G-11 Tuning

Preface:

This is not an official G-11 page, and there are differences in mounts delivered over the years.

Written down here are my own experiences and hints I got from other G-11 users. My mount dates 1997, in the meantime Scott Losmandy has made some changes. If you consider something obsolete, please feel free to inform me. I do not know about the newer models.

Nobody should get me wrong, I consider the Losmandy G-11 a very good, stable and nice mount. I recommend it fullheartedly and I'm sure I'd found something to grumble about with other mounts too. But it is mass produced and needs some care and adjustment. At last, you'll have much value for the money. And it allows you to put another OTA on it anytime.

1.) Backlash at the Elevation Axis.

Who of us doesn't like to shake a mount to see how stable it is. I did that during my first observations with the CG-11 and I was rather astonished that the elevation axis had a backlash of more than a half degree. Exactly said, toward north pole, because in the other direction the force of gravity works.

That may not be very disturbing when you use the mount at middle latitudes. The farther north (or south), the more the barycenter moves over this axis, decreasing the momentum you need for shaking the tube. This effect will get worse with a long tube, e.g. a refractor or a Newtonian.

I was told that this problem is well-known with Losmandy. The recommendation is to tighten the Allen head screws on both sides of the axis very firmly to increase friction.
You must be sure to make the last adjustments of the latitude toward the pole.

Having done precision engineering formerly, I don't like this kind of workarounds. Unfortunately, the design don't give us a chance to adjust something to minimize the backlash.

The problem arises from the difference in size between the internal brass bolt on the threaded rod of the Altitude Adjustment Knob and the two forks of the upper part of the mount in which it moves. The brass bolt is a piece of the bar and I found the two forks to have different opening measures.

I tried inserting some foils between the bolt and the forks. While the backlash was substantially smaller, some wobbling resulted.

I finally unmounted the brass pin and increased its diameter with soldering tin, on each side as much as needed for a tight fit. This can be done easier on a gas flame, f.i. with a small propane/butane gas bottle for plumbery than with a soldering iron. One should protect only the internal thread, so that no tin runs in.

I am very happy with the result. I used the G-11 often since this modification and noticed no backlash in this axis anymore. Pole height adjustment is very easy now.

Other fixes I know of:


2.) The Pole Finder.

There are two finderscope types: one from Losmandy and one from Celestron. If your borescope contains the words "Put Polaris Here" on the reticle, you have the Celestron version (like me) and kind of bad luck.

During polar alignment I found it annoying to accomodate my eyes focus between the etched drawing and infinity when looking at Polaris. Having had enough trouble with adjusting the etched reticle of the C-11 finder scope due to misfitting threads covered with lots of grease, I decided to do it the simple way: I put an rubber 0-ring used in plumbing fixtures on the end of the eyepiece's thread to keep it in the right distance.

To dim the brightness of the illuminating LEDs of both finders, I combined a cord switch with a variable 2.5 kOhm resistor (and some 100 Ohm in line). These switches are attached with Velcro to the mount respectively the finderscope mounting bracket.

I also use this dimmer as a power supply for my illuminated reticle eyepiece.

BTW: you can (with caution) use the three little Allen screws for exactly aligning the reticle plate of the finderscope when the lines crosses not exactly in the middle. Just point to an object and rotate the borescope. If the objects won't stay in the middle of the pole cross, you should adjust the reticle to fit exactly in the axis. Mine came pretty good adjusted, but you can save some time when drift aligning next time.

BUT: with the Celestron "Put Polaris Here" version it is impossible to match the stars exactly with the reticle pattern. The size of the reticle and the focal length of the objective do not match. You can only try to remember the places they should be.

3.) Vibrations from the Stepper Motors

I heard them (a characteristic clicking sound), I felt them with my fingers and I finally saw them when observing at high magnifications (300x und 600x). Cassini's division was more clearly visible with the tracking motor off. This effect was very obvious at daytime too, looking f.i. at antennas or wires.

What caused the vibrations ? All steppers show oscillations every step that must be damped by the design or the controling electronics. These oscillations can also lead to resonancy effects increasing their magnitude. BUT: sometimes things are much simpler: The step angles of the 48 halfsteps per revolution were not equal. In fact my RA stepper made: 2 little steps forward, one little step _backwards_!! and then a big 30° step forward. At high speed the steppers ran smooth and quiet, at tracking speed, about 2/3 rev./second I saw this behaviour.

The steppers of my G11 use an economical can-stack construction. The poles and coils are devided into two separate units which are stacked in a common can. It is obvious, that alignment between these stator sets is critical for the stepping angles. Alas, the design provides no means to achive and keep accurate adjustment. In fact, my outer stator could be moved easily without any tools.
BTW, for the experienced observer: the figure shows a stepper with 48 steps, the type used in my G-11 has only half as many stator poles and therefore steps.

I opened the cover of the motor and took the outer stator out of the can. Then I adjusted the upper poles of the inner stator as exactly as possible between the lower poles. Then I inserted the lower poles of the upper stator, adjusted it in the middle of an air gap of the lower stator, layed the coil in and added the outer poles.
Well, that was all I had to do. Instead of the 12 big tumbeling steps I saw before I had the 48 little smooth steps that should always have been there. But it took me months to get brave enough to disassemble the steppers.

4.) Backlash in RA and DEC

Both worm gears needed adjustment because of excessive backlash. They are not spring loaded, so you have to align the worms manually. Stefan Ritter explained adjustment at the ITT star party in Carynthia/Austria and lent me his special pliers:


After dismounting the stepper motors (2 Allen screws) you have to open the cover of the worm. It is recommended to press the 2 bearing blocks together (simply done with the pliers) and then adjust the worm tangential to the wheel. Tighten it slightly.
Now you have a great opportunity to "run in" your worm gear. I took an electric screw driver and drove the worm as far as possible to both sides. Readjust the worm. Repeat the procedure several times.

When reassembling the steppers, try to get the worm axis and the axis of the stepper reduction gear exactly in line. It can make movement smoother and is better for the little cardan joint.

Of course you can align the worm without using a special tool, too. With some practising, it is easy and you don't need to disassemble the steppers. Once a year f.i. you can repeat this procedure.

After the last adjustment check wether the backlash is gone and the steppers have enough torque to move at the highest slewing rates. Check at different points of the worm wheel.

It is better to live with a small amount of backlash than to introduce too much force between worm and wheel. In RA, tracking will take up backlash. You'll have to assure, that the worm has to work against gravity by keeping the scope slightly off balance. This will give good contact between the flanks.

In DEC, it is even more complicated. If there is some force between worm and wheel, the wheel seems to be pressed slightly off center. When making alternating guiding corrections, you can experience a short, quick movement of the guide star in the opposite direction, before it moves into the direction you want. TVC will not help you. If that happens, you have to allow some backlash to remove this nasty effect.

5.) Declination Setting Circle

The Allen screw for fixating the Declination circle, as mentioned in the manual, was missing. I dismounted the circle, drilled a hole at the intended place between 0° and 10° and tapped a thread into. I've done that especially to be able to proceed the NGC-Max initialisation, which begins with "Set DEC=0".

But, as Paul H. Bock pointed out, the DEC setting circle doesn't sit on an immovable part of the mount, but on the coupling to the DEC stepper motor. Therefore, the alignment of the DEC circle is maintained only as long as you move the telescope manually, but there will be a misalignment as soon you slew in declination electrically.

This design limits the usability of the DEC setting circle to relative, manual movements, f.i. from an object you want to identify to a well known object somewhere. You can use absolute coordinates, when you put an index mark on the DEC circle and an immovable part, f.i. the cover of the worm wheel and realign before use. I inserted a long srew with plastic cap as used for connecting computer peripherals in the threaded hole I applied. This helps for quick alignment and smooth positioning of the circle.

The accuracy of a simple index mark will be somewhat below the index vernier, so make this mark very sharp and fine and be sure to apply it after you carefully adjusted the DEC setting circle - a good job for a fullmoon night. Maybe you should choose a good compromise between the two possible positions of a GEM when locating a point in the sky by moving both axes 180 degrees.

It's up to you to decide if it is worth the effort. If you starhop solely, don't think about it. If you use Digital Setting Circles principially, an index mark applied could be necessary and sufficient. If you want to be able to use the setting circle, you can follow the above proposal.

And, by the way, forget what Celestron's CG-11 manual says about the DEC setting circle. It was directed at a former design.

6.) Sticky Bearings

Is your mount's manual movement a little coarse ? Hard to make minor movements ? It's a common problem, that can be solved very easily. The grease that is found inside feels a little like glue. Just take off the clutches, remove the encoder gear, but remember the sequence of the washers. Then you can pull out the axes. Try to turn the needles from the bearings. They should rotate freely. If not, take some fuel or other solvent, clean everything (axes, all bearings), until the needles are moving smooth. Put in some lubricant which is adequate for the temperature range in which you are operating your mount.
I would advise to check the bearings even if you did not recognize this problem yet. The needles could be ground a little flat if fixated, also the axes can run in a little, introducing some play.

Please email me if you find errors, have suggestions or if you would like me to include other tuning tips.

Back to my CG-11 page, or to my Astronomy pages

or to the TFH Berlin.

Last modified: Jan 01, 2001