Celestron Omni XLT 150 Telescope

150mm Newtonian Optics with StarBright XLT

The Omni XLT optics are made with aspheric shaping technology combined with hand-figuring, giving an image with virtually no spherical aberration, and carry the StarBright XLT coating system to enhance light transmission. The optical glass is hand-selected.

The 150mm parabolic primary mirror and 750mm 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 tube is 673mm (26.5") long and weighs 12 lbs (5.44 kg). The manual notes that a Newtonian's primary mirror is open to air and dust, so it needs a little more care than a closed tube. Celestron's Telescope Basics manual explains that a Newtonian reflector shows land objects upside down, or rotated depending on the angle of the focuser and how you hold your head; a diagonal would not correct this, and astronomers do not mind, since there is no up or down in space.

The Omni CG-4 Equatorial Mount

The Omni CG-4 is a German equatorial mount: once polar aligned, its axis of rotation is tilted parallel to the Earth's, so you follow the stars as they move across the sky with the right ascension (R.A.) axis. The manual lists ball bearings in both axes for smooth performance. Two slow motion control knobs (cables) make fine pointing adjustments in R.A. and declination; the longer one is for R.A. and fits either of the two R.A. shafts, and the declination knob goes on the shaft pointing toward the ground, within easy reach at the eyepiece. For rough movements, unlock the R.A. and Dec lock levers (counterclockwise) and move the telescope by hand. R.A. and declination setting circles, with a vernier scale on the R.A. circle, let you find objects by their coordinates.

The mount comes with two counterweights, 7 lbs (3.2 kg) and 4 lbs (1.8 kg), which slide onto the counterweight bar and lock with their side screws; the safety screw at the end of the bar keeps them from sliding off. The manual lists the mount with counterweights at 21 lbs (9.5 kg) and the tripod at 12.5 lbs (5.7 kg).

Balancing and Polar Alignment

Balance in R.A. first: with the telescope to one side and the counterweight bar horizontal, release the R.A. clamp gradually to see which way it rolls, and slide the counterweights until the telescope stays put. Then, with the R.A. clamp locked and the tube parallel to the ground, release the Dec clamp gradually, never letting go of the tube, and slide the tube forward or back in its rings until it stays still. These are general balance steps that reduce undue stress on the mount, and good balance is crucial for accurate tracking with an optional motor drive.

The latitude adjustment covers approximately 20 to 60 degrees. To raise the polar axis, tighten the rear latitude screw and loosen the front one; to lower it, the reverse. Make the final altitude adjustment against gravity, with the rear screw. For azimuth, move the whole tripod for rough adjustments and use the two azimuth knobs, which push against the tripod's peg, for fine ones. Once polar aligned, the mount itself must not be moved: you point the telescope only in R.A. and declination.

The easiest method, the manual says, is the latitude scale: point the polar axis due north using a landmark, level the tripod with the built-in bubble level, and adjust the mount in altitude until the latitude indicator shows your latitude. It can be done in daylight and limits the corrections you make while following an object. For a closer alignment at night, set the tube parallel to the polar axis (the declination circle reads +90) and adjust the mount in altitude and azimuth until Polaris is centered, which gets you within about a degree. The manual also describes finding the celestial pole, aligning in the southern hemisphere and the declination drift method for more serious work.

Finding Objects with the Setting Circles

To find objects with the setting circles, first align the R.A. circle on a bright star with known coordinates near the celestial equator: center it, then turn the circle until its coordinate lines up with the R.A. indicator (the zero mark of the vernier). Then look up your target's coordinates, move the telescope in declination and R.A. until the indicators read them, lock each clamp, and check the finder. The R.A. circle does not move with the telescope, so it must be realigned each time you use it, and the accuracy depends on your polar alignment. The vernier gives readings to one minute of R.A.

Tripod and Assembly

The tripod has 1.75" stainless steel legs and comes fully assembled, with an all-metal center leg brace that doubles as the accessory tray and pushes against the legs for rock-solid support. Its standard height is 33", extendable to a maximum of 47"; the manual notes it is most rigid at its lowest height. A metal alignment peg on the tripod head marks the side that faces north, and a bubble level is built into the mount.

The manual suggests a first assembly indoors. Retract the azimuth screws, set the mount on the tripod head over the alignment peg, retighten them and tighten the mounting knob under the head; slide the accessory tray over the central rod and tighten its knob. Thread the counterweight bar into the Dec axis and tighten its lock nut, point the polar axis north, then fit the counterweights, the slow motion knobs and the polar axis cap. With the R.A. and Dec clutches locked, slide the tube's dovetail bracket into the mounting platform, tighten the mounting knob and snug the safety screw.

Eyepiece and Finderscope

The 25mm eyepiece goes straight into the focuser and gives 30x, with a true field of 1.67 degrees, or 88 ft (27 m) at 1000 yards. The highest useful magnification is 354x and the lowest is 21x. Magnification is the telescope's focal length divided by the eyepiece's. The manual's rule is about 60 power per inch of aperture as the highest useful magnification under average skies, while most observing is done at 20 to 35 power per inch. To focus, turn the focus knob directly below the eyepiece holder: clockwise for a more distant object, counterclockwise for a closer one.

Align the 6x30 finderscope in daylight on a target more than a mile away, to avoid parallax: center it in the telescope, then turn the finder bracket's adjustment screws until the cross hairs sit on the same spot. The finder shows an inverted image, which is normal for most astronomical finderscopes.

Collimation

The manual describes daytime collimation using a collimation cap or Celestron's optional Newtonian Collimation Tool (94183): with no eyepiece and the focuser racked in, adjust the three secondary mirror screws until the whole primary mirror is centered in the secondary (never touch the center screw), then adjust the primary until the secondary's reflection is centered. On the primary cell, three large thumbscrews tilt the mirror and three small ones lock it; turns of about 1/8 make a difference, and 1/2 to 3/4 of a turn is the most the large screws usually need. A final check on a defocused star at medium to high power refines the primary.

Observing, Care and Accessories

Celestron's Telescope Basics manual suggests giving the telescope at least 10 minutes to cool to the outside air temperature, longer if the difference is large, and allowing about 30 minutes for your eyes to fully dark-adapt, using a red LED flashlight at low brightness if you need light. Because the Earth rotates, objects drift across the eyepiece, faster at higher magnification. Celestron's Telescope Basics manual suggests positioning the telescope just ahead of the target and letting it drift through the field. The manual also notes that nebulae do not look like the colourful long-exposure photographs in books: the eye is most sensitive to green, so most nebulae appear greenish-grey in the eyepiece.

To clean the optics, remove dust with a camel hair brush or a few seconds of pressurized air sprayed at an angle, then use an optical cleaning solution on white tissue with low-pressure strokes from the center outward, never in circles. Dew can be cleared with a hair dryer on low or by pointing the telescope at the ground until it evaporates; if moisture condenses inside, remove the accessories and leave the telescope pointing down in a dust-free place. Replace all covers after use, since the cells are not sealed. Internal adjustments and cleaning should be done only by Celestron's repair department.

For tracking, options include the Dual Axis Motor Drive (93522), designed for the Omni CG-4, which controls the tracking speed with four rates (1x sidereal, 2x for guiding, 4x and 8x for centering), and a polar axis finderscope with a cross hair reticle to speed polar alignment. Both are sold separately. Starry Night Special Edition software and the SkyPortal app come with the Omni XLT.