Losmandy GM 8 S German Equatorial Mounting System

By Chuck Hawks with Gordon Landers

Hollywood General Machining, Inc. in Los Angeles, California USA manufactures Losmandy astronomical products. They have been making state of the art German Equatorial mounts for astronomical telescopes for a long time. Losmandy mounts are highly regarded among amateur astronomers. Indeed, they are considered the Cadillac of German equatorial mounts.

GM 8 S mounting system

The GM 8S mounting system that is the subject of this article includes the GM 8 equatorial mount head, weight shaft with 7-pound counterweight, LW tripod and digital dual axis drive system. The GM 8 equatorial head that is the centerpiece of this mounting system is the most popular of the Losmandy mounts. It is made entirely from machined aluminum and stainless steel with all stainless steel hardware and a black anodized finish. The 2009 MSRP of the GM 8 S mounting system is $1495 and it can be purchased from Losmandy dealers or direct from Losmandy via their web site at www.losmandy.com

A polar alignment scope ($180) with LED illuminator and a reticle suitable for use in both the Northern and Southern Hemispheres is an optional accessory. The alignment scope screws into the center of the mount after the mount's aluminum protective caps have been removed.

The Gemini Retro-Fit Kit ($1595-$1795) converts the GM 8 S to a computerized go-to positioning system. Another popular accessory is the VSP, a GM 8 saddle plate (shoe) for the popular Vixen style dovetail mounting rails used on Celestron , Orion, Vixen, Stellarvue and other popular brands of telescopes. A clutch knob set ($125) with three lever bars per knob makes setting the mount's RA and Dec clutch tension easier. Extra counterweights of 7, 11 and 21 pounds ($50-$100) are available. See the Losmandy web site for the complete (and extensive) list of GM 8 mounting system accessories. (All prices in 2009 dollars.)

Secured by two hex head bolts to the top of the mount is a saddle (shoe) designed to accept Losmandy D-series dovetail mounting plates. This is a quick release mounting system commonly employed for portable telescopes and mounting plates are available from Losmandy to fit most telescopes. In addition, Losmandy mounting plates for their instruments are available from many telescope companies. The standard Losmandy mounting saddle can be replaced by the VSP saddle for use with telescopes supplied with Vixen mounting rails.

The promotional photographs do not do the GM 8 justice. Seen in person, it is impressive and obviously machined to a high level of precision. It is also extremely rigid.

GM 8 head
Illustration courtesy of Losmandy Astronomical Products.

Here are the Losmandy GM 8 mount head specifications:

  • One 2.100" diameter needle thrust bearing.
  • One 2.000" diameter needle thrust bearing.
  • Two 1.500" diameter needle bearings.
  • 2.812" diameter, 7075 aluminum worm gear, 180 tooth.
  • Dual supported, ball bearing housed stainless steel worm-heat treated and ground.
  • Variable slip clutch, both axes. One knob design.
  • 1.250" diameter aluminum shafts.
  • Fully locking, unique tangent arm design altitude adjustment, 0-64 degrees, large knob for easy alignment.
  • Laser engraved latitude scale in 2 degree increments.
  • Single knob azimuth control, bi-directional +/- 8.5 degrees.
  • 3.500" diameter laser engraved setting circles 6 minute R.A. / 2 degrees DEC. R.A. circle engraved for Northern and Southern Hemisphere.
  • Porter slip ring design.
  • 160 oz/in stepper motor, 1.0 arc sec. step.
  • Spit Dovetail saddle-plate design. Accepts "D" series dovetail plates. Dovetail plate drops in and locks allowing for interchaning of any tube assembly, reducing the need for DEC weights.
  • Removable stainless steel weight shaft w/safety knob.
  • 7 lb. counterweight.
  • 11 lb. and 21 lb. extra counterweights available.
  • Through the axis polar scope, works in Northern and Southern hemisphere. Polar scope optional.
  • Instrument weight capacity 30 lbs.
  • Equatorial head weight 21 lbs.

The GM 8 comes with integrated dual axis drive motors and a right ascension/declination hand controller. It has variable tension slip clutches on both axis to allow manually aiming the telescope, but does not have manual slow motion controls. Minor aiming adjustments can be made only with the electric hand controller, by means of its four buttons (up-down in Dec and right-left in RA). This is a typical system for motorized astro telescope mounts, but I miss the manual slow motion controls that are standard on Celestron CG-4 and Vixen GP mounts. It just goes to show that there are no perfect mounts, but the GM 8 is close!

Digital dual-axis drive system

The GM 8 comes with electric motor drive for both the RA and Dec axis. This dual axis drive system includes the RA and Dec motors, as well as a Control Panel, compact four-way Hand Controller and all connecting cords. A 12 VDC, 500ma power supply is required to run the system (not supplied). The control panel conveniently attaches to a small bracket at the top of the LW tripod by means of a pair of thumbscrews and the hand controller is connected to the control panel by a coiled cord.

Electronics specifications:

  • Diamond push button pattern.
  • R.A. and DEC. reversing switches.
  • Three guiding rates; 30%, 50% and 2x sidereal rate.
  • Three setting rates; 4x, 8x, 16x sidereal rate and super fast 32x HST (high speed transfer).
  • Quartz tracking rates: sidereal, solar, lunar, king.
  • Periodic Error Correction (PEC).
  • Programmable DEC. Backlash Compensation (TVC).
  • Control panel dimmer.
  • Accepts auto-guider systems.
  • Northern and Southern Hemisphere operation.
  • 12 volt DC - 500ma power use.
  • Tiltable control panel for easy access.
  • Small hand control box for comfortable use.

LW tripod

The LW tripod upon which the GM 8 usually sits is a large, two section, box leg, aluminum tripod. It is simple and rather crude, with hand knobs to tighten the lower leg extensions. This tripod looks as if it was designed and bolted together in someone's garage, but it is very stable and reasonably lightweight for its size. Obviously, Losmandy does not have an outside tripod supplier.

GM 8 S complete mounting system
Illustration courtesy of Losmandy Astronomical Products.

Here are the LW tripod specifications:

  • All machined aluminum, black anodized.
  • Tripod height, adjustable from 27" - 43" (from ground to top of tripod, not including height of GM 8)
  • Fold-up tripod design for easy transporting.
  • Tripod weight 15 lbs.

Perhaps the best feature of the LW tripod is the large, straight-sided cup welded to its top. The GM 8 head slips flush into this cup and is held securely (as long as the weight shaft and counter weight are not attached) while the three bolts that lock the mount to the tripod are screwed into place through the sides of the cup. This is a more secure system than is used by the Celestron CG-4 and Vixen Great Polaris mounts, which slip over the head of their tripods and are more prone to toppling over before their central mounting bolts are tightened.

The friction leg locks are inside the large upper leg box sections, rather than being in a fixture attached to the bottom of the upper leg sections, as is typical of most tripods. This means that the considerable space occupied by the leg locks necessarily reduces the diameter of the lower leg sections, degrading the strength and stability of the tripod. (A tripod is only as stable as its smallest diameter leg section.) The upper leg sections are huge, but the lower leg box sections are only average diameter for a tripod of this overall height. (Note the difference in the size of the upper and lower leg sections in the photo above.)

The folding center brace is hinged to the three legs and threaded at the center for an optional accessory tray. This brace is loaded in extension only. In other words, it keeps the legs from sliding apart, but adds nothing to the rigidity of the tripod, unlike the compression center braces used in the equivalent Celestron and Vixen tripods. On the other hand, the LW center brace is more convenient, as it does not have to be removed to fold the tripod legs.

Set-up and use

GM 8 diagrams
Illustration courtesy of Losmandy Astronomical Products.

Assuming that you are observing from the Northern hemisphere, as we do, set-up the LW tripod with its "north leg" oriented toward the North Star and adjust the legs for whatever height you find convenient. Next, place the GM 8 head in the top cup of the tripod and rotate it so that the holes in the base of the mount align with the holes in the cup at the top of the tripod. Secure the mount in place with the three supplied mounting bolts. Level the tripod using the two built-in bubble levels.

Set the mount's altitude adjustment, a single large hand knob at the front of the mount, for the latitude of your observing site. Loosen the mount's azimuth locking bolts (thumbscrews on each side of the mount) and center the mount using the left and right worm-drive knobs at the back of the mount. (If you are using the optional polar alignment scope, these altitude and azimuth adjustments are used to precisely polar align the scope.)

Screw the counter weight shaft into the bottom of the mount's declination axis. Unscrew the safety stop from the end of the shaft and slide-on the counterweight, tightening its locking bolt to hold it in place, and then replace the safety stop screw. (I always mark the shaft for the proper location of the counterweight so I don't have to rebalance the scope every time I set-up the mount.)

Attach the electrical control panel to the back of the tripod/mount system with the two supplied wing screws. Select the tracking rate (usually sidereal) and slewing speed. (I start at 16x.) Connect the RA and Dec control cords and the hand controller. Then connect a 12 VDC power supply. (A Celestron Power Tank works well.)

You are now ready to attach your telescope to the GM 8 using the Losmandy quick release plate you have previously attached to your scope. Loosen the knob at the side of the mounting platform so you can slide in the telescope's mounting plate. Slide the plate into the mount's shoe and tighten the knob to secure it in place.

If you have eyeball aligned the north leg of the tripod in the direction of the North Star with reasonable accuracy, you are now ready for visual observation and a more precise polar alignment should not be necessary. If you feel the need for more accurate polar alignment, I suggest purchasing the optional polar alignment scope and following the supplied directions.

Finally, slide the Control Panel's power switch to "on" and adjust the RA and Dec clutch tension knobs for comfortable manual slewing without creep. The RA adjustment knob is concentric with the polar axis finder and the Dec clutch knob is at the top of the counterweight bar. For more details, refer to the supplied instructions.

The Losmandy GM 8 / LW tripod mounting system works exactly as it should. It is rock steady and the telescope is held securely. With a Stellarvue SV105R Raptor (a 4" refractor) mounted on the GM 8, we manually slewed the scope around so that it pointed at whatever object we chose to observe and used the hand controller to refine our aim. Tracking is automatic, you don't need to turn the motors on and off or fiddle with clutches to manually slew the telescope from object to object. In use, the GM 8 is a simple, straightforward and very precise German equatorial mount. No one with a basic understanding of how German EQ mounts work should have any trouble using the GM 8.

Conclusion

The Losmandy GM 8 is a superior mount for its size and weight. I'd give it an "A" rating. The LW tripod is crude, but strong and it gets the job done. It gets a "C" rating, because of its lack of sophistication, not because it is unsteady. Somewhat surprisingly, this made in the USA, GM 8 S mounting system is priced competitively with equivalent mounting systems made overseas. If your telescope weighs less than 30 pounds and you are looking for the Cadillac of German equatorial mounts, you should checkout the Losmandy GM 8 S. Those with instruments weighing 30 to 60 pounds will be interested in the larger, but similar in concept and operation, Losmandy G-11 mounting system.




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Copyright 2009 by Chuck Hawks. All rights reserved.



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