The central brass bearing acts as the axis pivot for the telescope to rotate upon, and at the same time it ensures an accurate gap is maintained between the digital-encoders of the IntelliScope azimuth sensor, and the magnetic disc below that creates the magnetic flux that the encoders react with to give the readings for the computer to know which way the scope is pointing (see photo below).
This gap distance is CRUCIAL for the IntelliScope computer system to work properly, and the bolt that passes thru the whole assembly must be tight enough to;
a) fix the magentic disc firmly against the floor-standing base unit to ensure no movement (so the two rotate as one part), and
b) squeeze the floor base tight enough against the scope base for the critical gap to be correct (but not too tightly!).
At the same time the majority of the telescope weight is shared onto three white circular Teflon bearings, each about 1 inch diameter, and this is the part that is oft-described by suppliers adverts as providing "buttery-smooth" motion when rotating the telescope in azimuth. These Teflon bearings are static material. There are no ball-bearings or moving parts. Just Teflon pads fixed to the bottom base unit which the upper base slides over when rotated.
Well, yes, when you first build the scope you certainly think it seems very smooth. But just you wait until you put in the supplied 10mm eyepiece, plus a 2xBarlow to give a nice high magnification of 240x, i.e. bringing you sufficiently close to a planet to just start making out details.
At this magnification, once you've positioned the planet in the eypiece, it will march from one side of the field of view to the other in about 40 seconds due to the rotation of the Earth (faster with more magnification because you're observing a tighter region of sky). So about every 40 seconds you will need to nudge the scope slightly to follow the planet, and this is when you discover that "buttery-smooth" really translates to "quite jerky actually"!
You see the problem is that even though Teflon is chosen as the main weight bearing substance because it has a very low friction, there is still something called "stiction"; the natural stickiness of a material that has to be overcome before the telescope base will start to rotate (something to do with Newton's Laws of Motion!).
Once moving it is quite nice and smooth, but getting it to start moving means you have to apply not much, but sufficient pressure to push the telescope tube to overcome the stiction, but since the telescope is quite heavy and pushes down fairly hard on the Teflon pads, the pressure required to start rotation usually results in you pushing just a tad too hard and suddenly the planet you were just viewing has now shot out of view!
So then you have to carefully bring it back into view again in the reverse direction, and overcoming the stiction again. You soon learn to be quite delicate with your movements. Reading this paragraph it might sound like you have to use considerable pressure to make the scope move. No, not really, we're talking about very light pressure, just a gentle push, but when you're trying to make a tiny delicate adjustment to keep the view centred then gentle pushes actually result in large movements in the eyepiece.
This "overshoot" is a constant and frustrating battle at high magnifications that can occur every time you move the scope tube unless minimum friction and stiction are maintained.
No wonder Dobsonian owners have asked so many times in forums what they can do to improve the situation. I suppose this is just one of the downsides of paying for a large aperture scope, while cutting costs on the type of mount used.
If you want precision you've got to pay for it, and this is one of the things you should consider before purchasing a Dobsonian, because really Dobsonian's are intended more for gathering light from dim objects like nebulae and galaxies (which often cover larger areas of sky), and observing sweeping wide-field views of the night sky - as opposed to high-precision, high-magnification study.
Likewise, soon after getting my scope, I joined the ranks of fellow XT owners in search of "bearing Nirvana" !!
Before you do anything to your scope to try and improve the azimuth bearing, think very carefully. I highly recommend reading Maintaining and Modifying your Orion SkyQuest Dobsonian's Bearings by S. Waldee, which is a long but excellent article on the problem, and stresses that you should proceed with caution before using any of the following suggestions. Stephen's article relates to the older "classic" style of Skyquest XT scopes, but it applies equally well to the latest models of Dobsonian from Orion, except that care is required to ensure any mod does not affect the IntelliScope system's digital encoders, which weren't around in the days of the Classic XT's.
Remedies to the sticky azimuth bearing are many, varied and often best in combination:-
- Armor-All - This is a polymer-based car dashboard cleaning product which leaves a completely dry, but very slippery feeling to any surfaces it is applied to. Clean the underside of the base which slides upon the Teflon pads, and do the same to the pads themselves. This appears to be the favoured "quick-fix" method. I have tried it and it definitely improves the azimuth motion, but you keep thinking it could be better. Requires frequent re-application. Should be able to obtain this or similar product from all motor car accessory shops.
- 3-in-1 Oil - General purpose light household oil. This helps to a certain extent to begin with, but it runs the risk of dust and dirt particles sticking to it, and if this includes grit, it can end up scoring the Teflon pads making them even less frictionless! So perhaps a remedy to avoid. I have tried this and it seems better than Armor-All in some respects, i.e. it is liquid, so makes the stiction-start easier, but I don't really like the idea of it picking up dirt. I ended up thoroughly cleaning the pads again. A little oil on the central bearing bolt can help though.
- Teflon based lubricants - Many bicycle / mountain bike supply stores provide a variety of lubricants for wheel bearings and chains. This is quite a good form of liquid lubricant to apply as it semi-dries, so is less likely to pick up dirt. I have tried this and it seems about the same as Armor-All for the effect, and likewise requires re-application occasionally. They are generally a very watery white liquid, which contains very fine particles of Teflon. I used a clean cloth for liberal application of the lubricant.
- Milk-carton washers. Many folk cut plastic milk-bottle cartons into suitable sized washers, and drill/bore a hole in the centre. Alternatively you might find nylon washers available from your local hardware store. A number of these washers are then stacked together to provide just enough lift on the central brass bearing so as to bear some of the scopes' weight itself (normally the central pivot is non-weight bearing). The washers thereby raise and relieve the bulk of the weight from the Teflon pads, making it easier for the scope to rotate on them. I tried this and again it improved the situation - probably the best so far, but still slight jerkiness persisted.
WARNING: Be careful which parts you place the washers between - you may cause tilt of the whole system resulting in destruction of the encoder sensors.
- CDROM washers. Same idea as for the Milk-carton washers above, CDROM's have been used as weight bearing washers on the central pivot. WARNING: Be careful which parts you place the washers between - you may cause tilt of the whole system resulting in destruction of the encoder sensors.
- Un-tightening the central bolt. At one time I made the mistake of having the central bolt in conjunction with a thick washer that I had added on the top baseplate done up far too tight. This was binding the central pivot a surprising amount! Loosening it just a fraction helped ease things up. So pay heed to the XT instruction manual when it says to tighten the bolt only a fraction more than initial contact - the idea of tightening the central bolt is only to give sufficient pressure to hold the magnetic disc for the digital encoder stationary against the floor base mount while maintaining accurate disc-to-sensor gap (if the two rotate at all in relation to each other then the Computer Object Locator would get false readings).
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. 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. The problem of over-tightening 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). So please make sure you don't make the same mistake.
- Addition of a Lazy Susan turntable bearing. This is definitely the most free-moving solution, since it introduces proper ball-bearings to bear the weight throughout the full 360 degrees (no longer on just 3 pressure points). After a bit of research I found a Lazy Susan turntable bearing which was just the right height to support the telescope, yet maintain the critical distance required for the digital encoders, and it cost less than £5. Now the azimuth rotates incredibly smoothly - but in itself presents new problems because it moves far too easily - even the wind will blow the scope around, so now a simple brake has to be employed! My thanks goes to Stephen's article for the information regarding the Lazy Susan mod.
I have now created a new page explaining How I added a Lazy Susan Turntable Bearing to my Orion XT10 Dobsonian Telescope.
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 you or the wind nudges the scope accidentally.
I get the impression from reading other owners comments that these trials and tribulations are something they are willing to experiment with, and tweak their setup until they are satisfied with it. After all, astronomy is the kind of hobby where you gradually build up your equipment, and constantly improve things with the resources and finances available at the time. If you are that kind of person then maybe this route will suit you? But if you are not a tweaker/fiddler, and don't think you could put up with this irksome problem, perhaps you should consider something other than a Dobsonian mount, e.g. an Alt-Az or EQ mount with motorised tracking instead.
Certainly if you are serious about deep-sky astrophotography you should dig deep into your pockets and get a tracking mount. But likewise be warned that astrophotography can be a slippery path to much expense before you get DSO photos to be really proud of, and wow your friends!
Looking On The Positive Side
Bear in mind the "stiction" issue is less of a problem with lower magnifications because you don't have to be so precise with your movements. Remember that astronomy is not necessarily about getting maximum magnification. It's more about light gathering. At low magnifications with SWA eyepieces (Super Wide Angle, e.g. better than 60 degree fields of view), you get the amazing "space walk" feeling as if you're really out there.
Lower magnification eyepieces give brighter views (and a better chance of seeing dim nebulas and galaxies), and when viewing some star clusters and a wider expanse of sky, they literally take your breath away, appearing as diamonds glittering on black velvet.
The major advantage of Dobsonian mounted scopes is their simplicity. You can be outside and viewing the stars very quickly, whereas equatorially mounted scopes require more time to setup their polar alignment. This is why Dobsonians get used more often, are suitable for beginners, or the technically challenged, and therefore more appropriate for kids/friends to enjoy too.
Personally I enjoy this hobby very much, and on dismal cloudy nights when it is not possible to view the heavens, I usually find some other way to amuse myself and improve or experiment with the equipment I have.
Sidenote: The supplier of the 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
Other Topics in this XT10 Review: