finely geared motorized tracking. The simplest method is holding a phone or a point-and-shoot camera up to the eyepiece, which is easier with the low-power eyepiece and a steady hand. Celestron also offers T-adapters and T-rings to use a DSLR body in place of the eyepiece, and planetary cameras that record video on a laptop and stack the frames into a sharper image; these are sold separately.

Care

To clean the optics, remove large particles with a camel hair brush, blow away loose debris with a squeeze bulb or a few seconds of canned air held upright and at an angle, then use a 50/50 mix of isopropyl alcohol and distilled water on white unscented tissue, with low-pressure strokes from the center outward like the spokes of a wheel, never in circles. Dew can be cleared with a hair dryer on low or by pointing the telescope at the ground until it evaporates. Replace the covers after every session, since the tube is not sealed; internal cleaning and adjustments should be done only by Celestron's repair department.

Software

Starry Night Basic Edition software and the SkyPortal app come with the AstroMaster. Starry Night is a way to learn about the night sky and plan observing sessions, and you don't need it to use the telescope.