Celestron C8 OTA - CGE Dovetail

8" Schmidt-Cassegrain Optics: 2032mm at f/10

Celestron's SCT manual describes the Schmidt-Cassegrain as 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 203.2mm aperture and 2032mm focal length give 843x the light gathering power of the eye, a limiting stellar magnitude of 14 and a resolution of 0.69 arc seconds (Rayleigh) or 0.57 arc seconds (Dawes). The secondary obstruction is 64mm (2.5"), 31% by diameter and 9.77% by area.

The aluminum tube is 432mm (17") long and 238mm (9.37") in diameter and weighs 12.5 lbs (5.67 kg).

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.

Visual Setup: Visual Back, Diagonal, Eyepiece and Finder

The visual back comes installed on the rear cell; a 25mm 1.25" eyepiece, the 1.25" star diagonal, a 6x30 finderscope and a front lens cover are included. The diagonal slides into the visual back and the eyepiece into the diagonal, each held by a set screw, and loosening the visual back's set screw lets you rotate the diagonal.

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.

The finder bracket screws to the two holes in the rear cell, with its rings over the tube, and three nylon-tipped thumbscrews hold the finder, which has a cross-hair reticle. Align it in the daytime on a target more than a mile away, using the screws on the right and top of the bracket; the straight-through view is inverted, which is normal.

Focusing and Magnification

Focusing moves the primary mirror, which slides on the primary baffle tube; the focus knob sits on the rear cell just below the star diagonal, and it takes about 30 turns to go from close focus to infinity. Make the final focus turn counterclockwise so the mirror moves against gravity, which minimizes any mirror shift, and start on a bright object such as the Moon or a planet, since out-of-focus stars are very diffuse.

Magnification is the telescope's focal length divided by the eyepiece's, 2032mm here. The general rule allows 60 power per inch of aperture as the highest useful magnification, 480x for this tube, with most observing done at 20 to 35 power per inch, or about 160x to 280x. The included 25mm eyepiece gives 81x, and the lowest useful magnification is 29x.

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.

Imaging: Fastar and More

The 8" tube is Fastar/HyperStar compatible: the secondary mirror can be removed and replaced with a lens assembly, sold separately and not included, so that a camera works at the f/2 focus of the primary mirror at the front of the telescope. Celestron's SCT manual says f/2 needs exposures about 25 times shorter than f/10, with a field of view 5 times larger and the object 1/5 the size. The secondary mirror should never be removed except to install the Fastar lens assembly.

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. Since the rear cell is not sealed, keep its cover on when the telescope is not in use. If moisture condenses on the inside of the corrector, remove the accessories from the rear cell and leave the telescope pointing down in a dust-free place.

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.