Meade Instruments Corporation

LIGHTBRIDGE™ 12 Truss Dobsonian Telescope

COMPANY SEVEN PROMISES A GIANT DOB FOR EVERY SMALL CAR!

Dobson style telescopes promise a great degree of observing satisfaction especially when considering their the moderate cost. Many economical versions of these telescopes have come and gone over recent years, but those sold by Company Seven endure as they are fine tuned and backed by one of the most credible and competent sources of telescopes in the marketplace. For 2006 we add the new Meade Instruments Lightbridge™ to our inventory. This is a new contemporary telescope with great engineering features tweaked by Company Seven and aimed at bringing the deep sky regions within you reach!

Right: The Meade Instruments Lightbridge™ 12 Standard telescope model (37,161 Bytes).
Click on image to see enlarged view (97,806 bytes)

The Lightbridge™ telescopes are designed for those who seek the best possible view of the fainter deep sky celestial objects, balanced with unprecedented portability and ease of use all while on a comparatively modest budget. Available in 8, 10 and 12 inch apertures Company Seven endorses the 12 inch model as making the most sense especially when considering the work involved to assemble these systems. The nearest competitor to the title of "Best Economical Amateur Dob" would those made by Orion Telescopes, either their SkyQuest™ XT 12. It's cousin the XT12 Intelliscope™ adds the capability to locate any of more than 14,000 fascinating celestial objects with a nudge and some pushbutton ease. Each of these telescopes offer much in the way of good performance and features. It has been said by many in our hobby that regardless of bells and whistles "bigger is better" and in practice this is true - as long as you can take the telescope to where you want it, when you want it: this is where Lightbridge™ comes in.

Space Is Infinite, Your Back Seat Is Not!

        

What Characterizes A Newtonian Reflecting Telescope?: It was a Scottish astronomer, James Gregory (1638-1675) who published the first known design for a reflecting telescope in his book Optica Promota of 1663. However, it was Sir Isaac Newton who first made a working model in 1668 and he continued refining the design well into at least 1671. The optical arrangement of the Lightbridge™ employs a Parabolic Primary Mirror working with a smaller Secondary Mirror to gather incoming light; this is known as a "Newtonian" arrangment.

Right: Newtonian reflecting telescope optical arrangement with the incoming light path shown in yellow (38,882 bytes).
Click on the image to see enlarged view (187,513 bytes)

The light entering the telescope from the front is gathered by the Primary Mirror and then reflected forward in a converging beam to a Secondary Mirror (an elliptical flat) that is positioned near the front of the telescope and tilted at a 45 degree angle deflecting light to the Focuser. The light cone forms a virtual image at a plane just beyond the focuser. An eyepiece to enlarge the image or a camera for photography is slipped onto the Focuser to view or capture the image.

A well made reflecting telescope offers the best view per the dollar spent of the faint, deep sky objects where light gathering power is essential. The Primary Mirror is the factor by which most reflecting telescopes are described hence a 12 inch telescope has a primary mirror of 12 inch (30 cm) in diameter. Light gathering power increases are more important and come quicker than one may think. Consider how a 6 inch f/8 - a 48 inch (122 cm) long reflecting telescope may show objects about 335X fainter than the unaided eye. While this 12 inch Truss telescope (with similar physical length) provides about 1,400X the light gathering power over the unaided human eye: in astronomy size can matter!

The Dobsonian Telescope: Historically most Newtonian telescope optical tubes were fashioned with a cylindrical tube usually made of wood, fiberglass, metal, or cardboard. By the 1970's a variation of the traditional Newtonian pioneered by San Francisco sidewalk astronomer John Dobson was becoming known. Dobson's goal has been to provide light gathering power in an easier to transport and less costly alternative to the traditional Newtonian arrangement. The Dobson arrangement (also termed "Dobsonian") relies on simple to manufacture materials and techniques. Instead of the unwieldy and heavy German Equatorial Mount traditionally used to support a Newtonian tube, a simple and lighter weight Trunnion and Carriage platform is provided with simple bearing materials to facilitate smooth movement of the optical tube in altitude and azimuth. When made well then this can support the optical tube rigidly and it simplifies set up and use of a telescope.

Furthermore, instead of a full thickness 1:6 ratio Primary Mirror a lighter weight, a thin 1:12 mirror is usually used. This cut the mirror weight about in half and when properly supported, a thin mirror can provide performance rivaling some traditional thick mirrors. The larger mobile Dobson telescopes evolved into truss arrangements where the telescope optical tube is taken apart for transport into sections engineered to be more manageable and portable. The Lightbridge™ are distinguished most importantly by their compact profile and mechanical arrangement which permits quick disassembly of the Optical Tube into smaller components to facilitate transport and assembly in the field.

Lightbridge™ Framework To fashion the upper and lower assemblies which will house the mirrors a thin sheet of steel is rolled into a tube. Aluminum reinforcing rings are fitted onto each end of the assembly. Finally, the two assemblies are held together in the field by six rigid cylindrical alloy 1 inch / 25.4 mm diameter Truss Rods arranged in three pairs. The lower tips of the Rods are attached into six slots in the lower assemblies, and these joints are then secured by three large black plastic hand knobs. The Truss top brackets mate their slots onto the telescope upper assembly, and the remaining three knobs are tightened to secure the Truss in place. This is supposed to provide a telescope that can come apart, then later snap together requiring little if any adjustment to the telescope mirror positioning. Anyone buying a Truss tube reflecting telescope must count on having to learn how to collimate the system or else sooner or later there will be a train wreck..... We will be happy to teach you how to collimate your telescope. Company Seven does also sell precision collimating tools, and our staff will provide private instruction at no additional charge.

The Truss Rods are provided in a dull satin metal finish. The circular aluminum side bearings and other bright aluminum parts are anodized. The metal components of the upper and lower sections are finished internally with an anti reflection flat black paint, with the exterior painted in an appliance white enamel. Since the Truss arrangement exposes the Primary Mirror and to a lesser degree the Secondary Mirror to glaring lights in the vicinity, and increases the possibility of dew or frost build up Company Seven will offer stepped optional solutions for our customers:

• A fitted black Lycra fabric shroud which will surround the Truss rods and block light and mositure from the mirrors, or
• 12 volt DC powered regulated heater elements for the mirrors.

The comparatively thin wall steel construction permits the designer to select a smaller secondary mirror obstruction. This assembled optical tube assembly holds optical collimation well even with temperature or humidity changes. However, Company Seven does not recommend you carry the optical tube assembly by the Truss Rods, instead grip both the upper and lower assemblies. If you intend to carry the assembled Lightbridge™ optical tube then you will find it is more easily managed with our optional padded carrying bag.

The Engine of the Lightbridge™ is a full thickness (1:6 ratio) mirror. The mirrors provided with our first production Lightbridge™ 12 were about on par with what we have experienced coming with telescopes in this price class; certainly enough performance to assure most buyers will be happy for some time to come. At the time this article went on line we had not yet seen more than this example Lightbridge™ and so it would be premature to predict the consistency of the optics. We are pleased that Meade Instruments has provided another trendsetting product, and have provided with it a balance a reasonably good specification with cost. An amateur who seeks even better performance that is most likely to noticed when operating at higher magnifications could procure a replacement mirror set made by a third party - this would be likely to cost as much or more than the entire Lightbridge™ telescope. Or one could have provided mirrors refigured. But these changes are not without some associated risks. Most Dob users will not care about making this 12 inch telescope perform much better if this improvement is costly and obvious only at silly high magnifications - after all one has to manually nudge the telescope along to keep it on a celestial object.

Among the considerations of a well made reflecting telescope are that the mirrors be precisely positioned within the OTA and be adjustable in terms of centering, tilt, and spacing. The proper alignment of the one mirror to the other, and their placing the focal plane (the point where the image comes to focus) at the correct position beyond the focuser are critical. The optical axis of the telescope should be closely parallel to the mechanical axis of the tube assembly too. Company Seven chose to offer this telescope in part because of the good quality of mechanical components and our capability to assemble these telescopes precisely when requested.

Portable Light Gathering Power! The Lightbridge™ 12 provides the light gathering power and fields of view necessary for the clear observing many of the solar system and popular deep sky objects. With about 1,400X the light gathering power of the unaided eye, the Lightbridge™ 12 provides an increase of about 2.3X the light gathering power of the 8 inch reflecting telescopes, and 1.5X that of a typical 10 inch model.

The lower assembly houses the 12 inch f/5 Primary Mirror. This mirror is made to provide good versatility with not only aperture but with a comparatively low inherent magnification. The mirror measures a focal length (measure of the image scale or magnification) of about 1,500 mm so for comparison consider this telescope can show four times the area of sky that an 12 inch f/10 3,000 mm telescope would. Made of stable 2 inch / 50 mm thick Borosilicate Crown glass, this material provides a satisfactory balance of cost with good chemical, thermal, mechanical and optical properties.

Left: Bottom of Lightbridge™ telescope Primary Mirror in it's Cell. Note Cooling Fan at center with plug connector at about 7 o'clock; the Fan is provided only with the Lightbridge™ Deluxe version (57,616 bytes).
Click on image to see enlarged view (213,485 bytes).

The Primary Mirror rests on a nine point floatation Mirror Cell with support distributed by means of three groups of three pads across the rear of the Mirror. Three padded restraint clips 120 degrees apart at the front of the Primary Mirror keep it from falling forward or shifting position. The Mirror Cell permits the user to adjust the tilt of the Primary Mirror by means of three screws accessible at the rear for tilt adjustment, while three other screws lock the tilt mechanisms in place.

To facilitate collimation the factory provides a center reference mark on the Primary Mirror. Incidentally, this mark is located at the center of the mirror in an area obstructed from incoming light by the Secondary Mirror and so this mark has no impact on the optical performance. We checked the math regarding this telescope design after we assembled it and found the actual fit of the elliptical Secondary Mirror in both axes to be very close to nominal. The front aluminized surface of the Primary Mirror is protected by a stylish lightweight molded plastic slip on cover.

The Lightbridge™ Deluxe telescopes are provided with a 6 Volt DC powered Cooling Fan positioned so that it draws air across the back of the mirror thus helping to draw heat away from the it. The fan is used only initially to help acclimate the mirror to local temperatures, this should occur within about an hour of taking the telescope out doors. The Cooling Fan set includes a corded battery holder that accommodates four "AA" batteries, the cord plugs into a female socket. The fan has no protective grid so keep loose objects away from the area of the Carriage below the OTA. And also note this Fan is made of fragile plastic, and is vulnerable to impact if the telescope is set aside upright. This fan is not provided on the Lightbridge™ Standard telescopes.

Right: Lightbridge™ telescope "Business End" showing Secondary Spider and Mirror Holder Assembly (57,616 bytes).
Click on image to see enlarged view (213,485 bytes).

A Hands-On Telescope Working With Gravity: The Dobsonian is a telescope you can push around - literally. You point it by simply nudging the optical tube on its carriage up or down (altitude), and left or right (azimuth) by hand around a center pivot. Dobsonians are not equatorially mounted and so they are generally moved by hand. The motion on both axes should be smooth letting you guide the scope with just the lightest touch from one part of the sky to another. You do not need to loosen and tighten clamps when targeting objects, as you must with other mounts.

The center of gravity lies low and directly over the center of rotation in both directions of the Carriage, so no matter to where the telescope is pointed the weight is evenly distributed through the mount and down to the ground. This characteristic, and sound design results in almost complete elimination of unwanted vibration and oscillation: the inevitable downfall of many poor telescope mounts. Whereas in many economical telescopes the image seems to bounce around forever after the scope has been touched, this is not the case with well made Dobsonians.

The Dobson design remains the first choice among serious deep sky observers, and it is not uncommon to find custom made "Dobs" of 8 to 36 inch aperture in service at dark sky observing sites around the world. Depending on your goals and budget it may make sense to consider an even larger, more refined truss Dobson style telescope including those by Astro-Systems.

The Carriage of the Lightbridge™ consists of white plastic laminate clad boards that are bolted together easily with the provided hardware. Both 6-¼ inch / 15.9 cm diameter side elevation bearings are provided installed on the optical tube at the factory. These are precisely machined of aluminum, polished so that they glide effortlessly on the 8-¼ inch / 21 cm long strips of felt bearing material glued into the Carriage Fork side cutouts. This arrangement will provide smooth motion of the telescope as long as the felt pads are kept free of sand or other contaminants. The 17 inch / 43.2 cm wide spread of the Lightbridge™'s ¾ inch thick Fork side cutouts translate into good stability translated through the Base Plate, through three 1 inch / 25 mm wide pads to the ground.

While the Lightbridge™ Standard telescopes are provided with Teflon pads for their azimuth bearings, we suggest you pay the difference to buy the Lightbridge™ Deluxe since among its improvements are the substitution of three part metal Azimuth Roller Bearings in place of the Teflon pads provided with the Standard model. These Roller Bearings are notably smoother and provide better longevity than the Tefllon pads.

There is no elevation friction control and so this telescope may at times require the user to employ the tried and true technique of attaching a pouch of lead shot or other weight to counterbalance notably heavy payloads including optional Finders, or heavy eyepieces.

Meade has really set a new standard for portability not only with this Truss arrangement, but with the Carriage of the Lightbridge™ that has so low a profile that it will fit into the trunk of many automobiles. Unlike the 30 inch / 76 cm tall and 26 inch / 66 cm wide Carriage of the Orion XT-12 Intelliscope for example, the Lightbridge™ Carriage is just 20-¾ inches / 52.7 cm tall overall, and at its Base Plate is 25 inches / 63.5 cm in diameter. With the full thickness mirror adding so much ballast low in the tube, one can understand better why the balance point of the Lightbridge™ can be so much lower to the ground than some other Dobson telescopes.

Right: Three 12 inch telescopes displayed at Company Seven (L to R): XT12 Intelliscope™ 12 inch f4.9, Meade Lightbridge™ 12 f5.1 Deluxe,
and an example of a first rate slightly larger Dob - the Astro-Systems 12-½ f5.3. (56,972 bytes).
Click on image to see enlarged view (149,503 bytes).

Fully Machined 2 Inch Crayford Focuser This Focuser is among the more striking looking aspects of the Lightbridge™ telescope. This is a relatively new arrival to the USA with variations of this focuser being provided on some other telescopes coming from China too. The Crayford design takes some practice to get used to since unlike more common Rack and Pinion focusers, this is a friction drive providing frequent tension as well as traditional drawtube lock adjustments. Along the bottom of the Focuser body are the focuser drawtube lock (knob closet to the telescope) with a traditional tension adjustment (knob closet to the eyepiece). The Focus Knobs are machined of aluminum and with a diameter of about 1-7/16 inch 40 mm and are easy to handle even when wearing gloves. The drawtube lock and tension adjustment knobs however are smaller and closer together, so some practice will be needed to manage these with gloves on.

This focuser accepts the heavier low magnification wide field of view 2 inch diameter slip fit eyepieces, and with the provided 2 to 1.25 inch Reducer it also accepts 1.25 inch eyepieces. As a nice touch the male end of the Reducer is machined to accept thread in 48 mm filters so that you can buy one filter for use with both 2 and 1.25 inch accessories but note however, some 2 inch threaded filters including those distributed by Orion may only thread in one turn or so.

Left: Lightbridge™ Focuser with its provided 2 to 1.25 inch reducer shown. Note aluminum focuser drawtube lock knob (closet to telescope) and tension adjustment (closet to the eyepiece) knobs (66,675 bytes).
Click on image to see enlarged view (234,603 bytes).

The focuser body is black anodized, the drawtube has a polished aluminum exterior finish while the interior is lined with antireflection baffling threading that is painted flat black to attenuate unwanted reflections. Ball bearings are installed within the focuser body to support the drawtube alignment. The drawtube moves smoothly along the bearings; this is greaseless design and thus will reduce maintenance and reduce contamination of the mechanism. We have observed these focusers on other telescopes in the field and noted how over time the friction of the ball bearing against the drawtube will produce some wearing of the drawtube finish, while noticeable this has no impact on performance.

The drawtube tension adjustment is turned until it is fairly tight so that the drawtube can be drawn in or out by turning the focuser pinion hand knob. The drawtube lock will usually be tightened to prevent it from moving in or out when supporting heavier payloads. It is almost impossible to keep from having to periodically adjust the focuser tension, and even when at its best the user will encounter notable drawtube position movements in or out when installing or removing eyepieces; if you overcome these drawbacks, then the focus movement can be smooth and can be precise. However, most novices will do better with a Rack and Pinion 2 inch focuser like that provided with the Orion Dobs.

There have been a number of popular geared fine focus kits for telescope focusers in recent years. The Starlight Instruments Feather Touch^® offered for our Astro-Phyiscs telescopes is one successful example. These accessories provide the capability to achieve very fine tuning of focus, and these may reduce the possibility of introducing unwanted vibrations when using conventional focusers. There are no kits approved by Company Seven for these Crayford Focusers - yet.

As nice as this Focuser is in some ways the base has no mechanism so that a user can easily adjust its tilt to facilitate collimating the telescope. There is a way that one could loosen and then shim the focuser with spacers, but this technique tends to be tedious and inconvenient. The assumption seems to be that the user will be able to render the focal plane parallel to that of an eyepiece field stop - we do not like to assume. If the maker of this focuser for Meade were to provide the capability to adjust the tilt, then it might be easier for some people (with good skills) to refine collimation; on the flip side this could create more opportunities for the inexperienced user to create mischief.

If the Lightbridge™ 12 were equipped only with a 1.25 inch focuser, then the maximum actual Field of View seen would be only 1 degree - about twice the diameter of the full Moon. With 2 inch capability provided for Lightbridge™, a good 35 mm 2 inch eyepiece could provide 1.5 degrees at 44X with a more practical 7 mm Exit Pupil. By using an optional even lower magnification eyepiece then one could reach down to 26X observing areas as large as about 1.7 degrees across; but keep in mind that then the Exit Pupil (diameter of the cone of light coming out of the eyepiece) would be greater than 11 mm and so for most people this will not be as good a choice as using an eyepiece that provides an Exit Pupil of about 7 mm or less. The most desirable high performance low power eyepiece for the Lightbridge™ 12 would be the TeleVue 31 mm Nagler Type 5 eyepiece (nicknamed by the astronomy community the "Terminagler") which would provide 1.6 degrees field of view at 49X with a more practical 6.2 mm Exit Pupil. For most people the Lightbridge™ 12 does reach focus with this monster eyepiece, although some people may need to buy an optional 2 inch extension tube for this and some other 2 inch eyepieces.

Left: Lightbridge™ Focuser with optional TeleVue 31 mm Nagler Type 5 eyepiece (49,158 bytes).
Click on image to see enlarged view (207,747 bytes).

Fast focal ratio reflecting telescopes tend to exhibit a subtle optical anomaly that makes stars towards the edge of the field of view appear elongated as though they each have a small tail pointing out and away from the center of the field of view. The average amateur does not see this or know to look for it. This can not be solved by eyepieces however, for those who seek even more perfect images at lower magnifications, consider the TeleVue 2 inch "Paracorr" Coma Correcting Lens. This lens housing resembles a short pipe installed between the focuser and the eyepiece. It will reduce the effects of coma and will simultaneously boost the focal length of this telescope from about 1,524 mm to about 1,550 mm thereby causing some reducing of the field. But more critically, one will have to consider the leverage effect of adding distance between focuser and heavier eyepieces.

The factors of aperture and the 2 inch focuser are particularly important to people who seek better views of the extended, faint deep sky objects (nebulae, star clusters, galaxies, comets, etc.). In effect the stellar magnitude reach is extended from about 13.8 of an 8 inch telescope to about 14.7 with this 12 inch. And still this telescope affords the contrast and clarity qualities required to observe the moon, and planets; the Lightbridge™ actually can reveal changes on the planets throughout the observing season! With a comparatively moderate central obstruction and fewer optical surfaces, the contrast and brightness can actually surpass more expensive designs including 8" and 10" Schmidt-Cassegrain telescopes.

Red Dot Sights The Lightbridge™ Standard telescopes include a simple red dot sight. This is a device that projects a red dot onto a transparent panel giving the appearance of a red dot floating in space ahead of the telescope. Unlike conventional refractive finder telescopes, the sight imparts no magnification and so one can see the entirety of the night sky making it easy to navigate by "star hopping" from one celestial reference point to another with the assistance of a wide angle eyepiece on the telescope. In order to use the Sight one installs it onto the telescope, then point the telescope onto a distant target centering the target in the telescope eyepiece. Now without moving the telescope, turn the sight dial to "on" and carefully adjust the Sight on its bracket up or down, and left or right so that the red dot is pointing directly over the target centered in the eyepiece. Now you can employ Sight along with charts of the night sky to find objects to view in the telescope.

Left: Lightbridge™ Deluxe red illuminated sight with provided rubber slip on cover shown above left (57,002 bytes).
Click on image to see enlarged view (172,681 bytes).

The Lightbridge™ Deluxe telescopes are provided with a more sophisticated red illuminated sight. This sight provides not only alignment adjustments, but brightness control and the choice of four reticle patterns: Red Dot, Cross, Bulls Eye, and Diamond. It is also provided with a protective rubber slip on cover; you might punch a small hole well into the flap of the cover and run a short string from it to the Sight so that the cover is not lost. Observer's may need to keep the sight covered when not in use to prevent dew or frost accumulation, or simply attach one of our Kendrick Dew Prevention Systems. The sight is installed onto the Lightbridge™ by a quick release mechanism and hand knob, so the sight may be quickly removed or installed with no tools.

The Lightbridge™ includes a convenient Accessory Tray which can be attached onto the side of the Carriage. This provides holes fitting as many as three 1.25 inch accessories plus one accessory of 2 inch diameter. Unlike accessory trays by Orion for example where their tray is has two keyhole shaped screw holes (customers may slide it up and out to remove the tray for transport), the Lightbridge™ tray is designed to be left fixed in place so use extra care not to strike it when hauling the Base about. And to keep dew or other contaminants from marring exposed eyepieces Company Seven suggests you keep protective eyepiece caps in place.

This is a telescope that will perform at its best only when the subassembly components are properly made and put together at the factory. Someone with little experience can decipher the translated, illustrated instructions and should be able to obtain satisfactory results. Judging by the assembly errors on this telescope, how far out of alignment the components were before we began to work to align the optics, we are left to surmise that even if the telescope is made well and assembled properly at the factory, some percentage of novices will not be able to assemble and collimate the telescope to obtain its best possible performance without some guidance. For those who seek the best possible result, Company Seven will offer optional assembly services so that you may pickup a complete and ready to go Lightbridge™.

Noteworthy features of the Lightbridge™ Standard telescope include:

A set of accessories provided to get you up and running on the first night out!

• Meade 26 mm Series 4000 Super Plössl 1.25 Inch Eyepiece (59X, 0.8 Degree Field of View)
• Red Dot Sight - makes locating objects quick and simple
• Eyepiece Holder with three holes for 1.25" and one for 2" eyepieces or accessories
• Easy On-Off Lightweight Plastic Slip Fit Primary Mirror Cover

A set of accessories provided to get you up and running on the first night out!

• Meade 26 mm QX Wide Angle 2 Inch Eyepiece 70 degree apparent field-of view (59X, 1.1 Degree Field of View)
• Advanced Red Dot Sight - dial to select any one of four reticle patterns and adjust brightness
• Three Part Steel Roller Azimuth Bearings (in place of standard Teflon bearing pads)

Innovative Design!

• Fast, compact f/5 design keeps the eyepiece height comfortable for most observers
• Compact profile takes up less space in your vehicle and so you are more likely to take it out
• Portable Truss arrangement with rolled steel tube construction with cast aluminum end rings
• Exterior is finished in durable white enamel, interior in antireflection Black
• Full thickness molded expertly finished Parabolic Primary Mirror made of Borosilicate Crown glass.
• Primary and secondary mirrors are aluminized and over coated with a protective transparent Magnesium Fluoride
• Primary Mirror Cell provides good ventilation for fast cool-down time, spring-loaded collimation thumbscrews
• Optics hand finished and tested to assure diffraction limited performance
• Silky smooth machined 2 Inch Focuser
• Focuser incorporates both drawtube lock and tension adjustments
• Slip Fit 2 to 1.25 inch Reducer, threaded for 48mm Filters
• Focuser and Reducer employ non marring clamps to secure accessories