The photograph below shows the Azimuth Bearing. I have already discussed the pros and cons of the standard Teflon pad bearings so I will not repeat that information here, but I would advise you to have read that page before you proceed as it provides more details about the problem, alternative solutions and some warnings to take heed of.
Remember that while you are trying your hardest to free-up the motion of the Azimuth bearing, stiction does have a purpose! You need it to hold the scope sufficiently still once aligned, in case gravity, you or the wind nudges the scope accidentally. Some people sill swear by Teflon or Magic Slider (teflon equivalents) bearings as giving the right balance between free motion and stiction.
DOBSONIANS are intended to be simple, cheap mounts. If you wanted high precision, you should have paid for a quality EQ mount!!!
But also just a positive sidenote: A (heavy wooden) Dobsonian mount is much more solid and often far less prone to vibrations than EQ mounts!
An example of the kind of issues discussed can be found on this topic thread at the Cloudy Nights Forum:-
Cloudy Nights Thread Link: Lazy Susan for Intelliscope
These issues can be summarised as:-
- Installing a Lazy Susan bearing can make the Azimuth movement too free, so this requires some form of friction mechanism or brake; centre bolt tightening, clutch mechanism, or simply some carpet.
- Concerns that tightening the central bolt may affect the operation of the digital encoders so preventing the Intelliscope Object Locator from working correctly.
- Ball bearing race mechanisms could develop flat spots.
- Type of bearing; ball or cylinder.
- Does the telescope have to be on level ground? i.e. gravity tries to move the scope to lowest point of weight distribution.
- Sourcing a good supplier of the right kind and dimensions of lazy susan that will work well with the Skyquest XT range.
- Will the addition of shimming washers be necessary, and will this affect the digital encoders?
- Addition of Lazy Susan should be non-destructive, i.e. the original Teflon pads can be left intact, so reverting to the original Teflon bearings is possible.
- Some folk still prefer Teflon or improving by replacing with EbonyStar Laminate for the azimuth bearing.
Thinking about a brake mechanism I have visualised some kind of spring-loaded clutch operated by a lever or handle which would normally be on, but when pulled would release the brake off, so allowing free movement. The trouble with this idea is that when aligning the scope through the finder scope or eyepiece, you might find that the very action of operating a lever would actually cause you to move out of alignment again, because at high magnification the tiniest movement can cause the object being observed to move out of the field of view. I keep returning to the KISS notion - Keep It Simple Stupid! Often the simplest things work the best.
Bearing Flat Spots
The bearings do have a notching effect sometimes, i.e. you move the scope but the bearings want to roll it back or fore slightly before it rests. While this will depend partially on the quality of the bearing, this might also be due to me making a silly mistake when installing the lazy susan. I drilled mounting screw pilot holes, with the bearing in place, and blew the resulting sawdust away, but some got into the ball bearings! Ooops. So something to be careful of for would-be modders.
This could account for the idea of "flat spots" mentioned by others, meaning they are NOT flat spots, but more likely to be dirt or particles which have got into the bearings. I think the low cost of the lazy susan might warrant a 2-yearly renewal?
The Lazy Susan bearings are generally not atall expensive, so any worries regarding flat spots or sticking of the ball race are of minor concern. If the bearing develops problems it can be replaced at minimum cost.
After installing my own LS (Lazy Susan) bearing, plus "carpet" brakes I did a centre of gravity test to see how much tilt was required from dead level before the scope became unstable and started to move "downhill". I placed various thickness items under one of the base feet, and when I used a book about 3/4" thick it was then that, with the scope tube at horizontal altitude, gravity would cause the scope to overcome the friction of the "carpet" brakes and rotate downward to its lowest point. I think this equates to a tilt of between 7 to 8 degrees. Normally I use my telescope in my backyard on the patio which is dead level.
A properly balanced scope will help alleviate any of these problems. Use of counter-weights to balance each end of the scope will help.
One of the main concerns people have with making this modification is the potential for damaging the tiny digital encoder chips on the azimuth circuit board. The fear is that of the magnetic disc coming into contact with either of the two electronic sensor chips that protrude slightly above the circuit board.
However it should be noted that the brass bush has dimensions designed to ensure that the exact required gap is maintained between the magnetic disc and the encoder chips, provided the central bolt is tightened to the correct torque (until the top washer just stops turning freely, then 3/16th's to 1/4 turn beyond that [taken from the manual] ), AND the magnetic disc is in contact with the bottom base board so that the disc is static while the encoder circuit board (and the scope) rotates above it.
When everything is tightened the disc is held firm against the bottom board, which sits on the ground.
The encoder chip circuit board (and the whole telescope) rotates above this, around the brass bush.
The brass bush should make it impossible to push the magnetic disc closer to the chips than is required. The only way damage can occur is;
- If the central bolt is done up far too tight so as to squash the brass bush, but this would require considerable, even excessive force.
- If the magnetic disc somehow becomes misaligned and tilted so as to come into contact with the chips, but again I think this would require considerable mishandling of the scope, or not enough care when re-assembling the azimuth bearing, for instance letting the magnetic disc slip off the brass bush registration feature, and not realising this has occurred, then proceeding to tighten the central bolt (see my notes on care when assembling the azimuth bearing). This could result in bending the magnetic disc, particularly if the weight of the scope has been allowed to bear down after incorrect re-assembly. Careful re-flattening of the disc may fix this.
- If additional washers are inserted in the wrong place (with the intention of bearing some of the scopes weight, so relieving weight from the Teflon pads), and this in fact causes the "critical gap" to widen. For example adding milk-carton or CD disc washers between the encoder circuit board and the brass bush, or between the circuit board and the top baseboard.
I know from research that some people have reported the Intelliscope not working properly after making modification to the bearing. Often the user thinks they have damaged the chips somehow. Personally I think it would take considerable force and/or rotational friction of the magnetic disc on the chips to actually damage them! Just be careful ;-)
My gut feeling is that by some mistake they have either caused the critical gap to widen or close up, rather than actually causing damage, but thinking they have damaged it, request a new encoder board without attempting to revert to the original build to see if it still worked.
The important thing to understand here is that the magnetic disc does NOT rotate. It is held stationary against the bottom base board, while the encoder circuit board (which is screwed to the top base board) rotates above the magnetic disc.
The key thing then, is to ensure that when a Lazy Susan bearing is inserted, which due to its thickness usually results in increasing the size of the gap between the top and bottom base boards, that additional packing in the form of extra washers needs to be inserted underneath the magnetic disc to hold it up against the brass bush.
This action must do two things;
- Press the magnetic disc upwards from the bottom base board so that its hole registers properly into the brass bush registration feature, also ensuring the disc does not slip off the brass bush (the disc simply slips over the bush, it does not screw on or attach in any way).
- Maintain sufficient friction against the magnetic disc so that it cannot revolve against the bottom base board, i.e. the magnetic disc and bottom base board become as one fixed unit.
Principally I am suggesting that most problems people encounter with this mod, are as a result of the magnetic disc slipping in relation to the bottom base board, or not being held upwards close enough to the encoder circuit board. Only occasionally is it due to actual damage.
Take it easy when making this modification and you should be fine. Don't force anything, and think carefully about what you are doing.
Problems only occur if poor assembly results in the two base parts becoming skewed off axis, i.e. perhaps the central nut is not done up tight enough which allows too much wobble of the two base sections to each other. If anything, a lazy susan makes this less likely to happen since it provides a wider diameter for support of the top base onto the bottom.
I do sometimes feel that the Teflon pads and stiction approach, if maintained well, offers better control for wide viewing. Its just the high magnification that requires the smoothness of a ball bearing.
I'm sure Dobsonian designers like Orion have considered ball bearings but felt that Teflon was a better approach. I don't think Teflon is particularly a cheaper method, unless a high quality lazy susan were used.
Generally I much prefer the smoothness of the lazy susan, but it can sometimes be annoying having to move so gingerly around the scope if I have aligned it on something, and want to change the eyepiece for example.
The LS I used raises the top baseplate slightly above the Teflon pads by about 1.5mm, so the pads are no longer in contact. This also means the mod is non-destructive - I can remove the LS and have it working in the normal Teflon way quite easily.
Installing the Lazy Susan bearing
So enough with the theory and chat. Let's get on and install the new bearing.
First mark centre lines with pencil at 90° to each other exactly at the centre of the main central bolt hole, and far enough outward to mark the diameter of the bearing. (It's easier if the central bolt is removed completely - also note the rubber washer in the photo below is something I have added, and this is explained further below).
Measure the diameter of the central opening of the bearing, then measure and mark this equally on each side of each centre pencil line. Offer up the bearing over the pencil lines, and align it dead central as accurately as you can. On my LS the 4 mounting holes showed the pencil line through them which helped make the alignment easier.
If you don't get the bearing dead centre this could cause binding resistance when the scope is rotated.
Mark through the mounting screw holes, then remove the bearing before you start drilling else sawdust could get into the bearing rollers.
Drill Pilot Holes
With chipboard I think it is much better to carefully drill a pilot hole half the diameter of the screw you intend to use, and only as deep as the screw itself (no need to drill right through). A pilot hole makes it easier for the screw to get started in the chipboard, and doesn't tend to distort the chipboard so much when the screw is tightened up, and this results in a much more secure fixing.
Here is a shot of the bearing mounted and screwed into position:-
New Spacing Washers Required
Here is a side profile of the bearing, which is 8mm high, and results in the top base board being lifted about 1mm clear of the Teflon pads. This implies that there will be a slightly larger gap between the bottom and top base boards, so an extra couple of washers need to be added to ensure the magnetic disc is held firmly against the bottom base board.
I found that a combination of a largish diameter rubber washer super-glued onto a metal washer provided the extra gap filler, while the rubber washer allows some compression for when the central bolt is done up. I super-glued it because the central hole in the rubber washer was far bigger than the diameter of the bolt, so I needed to stop it moving around.
Another side view of the Lazy Susan. I use a light silicon-based lubricant to spray into the ball race.
Re-arrange Order of Central Bolt Components
The next two photographs show the order of the various washers on the central bolt.
Original order: This photo shows the bolt with its component parts in their original order before adding the LS bearing (ignore the additional parts in top-left of photo).
Re-arranged for Lazy Susan mod:
This second photo shows the parts divided into their relative positions above, between and below the top/bottom baseboards.
Top section: Note that I have moved one of the original large metal washers to the middle section to be used with the new black rubber washer. I replaced that washer with a new, smaller washer which the central bolt nut will screw down onto, and holds the new nylon "clunk" washer in place (see notes about the clunk washer further below).
Middle section: You can see the addition of the black rubber washer, plus one of the original metal washers, at middle of the photo.
The original large metal washer is now used in conjunction with the rubber washer super-glued to it, and this combination sit upon the top surface of the bottom baseboard, below the magnetic disc. They ensure the magnetic disc presses firmly up against the brass bush.
Bottom Section: The main bolt and second original large washer remain the same, passing up through the whole assembly.
The Clunk Washer
This is another small mod that I made to the telescope base, independently of the requirements of this Lazy Susan Bearing mod.
I call it the CLUNK washer because its purpose is to stop a clunking sound when you pick up the Dobsonian base.
In the left photo you can see what I call the "clunk" gap created by the brass bush protruding above the top baseplate, between the top plate and the washer/nut of the bolt (although photo is sideways on).
You can see the yellow brass in the gap.
In the right-hand photo you can see a nylon washer that I placed over the brass bush to fill this gap, because whenever I lifted the base, the bottom section would drop down by this gap distance, making an un-nerving "clunk" sound.
When I first put this washer in place, I got a friction problem due to overtightening of the central bolt, causing additional stiffness in azimuth which was because originally I used a washer that was far too thick, so that when the bolt was tightened it caused friction, and acted like a brake on the azimuth motion. In this photo I have replaced the thick washer with one that is exactly the right thickness (i.e. it only just fills the gap).
Optional: Additional Braking from the Clunk Washer
Just be aware of the washer thickness problem so you don't make the un-modified Teflon bearing system bind.
However when a Lazy Susan is being installed, this can be used to advantage because it can act like a clutch brake mechanism, giving the tiniest bit of braking power to help restrict the otherwise very free movement of the LS bearing.
Place a nylon spacer and/or washer on the center shaft so that it that sits about 1/64" below the top of the brass bush (use a straight-edge across it). You want the spacer to be barely below it (about 1/64"). Then put a big fender washer under the lock nut on top of the brass bush to help spread out the nut's clamping force a bit. The lock nut presses the fender washer down on the nylon washer to produce the friction you want.
You can then slightly tighten the lock nut (and thus transfer friction from the nut/shaft to the base) to get any amount of drag you desire. If you keep tightening, you will begin to tighten the spacer down onto the base, producing even more drag (like a clutch) without crushing the bearing or cracking the base.
You may need to experiment with the exact size of washer used to provide the right amount of braking. Be careful not to overtighten the central bolt in your efforts to apply braking force, because that could squeeze the brass bush and magnetic disc assembly, causing distortion. I think it is best not to rely solely on this method for braking, which is why I suggest the use of carpet brakes primarily.
Simple Carpet Brakes
Getting exactly the right amount of tension to provide the perfect balance between freedom of movement, yet hold the scope steady in azimuth when stationery (i.e. prevent wind blowing the scope around, accidental nudges or movement on unlevel ground), is a matter of trial and error.
The simplest form of braking I have found is to cut up small segments of carpet which can be fixed to the bottom baseboard, and will apply just enough pressure of the carpet rubbing against the bottom surface of the top baseboard, so as to hold the scope still.
Whilst experimenting I decided to use some electrical insulating tape folded in a loop, so as to give "double-sided" stickiness, to fix the carpet to the base. This means I can easily remove the carpet block and replace with a larger or smaller piece if necessary. A larger area of carpet would provide more friction, and smaller less friction. If the tape holds securely enough then that's fine, otherwise I can use some alternative stronger adhesive once I know the correct amount of carpet required for the brake to be just right.
With the Lazy Susan Bearing in place, accompanied by its high-tech braking system(!), the Dobsonian base can be put back together again (remember my notes on care when assembling the azimuth bearing).
IMPORTANT: Once you have re-assembled the scope rotate the base very slowly listening carefully for any nasty noises, and feeling for any points of resistance that might be caused by incorrect re-assembly of the Dobsonian base. If anything feels or sounds wrong do not force it. Undo the central bolt immediately and check everything over to see what you might have done wrong.
Performing the Encoder Test
If your telescope includes the Intelliscope Computer Object Locator system, then you should perform the following test procedure to establish correct operation of the Azimuth encoder.
IMPORTANT: Following the modification and re-assembly of the base, you should perform an Encoder Test which is one of the "Hidden" Functions detailed in section 12 of the Intelliscope COL Instruction Manual (page 22,23).
Press and hold the Enter button while pressing the Power button, once powered on press FUNC, then use up/down arrow buttons to scroll to the ENCODER TEST, then press Enter.
The Encoder Test screen will display as shown below. The upper line corresponds to the Altitude Encoder, which we will ignore.
The lower line of the display corresponds to the Azimuth Encoders. The first two digits on each line denote the Amplitude from sensor chip 1 (showing hexadecimal value of 32), the second two digits are the Amplitude from sensor chip 2 (showing hexadecimal value B1). The 3-digit number shown after each digit-pair on each line is the Radius for each encoder (showing value 074). The four-digit number (00E1) is raw encoder "ticks", and is not useful for this test.
Slowly rotate the telescope in a full circle around on its base, watching the Amplitude sensor and Radius values rise and fall, and check that they keep within the tolerance values explained below.
If the readings for the Azimuth endoder test show Amplitude values above F3, or the Radius values go above 125 or below 30 then the critical gap is compromised in some way, and the readings will cause inaccuracies in the Intelliscope system.
Amplitude above F3 means "too close", and the central bolt may need to be loosened. If this does not help you may need to disassemble, inspect for problems and re-assemble. This is also a symptom that the magnetic disc has become bent, and so at some point in the rotation comes too close to the chips.
Radius above 125 means "too close" and below 30 means "too far apart".
If these values seem good that completes the Encoder Test.
Alt Azimuth Readings Test
Next we will use the Alt Azimuth Readings Test. Press FUNC, then use arrow buttons to scroll to the ALT AZM TEST, then press Enter.
Here we are interested in the rightmost figures. The top line corresponds to the Altitude Encoder which we can ignore (showing value +000.0), and the bottom lines is for the Azimuth Encoder (value +002.8), which we will check.
Rotate the telescope until the bottom right figure shows +000.0, then without moving the base make a small pencil lineup mark on the edge of the top and bottom base boards to note the starting or zero point. When you first power up the Intelliscope the sensor readings will be zero, so the startpoint would be different every time you do the encoder test (i.e. don't expect it to be the same place every time).
Note the start point. Rotate the telescope anti-clockwise slowly through a full 360 degrees. Check the ALT AZM TEST goes fully through from 0 to +179.9, then at 180° rotation the figure swaps to negative -179.9, and continuing rotation returns thru negative values to 0, back where it started.
When a full circle is complete and the reading shows +000.00 again, the pencil marks should be lined up again.
If the marks do not line-up, this indicates that there could be some slippage, possibly because the magnetic disc is not being held tightly enough against the bottom base board washer. Either the central bolt needs to be tightened slightly, or a slightly thicker bottom base board washer needs to be inserted.
Provided the line-up appears to be good, repeat the test rotating backwards and forwards several times and at different speeds, quite fast to prove that there is still no slippage.
You should now be confident that your Lazy Susan bearing mod is successful.
Of course, "the proof of the pudding is in the eating", and a proper nights viewing will be required to say whether the modification was worth it, and to determine whether tracking objects at high magnification viewing is easier, and more controllable. Some fine-tuning of the braking system may be required, either with different sizes of "carpet brakes", or by tightening or loosening of the central bolt if you employed the Clunker washer braking method too.
I hope these instructions will be helpful. If you think they could be improved please let me know!
Watch it in action
Click the link below to view a short Quicktime movie demonstrating the effect of the bearing on the Dobsonian base after the Lazy Susan has been installed.
Note that this movie was taken before the addition of the carpet brakes!
Click for Lazy Susan movie clip
Lazy Susan Bearing Suppliers:-
The supplier of my Lazy Susan bearing was a UK company called Isaac Lord, the stock code was 300LS for a Lazy Susan Swivel Bearing No. 12C 12", which has a depth of 8mm, which with some re-ordering of the washer positions on the base pivot bolt allow the digital encoders to continue to work properly. See the following page: http://www.isaaclord.co.uk/productDetail.aspx?product=373&subCat=000
The following link by by Mark De Smet describes The Intelliscope System in incredible detail - YOU MUST READ THIS!! It describes the Encoders, cabling, computer controller, RS232 computer communications with the Intelliscope, discussion of using the Intelliscope with other telescopes, software that work swith the Intelliscope, tips and tricks and mechanical dimensions. You will learn a great deal from this link.
Other useful links:-