4" Omni XLT 102 Equatorial refractor with Starbright XLT optical multicoatings

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CG-4 mount
The CG-4 German equatorial mount supplied with this Celestron telescope does not come with a motor drive as standard equipment, but its manual worm gear drive slow motion controls will let you smoothly track celestial objects for visual observing with only an occasional twist of the wrist. Adding optional dual axis motor drive #MDCG4D will keep objects centered in the eyepiece continuously. If family viewing is planned, a motor drive means you don’t have to worry about the object being observed drifting out of view while changing observers. This is especially handy when observing with small children or groups of people (at star parties or scout meetings, for example). If astrophotography is in the cards, the #MDCG4D motor drive/drive corrector is essential.
    The mount uses precision ball-bearings in both the right ascension and declination axes to assure smooth and hitch-free motion of your scope across the skies. Built-in non-powered setting circles are provided to help you find objects by using their right ascension (hour) and declination (degree) coordinates as shown in a star chart or atlas. The right ascension hour circle is equipped with a vernier scale that lets you adjust the telescope position in right ascension to an accuracy of one minute.
    The standard manual right ascension and declination slow motion controls have easy-grip knobs that make manual tracking and small position adjustments a cinch. The extra-large control knobs are positioned so that they fall easily to hand when observing, so you don’t have to grope blindly for the knobs when you need to make adjust the scope’s position. To make a casual polar alignment for visual use quick and easy, there’s a latitude scale and fine adjustment controls in both altitude and azimuth. The mount can be used over a latitude range of approximately 20° to 60°. If astrophotography is in your plans, an optional polar finderis available to increase the accuracy of your alignment.
    The mount’s adjustable height tripod has 1.75” diameter stainless steel legs for strength and rigidity. There’s a center leg brace that locks the tripod legs firmly in place for excellent vibration-damping. The center brace is drilled with several holes that will hold your 1.25” and 2” eyepieces conveniently at hand and up and out of the dew-soaked grass. The no-tool locking knobs that adjust the height of the tripod are located on the inside of the tripod legs. This keeps them from snagging clothing in the dark, a thoughtful touch sure to be appreciated. The tripod adjusts over a height range of 33” to 47”. As with all tripods, it is at its most rigid and stable when at its lowest height.
    Two counterweights are supplied to balance the optical tube. The sliding counterweights are locked in place with a single hand-tighten thumbscrew each, making it easy to rebalance the scope if you add heavy accessories. The mount is easily capable of handling the scope and any reasonable combination of accessories you might want to add.
    An adjustable dovetail slide bar allows the optical tube to be quickly and precisely balanced fore and aft on the mount, eliminating the need for an extra counterweight to balance a camera or other accessories. Setup and takedown times are exceptionally fast, as no tools are required to assemble the mount and tripod and only a single large hand-tighten knob holds the optical tube in place. A second smaller lock knob prevents the tube from sliding off the mount should the hand-tighten knob loosen.
    The CG-4 equatorial head weighs 10 lbs (4.5 kg). The 7 lb (3.2 kg) and 4 lb (1.8 kg) counterweights total 11 lbs (5 kg). The tripod weighs 12.5 lbs (5.7 kg). Total weight of the complete mount set up to receive an optical tube is 33.5 lbs (15.2 kg).
This Celestron telescope has:

• 102mm f/9.8 achromatic refractor optics
• StarBright XLT multicoatings for the highest possible light transmission
• CG-4 German equatorial mount with ball bearings and stainless steel tripod
• 6 x 30mm finderscope
• 25mm 1.25” eyepiece (40x)
• 2-year warranty

    The Celestron Omni XLT 102 refractor combines quality optics with a solid German equatorial mount – at a remarkably sensible price for a scope of its quality. It provides an excellent mix of performance, portability, stability, and features that any serious backyard astronomer can appreciate.
For the observer whose interests are the brighter solar system and deep space objects, the Omni XLT 102 has a lot to offer. It is exceptional for observing within the solar system. Its views of subtle lunar and planetary details are sharp and contrasty, bringing the planets to vivid life in the eyepiece.
It has a sensibly large aperture and diffraction-free images that make it surprisingly good for much deep space observing, as well. 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.

This Telescope’s Optical System . . .

  • Refractor optics: 4” (102mm) aperture, 1000mm focal length, f/9.8 achromatic doublet lens using aspheric shaping technology for images that have virtually no spherical aberration. The lens cell is fully collimatable for optimum sharpness. The optical tube length is a very manageable 39.5”.

  • Starbright XLT multicoated optics: Fully coated on all air-to-glass surfaces with multiple layers of magnesium fluoride and hafnium dioxide antireflection coatings for the highest possible light transmission and contrast. They are the same coatings used on Celestron’s largest most expensive optical systems.

  • Lens cap/aperture stop: A protective lens cap is provided to cover the objective lens of the telescope and keep it dust-free when the scope is not in use. This lens cap has a built-in aperture stop in the center. When observing the Moon and planets, leaving the lens cap on the telescope with the aperture stop removed will reduce the scope’s chromatic aberration (the faint halo of violet light or “spurious color” visible around very bright objects in all achromatic refractor telescopes). The scope’s resolution will be slightly reduced, but eliminating the spurious color provides a generally more pleasing image for most observers. The lens cap should always be completely removed when observing faint deep sky objects such as nebulas and galaxies, where aperture (light-gathering) is essential and chromatic aberration is not an issue.

  • Dew shield: Slows the formation of dew on the lens in cold weather to extend your undisturbed observing time. Also improves visual and photographic contrast by shielding the lens from off-axis ambient light (the neighbor’s yard light, moonlight, etc.)

  • Rack and pinion focuser: 2” focuser, with 1.25” eyepiece adapter. Dual focusing knobs with rubber gripping surfaces for precise image control with either hand. The large focus knobs are easy to operate, even while wearing gloves or mittens in cold weather. The eyepiece holder has built-in T-threads for attaching an optional T-ring and camera for prime focus photography.

  • Star diagonal: 90° viewing angle prism-type 1.25” star diagonal.

  • Eyepiece: Fully multicoated low power 1.25” 25mm (40x) eyepiece with a 1.25° field of view (two and a half times the diameter of the full Moon).

  • Finderscope: 6 x 30mm straight-through achromatic design, with a wide 7° field of view. Focuses by loosening the trim ring behind the objective lens cell, screwing the lens cell in or out to focus, and tightening the trim ring to lock in the correct focus.

  • 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 slow motion controls. There are 75 full color images of well-known celestial objects to help you identify them through your scope.

This Telescope’s Mount . . .

  • CG-4 German equatorial mount: The mount has setting circles in both right ascension and declination, worm gear drives and manual slow motion controls in both axes, a latitude scale and fine adjustment controls in both altitude and azimuth, two counterweights totaling 11 pounds so you can easily balance virtually any accessory load, and more. An optional dual axis DC drive/drive corrector is available for no-hands tracking of celestial objects and photography. It is not possible to upgrade to a computerized go-to drive system. For more details, click on the “mount” icon above.

  • Adjustable height tripod: The tripod has 1.75” diameter steel legs with a center leg brace for rigidity. It adjusts over a height range from 33” to 47”. Vibration damping characteristics are excellent. The center leg brace is drilled to form a convenient accessory tray that holds 1.25” and 2” eyepiece to keep them up out of the dew-soaked grass.

  • Dovetail tube mount: The scope’s optical tube rings are connected to a mounting plate that slips into a dovetail groove on the mount’s equatorial head. Setup and takedown times are exceptionally fast, as a single large hand-tighten knob holds the optical tube in place. A second lock knob prevents the tube from sliding off the mount should the hand-tighten knob accidentally loosen while observing.

  • Two-year warranty: All Celestron telescopes have a two-year warranty, double that of competitive scopes.

What can you see through this 102mm Celestron refractor?

    With a relatively short 1000mm focal length and a light grasp 212 times that of the sharpest dark-adapted eye, the scope is capable of producing surprisingly bright wide-field images of the faint fuzzies outside the solar system – nebulas, galaxies, open star clusters, and more.
But those objects requiring high power and high contrast – globular clusters, close binary star pairs, lunar and planetary images, etc. – are where this achromatic scope shines. Using optional eyepieces and/or a Barlow to boost the magnification, you can see subtle solar system details that are virtually invisible in smaller aperture scopes. You can study lunar craters, rilles, mountain ranges, and low contrast lunar ray detail. With reasonable seeing conditions, detail in Jupiter’s cloud belts and the Great Red Spot (actually closer in color to the Faint Pink Spot at this point in time) are visible, as are dusky markings on the face of Saturn and Cassini’s division in Saturn’s brilliant rings. Chromatic aberration is present in the scope when viewing very bright objects, as it is in all achromatic refractor telescopes, but using the lens cap/aperture stop mentioned above will reduce its visibility.
Optically and mechanically refined, and very reasonable in cost for a 4” refractor, this Celestron Omni XLT 102 has enough optical performance to keep you busy for the rest of your life.

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.

1.14 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.
43 lbs.
Heaviest Single Component:
The weight of the heaviest component in this package.
21 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.  
Planetary Observation:
Very Good
Binary and Star Cluster Observation:
Very Good
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:
2 years
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General Accessories
Drive Motors and Drive Accessories (1)
2-Axis DC drive/drive corrector for CG-4 mount telescopes
by Celestron
Visual Accessories
Miscellaneous (2)
Kit of 1.25" Plössl eyepieces and visual accessories
by Celestron
Kit of 1.25" eyepieces and visual accessories
by Astro-Tech
  • Fully Starbright XLT multicoated 102mm aperture 1000mm focal length f/9.8 achromatic optics
  • CG-4 German equatorial mount with setting circles, manual slow motion controls, and adjustable height steel leg tripod with combined spreader bar/accessory tray
  • 6 x 30mm straight-through finder
  • 2” rack and pinion focuser with 1.25” eyepiece adapter
  • 1.25” prism-type 90° star diagonal
  • 1.25” 25mm eyepiece (40x)
  • Dewcap
  • Dust covers
  • TheSky Level 1 star-charting CD-ROM software
  • Two-year warranty.
Celestron Omni XLT manual 3341 KB
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Celestron - 4" Omni XLT 102 Equatorial refractor with Starbright XLT optical multicoatings

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Celestron - 4" Omni XLT 102 Equatorial refractor with Starbright XLT optical multicoatingsImage of complete scope and tripod.Close-up of the CG-4 equatorial mount head showing the counterweights, slow motion control knobs, altitude and azimuth controls for polar aligning, dovetail bar and tube rings, etc.Close-up of focuser, finderscope, tube rings, etc.
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Our Product #: C102XLT
Manufacturer Product #: 21088
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Clear skies,

This Celestron Omni XLT 102 refractor puts good fully Starbright XLT multicoated achromatic optics on a solid German equatorial mount to provide an excellent mix of performance, portability, and features at a price that’s surprisingly low . . .

. . . our 39th year