Celestron AstroMaster 90AZ Telescope

What the AstroMaster 90AZ Is

The AstroMaster 90AZ is a 90mm (3.54") f/11 refractor on a manual alt-azimuth mount. You'll be ready to observe in minutes thanks to the quick and easy no-tool setup, and the pan handle points the telescope the way you naturally would: up and down, left and right. With the erect image diagonal it also serves for daytime land viewing.

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 90AZ is a refractor with fully coated optics. The 90mm (3.54") aperture and 1000mm (39") 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. It attaches with a CG-5 dovetail bar.

The Alt-Azimuth Mount

The manual alt-azimuth mount moves up and down and left and right, the way you naturally point. The pan handle Alt-Az control with clutch gives smooth and accurate pointing: turn the whole panning handle counterclockwise to loosen the altitude clutch, move the telescope, and turn it clockwise to lock. The left and right axis spins freely and does not lock, but the azimuth tension knob on top of the mount sets how stiffly it turns, clockwise to tighten.

Tripod, Assembly and Weights

The tripod has 1.25" steel legs and an accessory tray on its center brace. The height adjusts from 32" to 51" (812.8mm to 1295.4mm) with the mount. The tripod with the mount weighs 7.9 lbs (3.6 kg), the tube 4.4 lbs (2 kg) and the complete kit 12.3 lbs.

The mount and tripod come fully assembled. 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. With the mounting platform level and the altitude clutch locked, slide the dovetail bar under the tube into the clamp on top of the mount and tighten the large hand knob and the smaller silver safety screw. The quick setup guide adds an angled lock-down screw next to the fastening screw for added security.

Eyepieces, Diagonal 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 star diagonal goes into the back of the focuser and uses a small prism to turn the light at a 90 degree angle, for a more comfortable viewing position. The manual says it gives a 100% correctly oriented image, so you can easily use the telescope for daytime terrestrial observing. You can turn it to any angle before tightening the set screws.

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.

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.

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.

Solar Warning: Never look directly at the Sun with the naked eye or with an optic (unless you have the proper solar filter). Permanent and irreversible eye damage may result. Never use your optic to project an image of the Sun onto any surface. Internal heat build-up can damage the optic and any accessories attached to it. Never leave your optic unsupervised. Make sure an adult who is familiar with the correct operating procedures is with your optic at all times, especially when children are present.