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Celestron - C6 Acromat Refractor Optical Tube Assembly (for CG-5) - DISCONTINUED

Model#: 21020-OTA

Discontinued Product

Manufactured by: Celestron

Celestron - C6 Acromat Refractor Optical Tube Assembly (for CG-5) - DISCONTINUED


This is a discontinued item. We suggest the C6 Achromat Refractor OTA as a replacement.

An excellent choice for the serious astronomer, the C6-R is the largest telescope in its class with over 6 times the light gathering ability of a 60mm. This deluxe quality 1200mm focal length, f/8 German Equatorial mounted achromatic refractor is an incredible value and offers outstanding light gathering ability and stability. It features premium accessories such as a 9x50 finderscope with bi-directional alignment screws and a spring loaded pivot support, 20mm Plossl 1-1/4" eyepiece, 2" focuser.


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.


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).

  • 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.

Additional Information

Name Celestron - C6 Acromat Refractor Optical Tube Assembly (for CG-5) - DISCONTINUED
Manufacturer Celestron
Model 21020-OTA
Discountinued Yes