First thing you should do is read this: Dobsonian Motion
I have had my XT10 for 7 months and still have not found the perfect motion. I take that back, my altitude motion is perfect. My azimuth motion is pretty darn close. This should not discourage anyone from buying an XT10, it’s just that I often let better get in the way of good enough. The motions of the XT10 are ok right out of the box, but I think they can get a lot better with very minor modifications and they can be nearly perfect with only minor modifications.
I have tried every modification and snake oil remedy that has ever been mentioned on the newsgroups and even a few others (Crisco vegetable oil on the baseboard?). I will not cover each and every one of them here, but I will go through the modifications that have worked the best for me and my XT10. Keep in mind that motion, as it relates to dobsonian mounts, can be very subjective. What I like may not be what you like. I like a very light touch combined with extreme precision. These two requirements tend to run counter to each other, but I have found I can get them close to what I need. Sacrificing precision for a lighter touch is unacceptable. I would rather the motion require more force, but result in precise control than have a very light touch that zips around all over the place.
So, how do you know when you have great motion? You’ll know it when you see it, or actually, when you don’t notice it anymore. Once you have the motion set up to where you don’t think about it (and especially don’t curse at it), then you have found your nirvana of motion. You can reach nirvana either mechanically or through repetition. If you find that the stock motions have become transparent because you have been using them for so long, then you don’t need to mess with them. You don’t want to be like me, on a never ending quest for unachievable perfection. Well, maybe you do, but it can be distracting. Quit futzing with it when it becomes transparent.
I have heard relatively few complaints about the altitude motion of the XT10. Most of the complaints are with the balance of the OTA and the azimuth motion. There are a few things that the operator can easily control that can help with smooth and predictable altitude motion.
The first step to controlling altitude motion is to properly balance the OTA (Optical Tube Assembly). Brute forcing an imbalanced OTA by introducing more friction is the wrong approach. This will only lead to poor altitude motion. Please see my Balancing the OTA page for hints and tips for proper balance. Once you are properly balanced, you can come back here.
My XT10 came with the tension spring system. This is a brute force way of controlling the amount of friction in altitude. Attach both springs and you’ve essentially just increased the friction in the altitude system. Attach just one and you get a little less friction. Attach neither and you have a little less. This is a convenient way to match your XT10 altitude motion with your style and eyepiece collection. If you have Terminaglers (you know, the eyepieces that weigh as much as a small Chihuahua) or binoviewers, then you may want the extra friction afforded by the springs in order to keep the OTA in place while swapping ep’s. If you use strictly lightweight ep’s, then you can often get away with neither spring engaged. As mentioned earlier, you should not use friction to correct for poor balance.
You can replace the plastic altitude bearings with teflon ones (the stock azimuth bearings are teflon, the altitude bearings are plastic). Teflon bearing kits are available through several places, but the one that is mentioned most often is Astrosystems. They didn’t have the dob kit listed the last time I checked, so you might have to email them for the information. I ordered mine off of Astromart by putting up a wanted ad. You can also put Ebony Star laminate around the altitude hubs for even less friction. Ebony Star is made by Wilson Arts and is available at all home centers. Believe it or not, they know what you mean when you ask for Ebony Star.
The problem with replacing the altitude bearings and using Ebony Star on the altitude hubs is the friction is reduced significantly. Any imbalance will be greatly magnified after you make this mod. Swapping even lightweight eyepieces may cause the OTA to swing upward. So, I wouldn’t be too quick to replace these. If you do put in teflon altitude bearings, save the old plastic ones in case the teflon ones prove too slick.
What am I using? I have replaced the stock plastic bearings with virgin teflon and have wrapped the altitude hubs in ebony star. I do not have either of the tension springs installed. This leaves me with almost zero friction in altitude. This has given me the desired feather touch motion I am after. But, if you’ve seen the pictures of my XT10, you can see that there is a great deal of stuff on it.
To attach the ebony star to the altitude hubs, I just cut a strip about 3 inches longer than the diameter of the altitude hub and the same width. I wrapped it around the hub and held it in place with a small wood screw that goes through the two layers of overlapped ebony star and into the plastic altitude hub. Be sure that the screw is in a position as not to interfere with the rocker box.
From the top picture, you can see that I have done something to my altitude hubs. The wingnuts are there to allow for off-center attach points for bungees. The bungee you see attached in the photo is to keep the OTA from going vertical when it’s near zenith. This is covered in the balancing the OTA section.
The bottom picture shows my XT10 fully loaded with the ST80, Rigel, binoviewers, 24 Panoptics and a 32mm plossl in the ST80. This is the configuration that I balance for. It stays balanced at any OTA angle. The motion is exquisitely light and predictable. But swapping ep’s can be problematic. I can remove one ep from the binoviewer at a time and the OTA will stay put. Anything more and it goes up. Take the binoviewers out and it moves a lot. The bungee keeps it from smacking into the base. The good news is the eq platform keeps tracking while I’m swapping out pieces parts. So I can pull the OTA back down and easily find the target even at 400x.
This requires a fix, but I haven’t devised one yet. Some sort of locking mechanism for altitude that can be easily engaged and disengaged would fit the bill, but I haven’t discovered the answer. Yet. I know I am not willing to sacrifice my perfect motion with higher friction. I don’t do a lot of ep swapping, so I’d rather have sweet motion.
Don’t let this windbagged discussion on altitude motion scare you. If you don’t toucha nuthin’, it should be very good.
Perfect azimuth motion has been a much more elusive target. I really have tried everything. Here is what works.
1. Milk jug washers placed around the center bolt.
2. Ebony Star laminate
These two additions have resulted in nearly perfect azimuth motions on my XT10.
I have tried all of them. No kidding. I will not say that they aren’t good, because many of them are very good. But none of them gave me the nearly perfect motion I have now. I say nearly perfect because I feel that my required force is still a bit too high. Thankfully, the motion is very predictable, so I will leave it alone for now.
Fell free to try anything you think might help. You really can’t muck it up so bad that you can’t return to the original configuration. That’s the wonderful thing about these dobs.
So, here’s how I came close to perfection.
1. Milk jug washers:
I made mine about twice that big and not nearly so neat. The key is how many to use. Too few, and you don’t relieve any of the load off of the outer teflon pads. Too many, and your scope will jiggle like jello. My first try at the right number helped, but wasn’t quite right. I didn’t put enough on. The key is to get the base to wobble JUST A LITTLE BIT without the OTA on and then let the weight of the OTA compress the milk jug washers.
Other options for a center bearing include CD’s or a large piece of teflon. For these to work, you don’t want any wiggle before putting the OTA on as these media are not compressible like the multiple milk jug washers.
2. Ebony Star laminate
What a wonderful coincidence that the most widely used bearing surface on dobsonian telescopes of all brands has an astronomical name like Ebony Star. You can find this at most home/lumber stores and it is made by Wilson Arts. Expect to pay about $25 for a 2×2 sheet, which is what you need for the XT10 or GS dobs. The kind I got was very easy to work with and you could cut it with a pair of heavy duty scissors. I just traced the round bottom of the rocker box and cut it out with the scissors. I held it on with the large flat head screws that hold the base of the rocker box on. No glue here. You can glue it if you like, but I haven’t found that extra step to be necessary.
That’s it, you’re done. I did replace the stock teflon with some virgin teflon I had bought off of Astromart. The stock teflon would have been fine except that I had removed it and used it for other things and it was definitely the worse for wear. The replacement teflon looked to be the same quality as the stock stuff and was the same size and I put it in the same spot. So, if you haven’t futzed with the stock teflon, just make sure the staple is seated below the top of the teflon pad and leave it there. If you are replacing the teflon pads, the easiest way to hold it in place is with double sided carpet tape.
You may be anxious to try out additional pads in other spots. Feel free to give it a whirl. I had 16 pads that I tried in myriad configurations and ended up with the 3 pads in the stock positions. You can try Magic sliders as well, but the virgin teflon seems to work better with ebony star. Once again, I have tried it all. Virgin teflon on ebony star is the way to go.
Now there is one last addition that I have made that seems to work very well so far. I sprayed some silicon spray lubricant on the ebony star and found that I am that close to perfection. This silicon spray lubricant is what I use on our dog hair clippers and I sprayed it on a whim. Seems to work great. DO NOT USE ARMOR ALL. You can try anything else you like. You can always remove it with ammonia or Windex later on.
So, in summary, try anything and everything. But a great place to start is milk jug washers and ebony star laminate. Cost you about $25 and will make your motions like butter.
UPDATE 15 FEB 2004
The stuff on my Starmaster is much better for azimuth motion than ebony star. Here’s the info I found on it:
“The stuff is called FRP, or fiberglass reinforced plastic. You can purchase it in 4×8 sheets at Home Depot and Lowes. Glasbord is one brand of the stuff. If you go to Lowes, check back in the area where wall paneling is sold. FRP is used typically in commercial kitchens and bathrooms. It is quite slick and easy to keep clean.”
I can say for certain that this stuff is better than Ebony Star.