Celestron C6 OTA - CG-5 Dovetail

150mm f/10 Schmidt-Cassegrain Optics

The C6 offers 40% more light gathering than a 5" Cassegrain telescope. The Schmidt-Cassegrain is a compound, or catadioptric, telescope: a zero power corrector plate, a spherical primary mirror and a secondary mirror, with light travelling the length of the tube three times. The design offers large-diameter optics in very short tube lengths, which makes the tubes extremely portable.

The 150mm aperture and 1500mm focal length give 459x the light gathering power of the eye, a limiting stellar magnitude of 13.4 and a resolution of 0.93 arc seconds (Rayleigh) or 0.77 arc seconds (Dawes). The secondary obstruction is 56mm (2.2"), 37% by diameter and 14% by area.

The tube is 406mm (16") long and 180mm (7.12") in diameter and weighs 10 lbs (4.54 kg). The rear has a male SCT (2"-24tpi) thread, and focusing moves the primary mirror.

StarBright XLT Coatings

StarBright XLT is Celestron's coating system, built from three parts: enhanced multi-layer mirror coatings of aluminum, SiO2 and TiO2, with reflectivity fairly flat across the spectrum for both imaging and visual use; multi-layer anti-reflective coatings of magnesium fluoride and hafnium dioxide; and water white glass instead of soda lime glass for the corrector. Water white glass transmits about 90.5% of light uncoated, 3.5% more than soda lime glass, and reaches an average transmission of 97.4% with the StarBright XLT anti-reflective coatings.

StarBright XLT visibly increases contrast and light transmission, so you can discern subtle details while viewing the Moon and planets as well as faint galaxies and nebulae.

Mounting, Eyepiece and Focusing

The C6 can go on a multitude of computerized telescope mounts thanks to its CG-5 dovetail bar, and also works well on a basic manual altazimuth mount for night sky viewing or as a terrestrial spotting scope during the day.

The included 25mm eyepiece gives 60x; the highest useful magnification is 354x and the lowest is 21x. A 6x30 finderscope with bracket, a 1.25" star diagonal and front and rear dust caps are included.

Focusing moves the primary mirror, using the focus knob on the rear cell. Celestron's SCT manual advises making the final focus turn counterclockwise so the mirror moves against gravity, which minimizes any mirror shift, and starting on a bright object such as the Moon or a planet, since out-of-focus stars are very diffuse.

With the star diagonal, the image is right-side-up but reversed left-to-right; with the eyepiece straight in the visual back, it is upside-down and reversed. This is normal for the Schmidt-Cassegrain design. To find the true field of view, divide the eyepiece's apparent field by the magnification. The longer an eyepiece's focal length, the lower its magnification, and you will generally use low-to-moderate power.

Care and Collimation

Dust on the corrector can be removed with a camel hair brush or pressurized air, then an optical cleaning solution and white tissue. Dew on the corrector can be cleared with a hair dryer on a low setting or by pointing the telescope at the ground until it evaporates. Internal cleaning and adjustments should be done only by Celestron's repair department.

Celestron collimates every telescope at the factory and checks it before shipping. If it is jarred in transport, you can adjust the tilt of the secondary mirror with the three collimation screws on the secondary housing, the only adjustment available to the user. To check, defocus a star at least 20 degrees above the horizon with a high-power eyepiece: the dark shadow of the secondary should sit centered in the bright rings on both sides of focus. If you are unsure whether it needs adjustment, it is likely fine and best left alone. Before checking collimation, let the telescope reach thermal equilibrium with its surroundings, allowing 45 minutes if you move it between large temperature extremes, such as from a heated house into the cold night air.