The New Generation Telescope
The NGT successfully
combines large aperture with an equatorial mount in a portable telescope.
by Alan Dyer
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
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
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