Smooth Motion with Dob Base Mods
Submitted: Wednesday, 29th June 2005 by Mike Salway
Update 17/01/2005: Added slew knobs and carry handles, scroll to bottom of this page.
Dobsonian bases are cheap, quick and easy to setup and easy to use. With a tiny bit of extra work, you can ensure the movement is smooth and fluid, without jerking.
You have to find the right balance between movement being too smoth and light and overbalancing, and being too stiff that you're unable to have the precision movement you need to starhop or keep objects in the FOV.
Dobsonian bases use Altitude/Azimuth (or alt/az) motion, where Altitude is the up/down motion and Azimuth is the swivel and turn (left/right) motion. I just think of Alt/Az as tilt/swivel.
Currently all I've done to the altitude motion is to dis-engage the springs to allow lighter, smoother motion when tilting up and down. When I made the trolley, I started storing the scope in the upright position and my mistake, I left the springs engaged while being stored. This lead to the springs being stretched a bit, and as a result there was backlash when moving the scope up - it would drop back down ever so slightly.
So for now, I have dis-engaged the springs which allows for very light movement in the altitude direction. This would most likely be a problem for someone with heavy, expensive 2" widefield eyepieces, but i'm not lucky enough to be burdened with that problem :)
If balance ever becomes an issue (with heavy barlows or eyepieces), i'll just buy some new springs or attach a new ring pull further up the existing springs.
I'm not the first, and I won't be the last, to say that dobsonians can become sticky when swivelling in the azimuth direction. The teflon pads supplied with the scope aren't the best quality, and for a start, the staple holding one of my pads in wasn't hammered in far enough, and scratched the base from day 1! That was a quick fix, using a nail punch and hammer, I drove the staple in below the top of the teflon pad.
Still, dirt and gunk can build up quickly and swivelling the scope can become sticky and is a real pain when trying to track a planet at high magnification.
You can see from the photo on the left that the underside of my top plate was scratched due to dirt and gunk buildup on the teflon pads.. no wonder the motion was sticky!
So I decided to do the "milk bottle washer" mods to encourage smoother azimuth motion. This involves cutting a number of washers out of a 2 or 3 litre plastic milk bottle, and fitting them over the central bolt. The idea behind this, is that you stack a number of washers, just enough to alleviate some pressure off the teflon pads and allow for smoother azimuth motion.
I used a stanley knife, scissors and a circle template from my wife's scrapbooking gear to cut 8 circles out of the plastic, each about 2.5" in diameter. I cut the central hole big enough for the central bolt to fit loosely through.
After some trial and error, putting the base back together, putting the OTA on and swivel testing it, I've found that 5 or 6 washers is the right number for me - to take enough pressure off the teflon pads to allow smoother movement, but not many where it's resting too much on the washers and wobbles side to side.
My first light of the mod definitely proved it a success. Tracking at high power was much easier and smoother. The was a bit of wobbling when focussing, a combination of a cheap focusser but probably also having 1 too many washers in. I'll take one out and see how it goes, but overall i'm happy with the improvements, for $0 in cost!
Update 9th December 2004
My azumith motion started to get sticky again, so I purchased some "Super Sliders" for $10 from Mitre10. They're furniture sliders, usually put on the bottom of chairs so they slide across a hard floor easily.
I stuck these at various places around the base, and they basically replace the existing teflon pads. They are about 3-5mm higher than the teflon pads, so I also added 2 extra washers to the middle to keep the weight evenly distributed across the base.
Swivelling now seems nice and smooth again, light to the touch but not too slippery. Hopefully it'll get a first light when these clouds ever go away!
Update 17th January 2005. While this mod technically isn't a mod to the base, I thought I'd include it here anyway.
Most people drive their dobs by putting their left hand inside the open end of the OTA and pulling/pushing/lifting it to point to where they want it. I do the same, however there are times where it's not convenient to drive it this way.
If I have a hartman mask or a solar filter on, then I can't put my hand inside the OTA, and so I have to drive it by pushing/pulling the side of the tube, and it's not very accurate.
So over the weekend I installed 2 plastic draw knobs to the top end of my OTA, which I use to push/pull and drive my dob to where it needs to go. I stuck two pieces of double-sided tape where I wanted the knobs to go, and drilled the holes in the OTA. The tape helps to stop the drill-bit from sliding around on the curver surface of the tube when starting to drill the hole.
Once the holes were drilled it's simply a matter of screwing the knobs on, and voila, all done. I did take care not to let metal shavings fall inside the tube onto the mirror, secondary mirror or into the focuser barrel. I put an old pillow-case inside the end of the tube to catch the shavings.
It only cost a couple of dollars and 15 minutes to install, and provides for greater convenience when I have a mask or filter on the end of my tube. One knob would probably have been enough, but I added an extra in a position further around/underneath for greatest convenience at all viewing/imaging angles.
Base Carry Handles
Update 17/01/2005. When moving the dob in and out of storage, I use the trolley, but for moving short distances across the backyard to move out of the way of a tree, putting the scope back on the trolley isn't the quickest or easiest way.
In the past I'd just grabbed a hand each side of the OTA down on the base, gripped tight and lifted it slightly off the ground and did the crab shuffle to the new location. I've always felt a little uncomfortable doing it this way because of the potential for my hands to slip and send the thing crashing to the ground.
So I decided to put these carry handles on the base to allow me to get a better, more comfortable grip when doing the crab shuffle.
The handles were a couple of dollars each from Mitre10, and were easy to install simply by drilling the holes through the base and screwing it in. I chose handles that screwed right through from the back, rather than screwing in from the handle-side, to help try and eliminate the possibility of the handle ripping right through the particle-board and sending the scope crashing to the ground.
I wouldn't recommend using them to carry the scope (with a OTA still on) a long distance, but I think they'll be fine for short crab shuffles across the yard.
There's nothing stopping the OTA from being tilted so far up that the bottom of the tube (where the mirror is!!) crashes into the inside of the base.
A little knock is probably harmless, but an accidental crash could put off your collimation at the least, so it's best to ensure no damage is done by putting a "knocker stopper" on the inside of your base where the bottom of the tube meets it.
I just used one of the felt furniture pads that simply stuck on the base. You could use a piece of foam or whatever else you've got lying around.