AstroMaster 90 EQ, 3.5" Equatorial refractor

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This Celestron AstroMaster is a 3.5” (90mm) aperture equatorial refractor. It is surprisingly economical, considering the performance and features you get versus the price you pay.

Many beginners start out with an inexpensive 60mm (2.4”) toy-store refractor, although too often they find to their dismay that a scope that size is too small for serious observing outside the solar system. For not much more money than a 60mm scope, this 90mm Celestron telescope gives you 125% more light gathering for brighter deep space images (and a full 50% higher resolution than a 60mm for revealing planetary details!)

The construction of the optical system is first rate, as the AstroMaster has all-glass optical components, with high transmission optical multicoatings for enhanced image brightness and clarity. For the observer whose interests are the brighter solar system and deep space objects, the AstroMaster 90 equatorial has a lot to offer. Its 3.5” aperture has a light grasp 165 times that of the sharpest eye for nighttime and twilight use. Its sensibly large aperture and diffraction-free images make it surprisingly good for much deep space observing. Binary stars and globular star clusters are particularly well-resolved and vivid, with the contrasting colors of many binary systems showing nicely. The brighter nebulas and galaxies stand out against a darker sky background than is possible in a comparably-priced reflector with its light-scattering diagonal mirror.

The AstroMaster 90 can also resolve details thirteen times smaller than you can see clearly with your unaided eye. Combine that sharpness with its two eyepieces (a 20mm for 50x magnification and a 10mm for 100x), and you have the ability to see many, many lunar and solar system details that are simply invisible in a lesser scope. Its views of subtle lunar and planetary details are sharp and high in contrast, bringing the planets to vivid life in the eyepiece.

Some chromatic aberration is present in the scope when viewing very bright objects at night, as it is in all achromatic refractor telescopes.

The Celestron AstroMaster 90 equatorial refractor is an optically good and mechanically solid astronomical telescope at a very reasonable price. It may well be the ideal telescope for you if you’re a beginning astronomer on a budget who wants more than a toy telescope.

This Telescope’s Optical System . . .

  • Refractor optical tube: 90mm (3.5”) aperture. 1000mm focal length f/11.1 two-element crown and flint glass achromatic doublet. No plastic lenses.

  • Multicoated optics: The two-element objective lens has multiple layers of vacuum-deposited antireflection materials on all air-to-glass surfaces for the highest light transmission and good contrast.

  • Dovetail mount: A quick-release dovetail bar on the optical tube fits into a dovetail slot on the mount. The optical tube can be installed and locked firmly in place in seconds using a single no-tool knob on the mount.

  • Dew shield: A dew shield (an extension of the optical tube that’s threaded onto the front of the objective lens) slows the formation of dew on the lens in cold weather. This extends your undisturbed observing time. It also improves the contrast when observing objects on the ground during the day.

  • Rack and pinion focuser: The well-made 1.25” focuser has dual focusing knobs for precise image control with either hand. The good-sized focus knobs are easy to operate, even while wearing gloves or mittens in cold weather.

  • Star diagonal:The 90° viewing angle 1.25” star diagonal (eyepiece holder) allows comfortable viewing when looking overhead at the sky. It provides erect mirror image views (objects are reversed left for right) with the supplied high power eyepiece. This is not a problem when observing the stars and planets, but may be disconcerting during terrestrial observing, as printing (license plates, the names on boats, etc.) will be backwards. For most nature studies this isn’t a problem, though, because a duck still looks like a duck whether he’s facing left or right. An optional #6329 45° viewing angle image-erecting diagonal is available if right-reading images are essential. It gives you correctly oriented images and more comfortable extended terrestrial observing.

  • Two eyepieces: You get a high power 1.25” 10mm (100x) eyepiece and a low power erect image 1.25” 20mm (50x) with a 1° field of view (twice the diameter of the full Moon). The 50x erect image eyepiece lets you use the 90EQ terrestrially, as its images are not reversed as they are with most refractors. However, the equatorial mount will make it difficult to center and track objects on the ground. The erect image eyepiece will probably serve you better for lunar observing, as it will show you a familiar image of the Moon, oriented as you see it with your unaided eyes or binoculars. Both eyepieces are of a higher quality optical design than you'll find in most other telescopes in this price range. They have antireflection coatings on their lens surfaces for sharp images and very good contrast. Instead of providing low quality eyepieces that give unrealistically high (and generally unusable) 200-300x magnifications as most economy telescope manufacturers do, Celestron has chosen to provide higher quality eyepieces with sensible powers you can use and enjoy every time you take your AstroMaster 90EQ out to observe.

  • Red dot finder: A straight-through red dot illuminated finder allows easy non-magnified views of the sky and ground, with a projected red dot of light showing exactly where the scope is pointed at all times. Stars appear through the finder in the same orientation that they appear on star charts. There's no confusing reversal and inversion of the image as you'll find in an ordinary straight-through finderscope.

This Telescope’s Mount . . .

  • Equatorial mount: The scope’s sturdy CG-3 equatorial mount is designed for astronomical observing. By aligning the mount on the north celestial pole, you only need to turn one slow motion control knob to follow planets and stars as they travel across the sky. An economical optional #MDASTRO battery-operated right ascension motor drive takes the slow motion control out of your hand and makes following the stars a no-hands operation. Two counterweights on the opposite side of the mount from the telescope tube balance the weight of the optical tube and make it easy to move the scope effortlessly from one part of the sky to another. No tools are required to adjust the position of the counterweights to quickly and precisely balance the optical tube. A micrometer control lets you adjust the altitude of the scope mount to match your latitude for fast alignment on the north celestial pole with no tools required.

  • Setting circles: Setting circles (graduated scales marked in either hours and minutes or degrees) are provided in both right ascension (the east/west position of objects in the sky measured in hours and minutes) and declination (the north/south position measured in degrees). These allow you to align the scope on the approximate position of an object in the sky by using its r. a. and dec coordinates from a star chart or atlas – before you search for it in the finderscope and eyepiece. Setting circles can reduce the time it takes for you to find the fainter and more difficult deep space objects.

  • Manual slow motion controls: There are two slow motion control knobs conveniently positioned on the mount so they are easy to reach while observing. One controls the scope’s motion in right ascension (the east/west direction in the sky). Turning this knob enables you to follow the motion of celestial objects as they travel from east to west across the sky. The second controls the scope’s motion in declination (the north/south direction in the sky). Turning this knob enables you to correct for any north/south drift a celestial object may take as it moves across the sky, due to an improper alignment of the scope on the north celestial pole when you first set it up. The two controls combine to give you complete access to any part of the sky. They give you the ability to star hop from a known object to any other object by using a star chart. They let you center objects in the field of view, then track them effortlessly with only an occasional quick turn of the r. a. knob. As mentioned above, an optional motor drive is available for hands-free tracking.

  • Tripod: The lightweight pre-assembled tripod has 1.25” diameter stainless steel legs to provide a rigid and stable observing platform. It easily adjusts in height with no tools needed. The no-tool lock knobs that adjust the leg height of the tripod are on the inside of the legs so they won’t snag on clothing in the dark, a thoughtful touch that’s sure to be appreciated. Spreader bars lock the legs firmly open when the tripod is set up. The tripod includes a convenient accessory tray that attaches to the spreader bars to hold your eyepieces and accessories close at hand and up out of the dew-soaked grass.

  • Software: Comes with TheSky Level 1 sky-charting CD-ROM that has a database of 10,000 stars and objects it can plot and display on your Windows-based computer screen. That’s enough solar system and deep space detail to keep you busy observing for years, yet not so much that you’re overwhelmed by charts showing much more detail than your scope can usefully reveal. Custom sky chart printing lets you print out eyepiece finder charts to use with your telescope to help you locate and identify the planets and many famous and faint deep space nebulas, galaxies, and star clusters by star-hopping from object to object using your scope’s manual slow motion controls. There are 75 full color images of well-known celestial objects to help you identify them through your scope.

  • Two year warranty: As an expression of Celestron’s confidence in the quality of their products, the AstroMaster 90 is protected by Celestron’s two-year limited warranty against flaws in materials and workmanship.

Highest Useful Magnification:
This is the highest visual power a telescope can achieve before the image becomes too dim for useful observing (generally at about 50x to 60x per inch of telescope aperture). However, this power is very often unreachable due to turbulence in our atmosphere that makes the image too blurry and unstable to see any detail.

On nights of less-than-perfect seeing, medium to low power planetary, binary star, and globular cluster observing (at 25x to 30x per inch of aperture or less) is usually more enjoyable than fruitlessly attempting to push a telescope's magnification to its theoretical limits. Very high powers are generally best reserved for planetary observations and binary star splitting.

Small aperture telescopes can usually use more power per inch of aperture on any given night than larger telescopes, as they look through a smaller column of air and see less of the turbulence in our atmosphere. While some observers use up to 100x per inch of refractor aperture on Mars and Jupiter, the actual number of minutes they spend observing at such powers is small in relation to the number of hours they spend waiting for the atmosphere to stabilize enough for them to use such very high powers.
167x
Visual Limiting Magnitude:
This is the magnitude (or brightness) of the faintest star that can be seen with a telescope. The larger the number, the fainter the star that can be seen. An approximate formula for determining the visual limiting magnitude of a telescope is 7.5 + 5 log aperture (in cm).

This is the formula that we use with all of the telescopes we carry, so that our published specs will be consistent from aperture to aperture, from manufacturer to manufacturer. Some telescope makers may use other unspecified methods to determine the limiting magnitude, so their published figures may differ from ours.

Keep in mind that this formula does not take into account light loss within the scope, seeing conditions, the observer’s age (visual performance decreases as we get older), the telescope’s age (the reflectivity of telescope mirrors decreases as they get older), etc. The limiting magnitudes specified by manufacturers for their telescopes assume very dark skies, trained observers, and excellent atmospheric transparency – and are therefore rarely obtainable under average observing conditions. The photographic limiting magnitude is always greater than the visual (typically by two magnitudes).

12.3
Focal Length:
This is the length of the effective optical path of a telescopeor eyepiece (the distance from the main mirror or lens where the lightis gathered to the point where the prime focus image is formed). Focallength is typically expressed in millimeters.

The longer the focallength, the higher the magnification and the narrower the field of viewwith any given eyepiece. The shorter the focal length, the lower themagnification and the wider the field of view with the same eyepiece.

1000mm
Focal Ratio:
This is the ‘speed’ of a telescope’s optics, found by dividing the focal length by the aperture. The smaller the f/number, the lower the magnification, the wider the field, and the brighter the image with any given eyepiece or camera.

Fast f/4 to f/5 focal ratios are generally best for lower power wide field observing and deep space photography. Slow f/11 to f/15 focal ratios are usually better suited to higher power lunar, planetary, and binary star observing and high power photography. Medium f/6 to f/10 focal ratios work well with either.

An f/5 system can photograph a nebula or other faint extended deep space object in one-fourth the time of an f/10 system, but the image will be only one-half as large. Point sources, such as stars, are recorded based on the aperture, however, rather than the focal ratio – so that the larger the aperture, the fainter the star you can see or photograph, no matter what the focal ratio.

f/11.1
Resolution:
This is the ability of a telescope to separate closely-spaced binary stars into two distinct objects, measured in seconds of arc. One arc second equals 1/3600th of a degree and is about the width of a 25-cent coin at a distance of three miles! In essence, resolution is a measure of how much detail a telescope can reveal. The resolution values on our website are derived using the Dawes’ limit formula.

Dawes’ limit only applies to point sources of light (stars). Smaller separations can be resolved in extended objects, such as the planets. For example, Cassini’s Division in the rings of Saturn (0.5 arc seconds across), was discovered using a 2.5” telescope – which has a Dawes’ limit of 1.8 arc seconds!

The ability of a telescope to resolve to Dawes’ limit is usually much more affected by seeing conditions, by the difference in brightness between the binary star components, and by the observer’s visual acuity, than it is by the optical quality of the telescope.

1.29 arc seconds
Aperture:
This is the diameter of the light-gathering main mirror or objective lens of a telescope. In general, the larger the aperture, the better the resolution and the fainter the objects you can see.
3.5"
Weight:
The weight of this product.
23 lbs.
Heaviest Single Component:
The weight of the heaviest component in this package.
18 lbs.
Telescope Type:
The optical design of a telescope.  Telescope type is classified by three primary optical designs (refractor, reflector, or catadioptric), by sub-designs of these types, or by the task they perform.
Refractor
 
Based on Astronomy magazine’s telescope "report cards", scopes of this size and type generally perform as follows . . .
Terrestrial Observation:
Observing terrestrial objects (nature studies, birding, etc.) is usually possible only with refractor and catadioptric telescopes, and convenient only when the scope is on an altazimuth mount or photo tripod. Most reflectors cannot be used for terrestrial observing. Scopes with apertures under 5" to 6" are generally most useful for terrestrial observing due to atmospheric conditions (heat waves and mirage, dust, haze, etc.) that degrade the image quality in larger scopes. 
No
Lunar Observation:
Visual observation of the Moon is possible with any telescope. Larger aperture scopes will provide more detail than smaller scopes, thereby getting a higher score in this category, but may require an eyepiece filter to cut down the greater glare from the Moon's sunlit surface so small details can be seen more easily. Lunar observing is more rewarding when the Moon is waxing or waning as the changing sun angle casts constantly varying shadows to reveal craters and surface features by the hundreds.  
Great
Planetary Observation:
Very Good
Binary and Star Cluster Observation:
Very Good
Galaxy and Nebula Observation:
Fair
Photography:
Yes
Terrestrial Photography:
Photographing terrestrial objects (wildlife, scenery, etc.) is usually possible only with refractor and catadioptric telescopes, and convenient only when the scope is on an altazimuth mount or photo tripod. Most reflectors cannot be used for terrestrial photography. Scopes with focal ratios of f/10 and faster and apertures under 5" to 6" are generally the most useful for terrestrial photography due to atmospheric conditions (heat waves and mirage, dust, haze, etc.) that degrade the image quality in larger scopes.
No
Lunar Photography:
Photography of the Moon is possible with virtually any telescope, using a 35mm camera, DSLR, or CCD-based webcam (planetary imager). While an equatorial mount with a motor drive is not strictly essential, as the exposure times will be very short, such a mount would be helpful to improve image sharpness, particularly with webcam-type cameras that take a series of exposures over time and stack them together. Reflectors may require a Barlow lens to let the camera reach focus. 
Yes
Planetary Photography:
Yes
Star Cluster / Nebula / Galaxy Photography:
No
Warranty:
2 years
Reviews from Cloudy Nights (www.cloudynights.com)
These reviews have been written by astronomers just like you and posted on the Cloudy Nights astronomy forums . . .
Celestron Astromaster 90mm Review

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1. Russell on 9/9/2012, said: AstronomicsAstronomicsAstronomicsAstronomicsAstronomics
This 3.54" refractor is excellent. It is a quality, low-priced alternative to and nearly as large as the very popular 100mm scope's. It's only a tiny 10mm less ! But the price is quite a bit lower than those in the 4" (quality) class. A real winner. I've observed the Moon at over 250x up to 320x and the images were clear, sharp & contrasty. I'm happy ! Sometimes I think I'm viewing through my 5" !
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General Accessories
Drive Motors and Drive Accessories (1)
Single axis DC drive for Celestron AstroMaster and PowerSeeker telescopes
by Celestron
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$37.95 
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Eyepieces (2)
Omni 6mm 1.25" Plossl
by Celestron
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$29.95 
Omni 40mm 1.25" Plossl
by Celestron
Quantity:  
$49.95 
  • 90mm aperture achromatic refractor optical tube with multicoated optics, dovetail mounting bar, and 1.25" rack and pinion focuser
  • CG-3 equatorial mount with manual slow motion controls in altitude and azimuth and locks on both axes
  • 10mm (100X) and 20mm (50X) eyepieces
  • 90° 1.25” star diagonal
  • Red dot illuminated non-magnifying finder
  • Operating instructions
  • TheSky Level 1 CD-ROM software
  • No-tool adjustable height tripod with stainless steel legs and accessory tray.
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Celestron - AstroMaster 90 EQ, 3.5" Equatorial refractor

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Celestron - AstroMaster 90 EQ, 3.5" Equatorial refractorFull-length image of the scope on its tripod.Close-up of focuser, red dot finder, star diagonal, and eyepiece.Close-up of mount showing counterweights, setting circles, manual slow motion controls, and dovetail mount.
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The Celestron AstroMaster 90 equatorial refractor is a big step up from a 60mm refractor “starter scope.” While it is surprisingly close to many economy 60mm scopes in price, it is far above them in features, quality, and optical performance . . .





. . . our 35th year