ETX90 Observer 3.5" go-to Maksutov, tripod, case, and Audiostar computer

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   The fully go-to computerized Meade ETX90 Observer is designed to be the ideal ultra-portable telescope. It combines Meade’s unsurpassed optics with industry-leading electronics to deliver a truly exceptional viewing experience. The computer-optimized Maksutov-Cassegrain optics of the Meade ETX90 Observer yield remarkable high power lunar, planetary, star cluster, and binary star images. 

   The ETX90 Observer’s standard equipment AudioStar computer hand control is amazingly simple to operate. It will even help you align the scope on the sky. You don't have to know Castor from Pollux, or Albireo from Zubeneschamali. All you have to do is tell the AudioStar where and when you are on Earth, level the tube, and show it where North is located using the supplied bubble level/compass. The AudioStar computer will do the rest.

The Meade ETX90 Observer’s Optical System . . .

Maksutov-Cassegrain optical tube: 90mm (3.5") aperture, 1250mm focal length, f/13.8 focal ratio. Removable optical tube has a Vixen-style dovetail with a 1/4"/20 thread mounting hole for using the optical tube on a standard photo tripod as a terrestrial spotting scope. 

Fully multicoated optics: Anti-reflection UHTC (Ultra High Transmission Coatings) multicoatings on both sides of the Maksutov corrector lens, plus high light transmission enhanced aluminum multicoatings on the mirrors, provide a 16% increase in light throughput compared to standard coatings. UHTC multicoatings effectively add the equivalent of more than a quarter of an inch of extra light-gathering aperture to the performance of a scope with standard coatings, but at no increase in size or weight.

Dust covers: Dust covers protect the meniscus corrector lens and seal the Cassegrain focus on the rear cell.

Finderscope: The supplied straight-through non-magnifying illuminated red dot finder seems to projects a red dot of light on the sky or terrestrial background for astronomical alignment purposes and to help aim the scope when the optical tube is used as a terrestrial spotting scope.

Star diagonal: Built-in first surface flip-mirror. For astronomical observing, the internal flip-mirror directs the image to the eyepiece holder at the top of the scope. Touch a lever, and the flip-mirror redirects the image through the Cassegrain focus at the rear of the scope to an optional #64 T-adapter and T-ring for lunar or terrestrial photography.

Two eyepieces: One is a 1.25" 26mm Super Plössl (48x). The eyepiece field of view is 1.04 degrees, more than twice the diameter of the full moon. The second eyepiece is a higher power 1.25" 9.7mm Super Plössl (129x) for close-up lunar, planetary, binary star, and globular star cluster views.

The Meade ETX90 Observer's Mechanics and Electronics . . .

Dual fork arm altazimuth mount: The removable optical tube is aluminum. The rear cell, fork arms, and drive base are durable metal-reinforced injection-molded plastic that helps keep the cost of the Meade ETX90 Observer so very reasonable.

   There are three rubber feet on the base of the scope. These allow the ETX90 Observer to be placed on a suitably sturdy tabletop or flat surface for casual computerized observing without having to set up the supplied full length field tripod.
   The mount includes built-in right ascension and declination DC drive motors for tracking objects under the control of the supplied AudioStar computer. The motors are powered for up to 20 hours by six AA batteries (not supplied) in a battery holder that fits into an internal battery compartment in the telescope base. A 9V~12V DC input jack will let you plug in an optional AC or DC adapter to power the scope from household AC or a rechargeable battery pack. 

   When used in the default altazimuth mode, the supplied AudioStar computer hand control moves the scope in any of four directions (right/left and up/down) at any of nine speeds. The speeds range from 2x the sidereal rate for guiding, up to 5 degrees per second for locating objects and slewing, plus the sidereal rate for tracking objects. No manual slow motion controls are provided – or needed – as pushbuttons on the AudioStar hand control will do all the powered slewing and centering you might want. When set up on the supplied tripod for equatorial (polar) operation using the tripod’s built-in equatorial wedge, the AudioStar computer converts the right/left, up/down altazimuth directions to motions in right ascension and declination to correspond with the coordinates of objects on the celestial sphere.

AudioStar computer hand control: The supplied #497 AudioStar computer automatically slews to, and tracks, more than 30,000 near and deep space objects (although not all will be visible due to the 11.7 limiting magnitude of the scope’s 3.5" aperture). The objects include all 110 of the Messier objects; the complete 5386-object IC catalog, the complete 7840-object NGC catalog, the complete 109 object Caldwell catalog of the best objects for small telescopes; 16,800 stars from the SAO catalog including double starts, variable stars, and other stars of special note; the planets; 26 of the brightest asteroids; 15 periodic comets; 50 earth-orbiting satellites; the centroids of all 88 constellations; plus user-defined objects and guided tours around the sky.

   The AudioStar can show you these objects the very first night you use your scope – even if you've never used a telescope before! And a speaker built into the computer hand control lets the unique Astronomer Inside software provide you with more than four hours of fascinating audio descriptions of the objects you are observing. Astronomer Inside puts the experience and astronomical knowledge of a professional astronomer in the palm of your hand. 
   The AudioStar includes a dual-axis drive corrector with PEC (Periodic Error Correction). This allows long-exposure guided DSLR or CCD astrophotography in a polar mode, using the equatorial wedge built into the supplied steel tripod. 

   The AudioStar computer includes hundreds of special event menus, guided tours, a glossary, utility functions, and telescope status options. It also allows fast alignment of the telescope in either an equatorial or altazimuth mode, using any of three alignment methods, including Meade’s proprietary Easy Align method. 
Adjustable height tripod and tripod carry bag: The supplied full-length field tripod provides a rigid mounting platform for virtually any observing application, astronomical or terrestrial, and can be used for either standing or seated observing. The tripod has tubular steel legs and is adjustable in height from 26" to 44" when used in the altazimuth mode. 

   The tripod has a built-in equatorial wedge with an integral latitude scale and no-tool latitude (altitude) adjustment for use at any observing latitude between 20° and 90°. The 90° (altazimuth) position is for terrestrial observation and for quick set-up astronomical observing. Setting the wedge tilt plate to your observing latitude and aligning the scope on the celestial pole allows more accurate object location and sidereal rate tracking, but takes a little more time to set up.
   The ETX90 Observer drive base attaches to the tripod in seconds using two thumbscrews in the wedge tilt plate. The thumbscrews are tethered and spring-loaded so they can’t be lost in the field. The spreader bar that holds the legs open is spring-loaded as well, so setting up and taking down the tripod takes only a few seconds. A soft carry bag is provided to transport the tripod. 

Hard case for scope: A locking hard case with carry handle is provided to transport and protect the ETX90 Observer scope itself.

AutoStar Software Suite: This extensive software package is included as standard equipment with the Meade ETX90 Observer. It is designed to integrate the telescope with your PC or laptop computer for an enhanced range of performance features. The AutoStar Software Suite also includes a planetarium program for display on your computer screen. It includes all the standard planetarium program features for stand-alone use when nights are cloudy.
   In addition, when the ETX90 Observer is connected to your computer, using an optional cable, the AutoStar Software Suite lets you click on objects in the sky map displayed on your computer screen and have your telescope automatically slew to those objects. You can automatically generate tours of favorite objects with a simple point and click. The software lets you control all AudioStar functions from your computer or laptop. You can use it to create observing lists and download them to the AudioStar for use in the field when you don’t have your computer or laptop with you. An AutoStar Update Tool keeps your AudioStar hand control current by downloading the latest system firmware updates and comet, asteroid, and satellite data over the internet from Meade’s website.

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.
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).

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.

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.

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.

1.3 arc seconds
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.
The weight of this product.
18.9 lbs.
Heaviest Single Component:
The weight of the heaviest component in this package.
9.8 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.
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. 
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.  
Very Good
Planetary Observation:
Binary and Star Cluster Observation:
Galaxy and Nebula Observation:
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.
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. 
Planetary Photography:
Star Cluster / Nebula / Galaxy Photography:
1 year
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ETX90 Observer 3.5" go-to Maksutov, tripod, case, and Audiostar computer

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ETX90 Observer 3.5" go-to Maksutov, tripod, case, and Audiostar computerFull length view of the Meade ETX90 Observer on its tripod.View of Meade ETX90 Observer's rear cell.
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Our Product #: M90ETX
Manufacturer Product #: 205004
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The fully go-to computerized ETX90 Observer portable observatory has computer-optimized UHTC multicoated 90mm Maksutov-Cassegrain optics, AudioStar computer control, tripod, tripod carry bag, scope hard case, and two eyepieces – all at a remarkably sensible price.

. . . our 38th year