Celestron Omni XLT 102 Telescope
By Chuck Hawks and the Astronomy and Photography Online Staff
This is the standard (achromatic) Celestron Omni XLT 102 telescope, produced for Celestron by Synta Optical, as opposed to the Omni XLT 102ED previously reviewed by Astronomy and Photography Online. Substituting a conventional crown glass / flint glass doublet for the extra-low dispersion glass doublet of the 102ED reduces the discount retail price of the telescope by about 50%. There are also some other changes, such as the way the objective doublet is mounted and the use of a rack and pinion focuser instead of a Crayford, but the supplied CG-4 mount remains the same. This excellent German equitorial mount, by itself, makes the Omni XLT 102 package worth its online discount price, so you can hardly go wrong.
The XLT 102 comes with an attractive "gunmetal blue pearl" (my color description) main tube, the same as the rest of the Omni line. The front cell, focuser casting and mounting rings are painted white. The CG-4 mount wears a matching white finish, as does the tripod's head and leg locks. It is a good looking package.
This 102mm (4") telescope is the entry level refractor in Celestron's Omni XLT line, although its quality is identical to its larger aperture 120mm (4.75") and 150mm (6") models. Its advantage, in addition to a lower price point, is that a 4" aperture, f/9.8 refractor is a comfortably portable telescope and increasing the diameter of the objective lens quickly makes a refractor ungainly. The XLT 102's optical tube weighs 9.5 pounds, while the XLT 120 weighs 12.5 pounds and the XLT 150R weighs 16 pounds. The heavier telescopes also put more strain on the CG-4 mount that is supplied with all Celestron Omni XLT telescopes.
Here are the features of the Celestron Omni XLT 102:
Omni XLT 102. Illustration courtesy of Celestron International.
Here are the technical specifications:
The Omni XLT 102 is built on a seamless aluminum main tube and uses heavy-duty cast aluminum front and rear cells for structural rigidity. Dual one-inch wide rings are used to attach a Vixen style dovetail that mates the scope to the CG-4 German equatorial mount. Celestron's heavy-duty CG-4 mount includes a dedicated tripod with 1.75-inch diameter, two section, stainless steel legs. This high quality mounting system is an improved version of the Celestron Super Polaris German equatorial mount and it makes the Omni XLT series scopes a true long-term investment and a best buy.
The mounting system is at least half the value of any amateur telescope and its importance is usually overlooked by first time buyers. One of the things an amateur astronomer learns with experience is the crucial importance of a solid, accurate mounting system. Celestron's CG-4 German equatorial mounting system incorporates ball bearing races on both azis for smooth movements and precise manual slow motion controls. It should satisfy the needs of any visual astronomer.
Refractors are probably the most durable and trouble free of the common types of telescopes. Relatively trouble free operation and sharp, contrasty views, for which high quality refractors like the XLT 102 are noted, are important plusses for an amateur telescope. Unlike telescope designs using mirrors, refractors have no secondary obstruction to degrade resolution. Refractors also avoid most of the coma inherent in reflector telescopes. Coma is an optical aberration that causes point sources of light, such as stars, to become crescent shaped near the edge of the field of view.
Admittedly, an achromatic refractor with a two spherical element objective lens, such as the XLT 102, is going to produce some color fringing when viewing bright objects, usually in the green/yellow area of the spectrum. This is because it takes a minimum of three conventional optical glass elements to focus all three primary colors of light (red, green and blue) to the same point and achromatic refractors accurately focus only the red and blue colors. Color fringing can be minimized by incorporating an ED glass element, fluorite lens element, aspheric elements, three or more spherical elements, or some combination of the above in the objective lens system, but doing so substantially increases the price of the telescope. Lengthening the focal length also minimizes chromatic aberration and that accounts for the XLT 102's extra 100mm compared to the XLT 102ED. Most intermediate level amateur astronomers (the XLT 102's primary market segment) find that the relatively minor improvement in performance does not justify the relatively large increase in retail price occasioned by the use of ED glass.
A Celestron 25mm Plossl ocular is supplied with the Omni XLT 102, but you will need to augment its capabilities with shorter focal length (higher magnification) and longer focal length (wider field of view) oculars. Useful ocular additions would include a 32mm Plossl and an 8-24mm zoom ocular. We had both of these, including an excellent Vixen 8-24mm zoom ocular available for use in this review. Other oculars can be added as the need arises. In addition, a padded case for the optical tube, an ocular/accessory case and a neutral density "moon" filter will need to be acquired. For this review, we had on hand a rather complete selection of oculars ranging from 40mm to 4mm.
A couple nights of observation with the XLT 102 quickly revealed that the scope and mount are fully functional, but the 6x30, straight through, inverted image finder is a pain in the neck (literally) to use. A right angle, correct image finder scope or a red dot finder (or both), which are available from Orion and other sources, are practically necessities. Perhaps not necesssary, but very handy, is a green laser, such as Celestron's Laser Finderscope Kit.
Being reasonably familiar with the performance of 4" telescopes in general and Celestron telescopes in particular, we did not expect, or receive, any untoward surprises while viewing with the XLT 102. For convenience, we did our initial observing from the College Hill water reservoir in Eugene, Oregon, a location convenient for staff members Bob Fleck, Jim Fleck, Rocky Hays, Gordon Landers and myself, all of whom participated in this review. This location provides a good 360 degree field of view, but the seeing quality is reduced by the substantial sky glow from the city of Eugene.
In addition to the Omni XLT 102, we had on site a Bosch & Lomb Criterian 4000 (4" CAT) and an 80mm StellarVue APO refractor. The StellarVue and Celestron scope's focusers can accommodate two-inch oculars, but most owners will use 1.25-inch visual accessories, as we did in all three telescopes.
Setting-up the XLT 102 is straightforward. We transported the scope in three main sections, the optical tube (with finder removed), mount (with counterweight removed) and tripod legs (with center tray removed). Upon reaching the observing site, we set-up the tripod and attached the accessory tray. We next attached the CG-4 mount on top of the tripod and tightened the threaded center shaft to hold it in place; after that we slid the 14 pound counterweight about half way up its shaft and secured it in place. (The additional 7 pound weight will not be needed unless you attach a camera to the telescope.) We placed the telescope on the mount and tightened the main and safety thumb screws that secure the telescope's Vixen dovetail mounting rail to the mount. Last, we slid the finder scope and mount into its shoe on the scope's rear cell and tightened the thumb screw that retains it.
My simplified alignment technique for visual observing is to level the tripod with the north leg visually aligned with the north star and the mount centered in azimuth and set for the correct latitude (44 degrees in this case). That is all you need to do with a German equatorial mount to minimize declination adjustments for enjoyable viewing.
Having set-up the XLT 102, we were ready to observe some commonly viewed objects. Since we were reviewing an achromatic refractor, we concentrated on the two brightest objects in the sky, which were the nearly full moon and Jupiter. For both, we used a Celestron moon filter. We also took a quick look at the Hercules globular cluster (M-13), one of the most famous deep sky objects, and split the colorful double star Albireo. We used the 32mm Plossl ocular in conjunction with the Celestron's finder scope to locate objects, taking advantage of that ocular's relatively wide field of view, then switched to shorter focal length oculars for increased magnification as appropriate.
GLOBULAR CLUSTER M-13 - This large globular cluster has a stellar magnitude of 5.9. It is an impressive object when viewed through practically any telescope and the XLT 102 is no exception. M-13 looked good through the 32mm Plossl and better in the 25mm Plossl supplied with the scope. A 15mm Plossl was able to show individual stars at the fringe of the cluster, a real achievement given the amount of light pollution present.
ALBIREO - Located at the foot of the Northern Cross (Cygnus), Albireo is a double star with contrasting blue/gold component stars. Their stellar magnitudes are 3.1-5.1 and they are separated by 35 arc seconds. As expected, the XLT 102 easily split this double and revealed the proper colors in the individual stars, one of the strong points of refractor telescopes in general. Its performance on Albireo was essentially indistinguishable from the StellarVue APO refractor.
JUPITER - Jupiter and its four big moons is the largest and most impressive planetary system in our solar system. Since it is very bright, it allowed the use of relatively short focal length (high magnification) oculars. The seeing conditions supported the use of oculars as short as 9mm (111x) in the XLT 102. The Celestron provided good views of Jupiter, just what you would expect from a quality four inch refractor. There was some greenish color fringing visible, but we did not fine it intrusive and it did not detract from our enjoyment while looking at Jupiter.
Initially, the best views were provided by the 15mm and 9mm oculars. The 15mm showed the two central cloud bands and all four moons and provided the brighter view. Jupiter always looks like a miniature solar system to me and the 15mm portrayed that well. Switching to a 9mm eyepiece made the planet larger, but the seeing conditions did not allow much more detail to be seen. Finally we tried the 8-24mm zoom ocular and, set at about 12mm, it gave the best view of all. That is the advantage of a zoom ocular, you can adjust it for just the right magnification for the seeing conditions.
THE MOON - Our very own moon, the largest and brightest object in the night sky, provided the most impressive views of the evening. The moon was about 3/4 full, but the terminator's shadow was sufficient to emphasize details for our viewing pleasure. We could see a lot of detail in the moon's craters and mountain ranges using the 15mm (which showed the whole moon) and 9mm oculars. Once again, some residual chromatic aberration was visible on the moon's bright limb, but it in no way detracted from the grandeur of the view. Fortunately, we had a moon filter screwed into the barrel of the star diagonal, so the moon's brightness was not unpleasant.
It is worth adding that the XLT 102 provided sharp star images. The Omni XLT 102 clearly out performed the B&L Criterion 4000 catadioptric on the objects we viewed and held its own with the more expensive (albeit smaller) StellarVue APO refractor. The scope's focuser comes with large, rubber covered knobs and operates reasonably smoothly. It is much better than the Criterion's focuser, although inferior to the StellarVue's two speed Crayford focuser.
The CG-4 mount and tripod are steady and vibrations dampen quickly, even at full leg extension. When balanced correctly, which is very easy to accomplish, the mount slews almost effortlessly and the smooth slow-motion control knobs make it easy to center an object in the field of view. The RA and Declination locks are positive (turn clockwise to lock). It's a good mount for visual astronomy.
The Celestron Omni XLT 102 offers good optics and a solid equatorial mount at a bargain price. It is well designed, well made, versatile, durable and has sufficient light grasp to keep most observers occupied for years. I don't think anyone will regret buying one. It is probably the best $400-$500 (approximate 2010 online retail price) telescope system on the market today.
Copyright 2009, 2010 by Chuck Hawks. All rights reserved.