Tenmon Guide
April 1992

An English Translation of

New Face Test Report–4


45cm Aperture Newtonian Equatorial Telescope

Conducted by Kiyoshi Ishiwatari and our Staff

(Translation by Barry Gooley)

     The NGT-18 equatorial reflector, manufactured and marketed by Denver's JMI, looks at first glance, like a simple alt-azimuth dobsonian. However, in reality it is a new wave, honest to goodness large aperture equatorial mounted reflector that exhibits plenty of potential. Trying out the scope after first ridding ourselves of the stereotype that "it's just an advanced dobsonian", we found that the scope stands out from the crowd, having performance enough to really set it apart from the pack of large aperture reflectors on the market.

NGT-18 is a Rim-Driven Horseshoe Type Equatorial

     When people learned that the NGT-18 was going to be in the New Face Test Report, our staff was bombarded with lots of questions, feeling again the high level of interest in the scope. However, the questions tended to revolve around things like "how is that dobsonian?" or "an equatorial dobsonian is pretty neat, huh?" Therefore, although it bears a striking resemblance to a Dob, let's make it clear that the NGT is not a modified dobsonian type telescope. It is a fresh, new style large aperture equatorial reflector, utilizing a rim-driven, extremely compact horseshoe equatorial mount and a truss supported giant size 45cm tube assembly.
     The horseshoe type equatorial mount, derived from the yoke mount, places the DEC axis inside a horseshoe shaped drive ring, a little larger than the tube diameter to allow viewing even the far northern sky. Starting with the Palomar 5 meter scope, this design is used in large aperture equatorials all around the world. Horseshoe equatorials usually end up taking up all the space inside the observatory but, with the portable NGT-18, the DEC bearings are placed as for north as possible, resulting in an extremely (page 3) compact appearance. In an effort toward even more compactness, the tube assembly inside the ring has counterweights on the underside of the mirror cell, with the DEC axis bearings placed as close to the primary mirror as possible.
     A rim drive is also called a friction drive and does not utilize a worm wheel and gear system. This design has been seen on homemade telescopes in the US for a long time and is simple and economical, as can be seen in the illustration (on previous page).
     NGT-18 combines a rim drive plus horseshoe in an extremely well thought-out and well-finished mount that supports a 45cm tube assembly that can be broken down and reassembled. As the mount is quite compact, its resemblance to a dobsonian is a bit to its disadvantage.

Assembly Time is Five Minutes
An Incredibly Strong Equatorial Mount

     We heard from the NGT's Japanese dealer, Kokusai Kohki, that a brand new NGT-18 had come in so we went to take a look, hauling the scope up to an observing site in the nearby mountains. In this latest scope, a brand new focuser of original manufacture had been installed and screw holes placed in the lower truss supports, that allow the truss rods to be cinched down if needed. For the trip we took along the comet hunter Tsuruhiko Kiuchi. Kiuchi had been thinking of purchasing an NGT for comet hunting and was allowed to take the scope back with him to his home in Nagano for a longer test. We used some of his comments in compiling this report.
     The NGT-18 breaks down into the following four parts: (page 4)

          1) Equatorial mount and tub assembly
          2) 8 truss rods
          3) Upper diagonal cage with focuser
          4) Accessories and options

     The biggest part of the scope by far is the mount and lower tub assembly with its mirror, cell and counterweights (under the tub). The business end of the scope is all located here. As the drive ring is 91cm in aperture, it probably won't fit in most passenger cars. A van is recommended for transporting. A light van would probably hold a couple of these telescopes.
     Accessories include the electric focus and focuser controller among other small parts. The power supply comes from batteries enclosed inside the electrical panel of the drive base, and drive rollers and digital circles can be transported installed as-is on the scope, for easy takedown.
     Upon reaching the observing site, we first lowered the equatorial drive base on the ground. The truss rods are inserted with a "thunk" inside spring plate type holders that appear like ball joints around the top of the mirror tub. This method allows not only extremely fast assembly but also will probably allow freedom from having to realign the collimation every time. Strength of the plate springs seemed a little weak at first but the rod and ball joint combination proved surprisingly strong. If you still feel any uneasiness, it's possible to bolt the spring plates up against the truss rods ball.
     After the truss rods are installed, the upper diagonal cage is installed with four bolts and then you're done. This entire process took only about five minutes.

An Excellent 45cm Mirror Gives Sharp Images

     The rim drive equatorial proved to be much more rugged than we had imagined. Drive is with stepping motors turning one of two rollers that the drive ring rests on. In this section, a clutch transfers (page 5) the motor rotation to the roller. The DEC axis uses a tangent screw.
     Polar alignment uses the RA/DEC digital setting circle, aligning on a star of choice. However, except for photography, this DSC does not need to be polar aligned so it's possible to use the scope as an alt-azimuth mount. Also, although other DSCs are in use, the NGT-18 marketed by Kokusai Kohki uses their original Navigator setting circle.
     The Navigator comes with a 110 Messier data base, a star database, 1500 other objects plus a user settable data base. Checking an object's RA and DEC, it's possible to find objects one after another. With the mount's smooth motion and the revolving diagonal cage assembly, using the scope was a pleasure.
     The 45cm f/4.5 is made by the Denver maker, Galaxy. A null (zero) test performed when the mirror cooled off revealed a perfectly flat image with no zones. A mirror of this quality is pretty unusual. Due to the f/4.5 ratio, images degrade away from the center of the image but the central area was plenty sharp with lots of fine detail visible in Jupiter's disc.
     However, for a while after taking the scope out of the warm car interior, images were poor while currents wafted up from the primary mirror. Also, when the mirror first started its acclimatizing to the outdoor temperature, its changing shape resulted in seeing stars double for a short while. Checking with (page 6) a knife edge showed not only currents rising from the primary but currents emanating from hot spots on the diagonal holder. All this, not to mention upper atmosphere scintillation, resulted in some pretty blurry images. Even using the mini fan, it probably took a full 3 hours for the mirror to completely cool down and give sharp images. That (sigh) is the lot of large aperture mirrors.  Instead of a portable scope, put it in a dome to really use it.
     The NGT18 is a high quality large aperture reflector telescope with no major faults. The optics are excellent and the digital setting circles worked very well. This much potential poses a threat to all the large German and fork type equatorials out there.
     As we had thought of the scope as only for visual use, we didn't perform a periodic motion test, but by visual reckoning, tracking precision is not bad. We felt that the scope should not be limited just to visual observing, but be permanently installed inside a dome for hard-core prime focus photography. Yen 1,880,000 is not cheap, but for astrophotography, using other large aperture scopes costing much more, makes success a real problem due to poor tracking precision and lack of tube rigidity.


     I am involved in comet searching with giant binoculars.  I had been thinking recently that planned visual observations of comets far from the sun was impossible when I saw an ad in this magazine for the NGT-18 ('sure would like to take a look through one...', I thought).
     When the NGT-18 was selected to appear in a test report, I asked to tag along.  From a visual observer's point of view I attempted to test the instrument.
     Conditions affecting an instrument suited for visual searching include sturdy and smooth movement, an easy to reach focuser, wide field, sharp, settled star images and finally, overall simple construction.
     The mount uses a horseshoe design.  I am mainly concerned with visual use but the design is so compact that I never felt I was moving the mass of 45cm of aperture.  Also, with the exception of the truss rods, the entire scope compacts into a single assembly, allowing it to fit inside a mini van.  Moving the telescope was extremely smooth, very much like using a Dobsonian.  The focuser with the horseshoe mount was easy to reach with no worries, unlike German and fork type equatorials where the focuser can end up in places where it's difficult to observe in safety.  Also, the diagonal cage assembly revolves, allowing the viewing position to be freely adjusted.
     Stars were sharp and clear.  Stars were a little distorted near the edge of the field but hardly enough to be annoying when observing visually.   Rather than using it as a portable scope, I would prefer to use this incredible telescope permanently installed inside a dome.

Tsuruhiko Kiuchi