6" F/8 achromatic doublet refractor optical tube, CG5/AVX dovetail

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This Celestron optical tube has:

• big 6” aperture fully multicoated achromatic refractor optics
• collimatable objective lens cell for fine-tuning the images
• big 9 x 50mm finderscope
• 20mm Plössl eyepiece (60x)
• bright high contrast deep space views, plus excellent performance inside the solar system

This 6” achromatic refractor optical tube is a sensational value! It’s the same optical tube that’s used on Celestron’s best selling Advanced Series go-to refractor. It has a 150mm (6”) diameter multicoated doublet objective lens (1200mm focal length f/8). That’s the equivalent of a 7” reflector in terms of the amount of light that reaches your eyepiece. With 459 times the light-gathering capacity of the human eye, the optics are sharp and well suited to serious deep space observing of binary stars, star clusters, and the brighter nebulas and galaxies, in addition to excellent high contrast planetary and lunar images. The objective lens cell is fully collimatable, using push/pull collimation screws, so you can optimize your image quality.
Some chromatic aberration is visible on stars and objects of 2nd or 3rd magnitude and brighter, as you would expect in an achromatic refractor of such large aperture and fast focal ratio. However, the spurious color is well controlled and not unreasonable, thanks to careful optical design, and has little effect on deep space observing. Most observers feel the relatively limited amount of chromatic aberration visible is a small price to pay for the scope’s many other virtues of sharpness, contrast, and optical clarity.
For peak performance, some observers have found that upgrading the supplied star diagonal to a premium version, such as the Astronomy Technologies #AT1D dielectric diagonal, pays visible dividends in sharpness and contrast during high magnification observing.

Details About This Optical Tube . . .

  • Achromatic refractor optics: 6” (150mm) aperture, 1200mm focal length, f/8 air-spaced crown and flint glass doublet lens. The objective lens cell is fully collimatable for peak optical performance, via a series of push/pull collimating screws.

  • Multicoated optics: Fully coated on all surfaces with multiple layers of antireflection materials for high light transmission and good contrast.

  • 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.)

  • Dust cover: The 6” dust cover of the objective lens has a removable 4” diameter cap in the center. During lunar and planetary observing, it is often beneficial to leave the 6” dust cover in place, but with the 4” center cap removed. This effectively reduces the aperture to 4” and converts the telescope to an f/12 focal ratio. At this longer focal ratio, chromatic aberration is reduced, with only a modest loss in resolution. The same optical trick can be used to improve lunar and planetary image quality during nights of sub-par seeing by reducing the amount of unsteady air the scope has to look through.

  • Rack and pinion focuser: 2” focuser, with 1.25” eyepiece adapter. The 1.25” adapter has external T-threads that let you connect a 35mm camera to the scope for prime focus photography using an optional T-ring with no other camera adapter required. 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.

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

  • Eyepiece: Fully multicoated low power 1.25” 20mm (60x) Plössl eyepiece with a 0.83° field of view (over one and a half times the diameter of the full Moon).

  • Barlow lens: 2x 1.25” achromatic Barlow doubles the power of the supplied eyepiece. The Barlow has a built-in T-thread photo adapter for prime focus lunar photography (needs T-ring for photography).

  • Finderscope: 9 x 50mm straight-through achromatic design, with a wide 5.8° field of view. Spring-loaded bracket for fast no-tool alignment with the main scope optical tube. 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.

  • Dovetail tube mount: The optical tube comes with two split and hinged tube rings for installing the optical tube on your mount. The rings are connected to a dovetail mounting plate that slips into the standard dovetail groove on a Celestron CG-5 or Advanced Series go-to mount’s equatorial head. The dovetail will also work with the Meade LXD75 go-to mount and with Vixen equatorial mounts such as the Sphinx and Great Polaris.
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.
300x
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.

1200mm
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/8
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.

0.76 arc seconds
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).

13.5
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.
6"
Weight:
The weight of this product.
18 lbs.
 
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:
Great
Binary and Star Cluster Observation:
Great
Galaxy and Nebula Observation:
Good
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:
Yes
Warranty:
1 year
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  • 9 x 50mm straight-through finder
  • 20mm 1.25" Plössl eyepiece (60x)
  • 2x 1.25" Barlow with built-in photo adapter
  • 2" rack and pinion focuser with 1.25" adapter
  • 1.25" prism-type 90° star diagonal
  • Multicoated 150mm aperture 1200mm focal length f/8 achromatic optics
  • Lens shade
  • Dust covers
  • Tube rings with dovetail
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Celestron - 6" F/8 achromatic doublet refractor optical tube

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Celestron - 6" F/8 achromatic doublet refractor optical tube
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