Courtesy of
Astronomy
March 1991

PRODUCT REVIEW

The New Generation Telescope

The NGT successfully combines large aperture with an equatorial mount in a portable telescope.

by Alan Dyer

     Most backyard astronomers arenít long in the hobby before they begin their lust after large aperture. Big telescopes give bright views. They show familiar objects with photographic clarity and reveal faint objects beyond the grasp of lowlier telescopes.
     In the 1980ís telescope builders broke previous aperture barriers by designing Newtonian reflectors in simple, easy -to-build mounts. These "Dobsonians," named for the original designer, John Dobson, provided big scopes at low prices. Amateurs were in aperture heaven. But Dobsonians forced telescope users to sacrifice the one thing always considered essential to a "serious" telescope ó an equatorial mount. Dobsonians can not automatically track the stars
     Dobsonians canít be used to take astrophotos. Many amateurs would say, "So what? Iím just interested in looking." But as the Ď80s drew to a close, an increasing number of amateur observers found themselves wanting big aperture and tracking capability. And they wanted it in a package they could transport to dark sky sites.
     In response to the increasing demands of a sophisticated amateur community, Jim Burr, founder of Jimís Mobile Industries, designed what he has dubbed the New Generation Telescope. "I wasnít interested in building another Dobsonian," Burr told us. "Itís not my nature to build what everybody else is building. I want to build something that stands out in the marketplace." The NGT certainly does that.

Assembling an NGT

     The NGT-18 has an 18-inch f/4.5 Galaxy Optics mirror housed in a type of equatorial mounting unfamiliar to most backyard astronomers. The "split ring" mount incorporated into the NGT has long been a standard design for professional observatory telescopes and has been used by some amateur telescope makers. The design features a large horseshoe-shaped ring that forms the bearing surface for the right ascension motion. In the NGT, this surface is a metal ring 36 inches across, a size that ensures a very stable yet smooth-turning axis. The split-ring mount also has a much lower center of gravity, and is therefore more compact, than traditional fork and German equatorial designs.
     The bottom unit of the NGT contains the split-ring mount, the drive, and the "tub" that holds the mirror and cell. The cell is an ingenious design that uses an 18-point flotation system that does not have potentially distorting edge clips; instead the back surface of the mirror is glued onto eighteen metal cups. Side handles on the cell allow you to carry the mirror-and-cell combination as a single component.
     The assembled bottom unit weighs 163 pounds. This is a hefty piece of equipment. For transportation, the upper end unit nestles into the mirror tub, requiring a storage space for the packed-up telescope of 36 (width) by 30 (depth) by 32 (height).
     To lighten the load, you can remove the 58-pound mirror (still attached to its cell) from the tub. The mount can be broken down still further by pulling the 60-pound tub split ring assembly off the 45-pound steel base. This base has a rocker assembly for tilting the mount to various latitudes. The normal range is from 30ļ to 55ļ, but custom versions are available for other latitudes. Disassembled in this fashion (see the photo on page 82), each of the pieces is light enough for one person to carry, though not without some effort. A set of optional rubber wheels is available to aid in moving the telescope.
     The eight 47-inch poles that form the tube snap into ball-and-socket connectors around the rim of the tub. The cylindrical top end that contains the secondary mirror, focuser, and 8x50 finder scope bolts onto the paired tube poles. My first cautious attempt at assembling the NGT took about 20 minutes, but subsequent setups took half that time, remarkable for a telescope of this size. Unless you are very short, the entire assembly can be done without a ladder, a welcome feature.
     The principal frustration I encountered was that the pairs of poles tend to flop down easily. This can create a juggling act as you hold the 11-pound upper end in mid-air, grab the sprawling poles, and try to attach them.  The NGT needs a better method of keeping the poles paired together and a means of attaching them to the bottom tub that hold them firmly positioned on the vertical.
     The adjustment for the latitude setting is a knob located on the steel base. I found that before the tub and mirror were installed this adjustment was relatively easy, but once the weight of the whole scope was resting on the rocker base, I could not turn the latitude knob. To do a fine polar alignment, I had to use the leveling screws on the four corners of the base to adjust the height of the mount.  To adjust the polar alignment in the east-west direction I had to nudge the whole scope from side to side. Considering that the big benefit of the NGT is its excellent equatorial mount, I found the inconvenience of precise polar alignment (and the lack of any instructions for polar alignment in the manual) a deficiency.

Using an NGT

     Once you have it set up, the NGT is a pleasure to use. The maximum height of the eyepiece at the zenith is a convenient 78 inches, extremely low for such a large telescope. For most of the sky, no ladder at all is required to reach the eyepiece. For added convenience, the top half of the upper unit rotates to place the eyepiece at a comfortable position. The rotation motion was not as velvety smooth as I would have liked, but it worked fine and maintained collimation no matter where I placed the eyepiece.
     While the NGT doesnít balance with extra-heavy eyepieces (unless you attach some optional counterweights on the poles), you can place enough friction on both axes to stop the scope from falling when itís out of balance. The axes are still loose enough, however, to allow you to move the scope from target to target. The motions were smooth and without slop or jerkiness. Once you are on target you can lock both axes if you wish.
     However, while I felt that the mount itself was very solid, the all-metal scope still exhibited an inordinate amount of shake. After a good rap near the eyepiece, the vibration took 4 to 5 seconds to completely die out; plus the vibrations damped out gradually. The main source of the resonance seemed to be the spring clips that hold the bottom ends of the poles.
     The DC motors on the right ascension and declination axes are powered by two 6-volt gel cells that are built into the base. Built-in batteries are wonderful! I found that after the recommended overnight charge, the batteries powered the scope for about ten hours. As an alternative, the scope can be powered from a car battery.
     The NGT comes with a control paddle that allows you to guide the scope for photography in right ascension and declination using a 2x speed override or to slew the scope with a 4x speed. However, I feel that a 4x speed is still too slow for scanning and slewing. An 8x or 6x override is far better, especially on a big scope like the NGT where it is impractical to have manual slow motions.
     The NGT does not come with setting circles. Instead, the 18-inch model is supplied with Jimís Mobile DSC digital circles. There is an indicator and scribe mark on one declination axis to mark 90ļ to aid in polar alignment, a nice touch. Also standard on the NGT-18 is a variable speed motorized focus (the accessory that made Jimís Mobile famous). Once tightened properly it had little backlash or "deadband" but did jar the focuser drawtube when it reversed direction, causing a slight image shift.
     The collimation adjustments on the primary mirror require a wrench. Again, I would prefer to see the instruction manual provide detailed collimation instructions, as well as a set of collimation tools supplied as standard equipment. I did find that after reassembly from a complete breakdown (including removal of the mirror/cell unit from the tub), the scope was out of collimation, requiring 10 to 15 minutes of tweaking of the primary mirror to set it right.
     Another surprising deficiency was the lack of a fabric cover or "light shroud" for the tube, and essential item for keeping stray light out of any open tube telescope. Stray light can also enter directly into the eyepiece from the opposite side of the tube, lowering image contrast and fogging long-exposure photos. Burr told us a shroud and tube extension baffle will be offered soon to correct these problems.
     Now, the bottom line: what are the optics like? In a star test, I found the optics to be excellent. Out-of-focus star images were nearly identical on either side of focus, with no trace of astigmatism and no obvious spherical aberration or zonal errors. On our scale of A (excellent) to E (junk), the NGTís Galaxy Optics mirrors rate an A-. This is a telescope that will provide superb images of all subjects. The generous size of the secondary mirror (a 4.5-inch minor axis) is optimized for 35mm astrophotography. It provides a fully illuminated one-inch diameter field. Vignetting and coma would be visible, however, in photos taken with medium format cameras.
     For any prime focus deep-sky photography, youíll have to use an off-axis guider since the open framework of the NGT does not lend itself to the attachment of guidescopes or even piggybacked lenses or Schmidt cameras. But if fabulous prime focus shots are what you want, the NGT should do the job.
     In fact, one enthusiastic NGT owner and astrophotographer, Dave Young, wrote to tell us that his NGT-18 "was the best toy Iíve ever bought. My last few photo sessions have almost been religious experiences." Well, we canít promise that an NGT will change your life, but if you are looking for a transportable telescope with aperture and tracking capability, the NGT is an excellent choice. While it still has some rough edges to iron out, the NGT is the best equatorially mounted telescope weíve seen in the big aperture league.
     PS: If the NGT-18 is a bit rich for you, Jimís Mobile offers an NGT-16. It uses exactly the same mounting and tube hardware as the 18-inch model (so it is no smaller or lighter) but uses a 16-inch f/5 Galaxy Optics mirror. The NGT-16 sells for $7,400. They also offer a 25-inch for $22,500.