Thinking of re-building base for 10" skywatcher dob...

[mbaddah]

I'm not happy with the alt/az motion of my 10" skywatcher truss dob. Trying to observe Saturn at 200x last night proved more of a chore than pleasure. It's also quite heavy and taller than it needs to be, hence I'm contemplating rebuilding the base of it out of marine ply.

Just wondering if anyone has rebuilt their bases for one of these dobs and what their experience is? If anyone has any tips, suggestions etc... would be greatly appreciated. I'm not intending on building anything to funky, just keeping it very simple and basic. Something like the following:

http://stellafane.org/tm/dob/mount/rockerbox.html

Thanks :D:thumbsup:

[mental4astro]

A dob rocker box is relatively easy to make. I've made several, and they all work beautifully. They are the heart and sole of the Dobsonian model, but if the Dob principals are not followed properly, you end up with a dog of a mount, as you're expirencing.

I've attached a few pics of the ones I've made.

If you are going to go down this path, the things you need to keep in mind are:

* the height at zenith you want the EP to reach.

* the centre of gravity of the scope, and if you need to adjust the location alt bearings to make it work better.

* the rigidity of the box. A flimsy box = trouble.

* bearing material selection. Teflon and highly textured laminates work best.

They are simple mounts to make. Do it right, and they are magic.

If you'd like to talk a bit about these mounts, I'm only too happy to help.

Mental.

[mbaddah]

Many thanks Alex for those excellent points and lovely photos :) Greatly appreciate the help.

I've taken note of them and will get cracking on a plan.

[mental4astro]

Actually, before picking up a hammer and saw, have a look at what you could do to improve the mount you have first.

Are the bearing surfaces clean? Give them a good and gentle clean and apply some good quality car wax to them. The Teflon pads only clean, don't apply wax to them! You will be amazed what a differnce a little clean and wax will do!

I just waxed the alt bearings of my bid dob, having being one year since the last clean and application. What a difference! Buttery smooth and just the right amount of 'stick' (which is bugger all, but just enough to both hold the scope in place, and move it with the slightest touch without bouncing or kick back).

If this fails to improve the situation, next look at changing the position of the Teflon pads to a more "textbook" one. Later to changing them to a more "textbook" size. It will be cheaper too. Teflon is inexpensive to buy if you cut it yourself. Easy to cut too, just like the plastic of and ice cream tub.

The last step, before totally building a new mount, if you've got balance issues in the alt., look at changing either the position of the current bearings ON the OTA (raise them if bum heavy, drop them down the tube if top heavy). You might find it an easier solution to make NEW alt bearings too, fitting them over the existing ones, making them larger in diameter too, and then modifying the rocker box to accomodate the new bearings. This is far less work than making a whole new mount.

[mbaddah]

I'll definitely give the car wax a try see how it goes. I'll have a look at the teflon pads this weekend to see whether they need a clean or not (I have the argo encoders installed so will require bit more fiddling).

Would it be possible to move the altitude bearings on the skywatcher truss dob? Either way I think it wouldn't be possible to fix in this manner as the scope is top heavy and would require me to move the bearings further up, which is not possible due to them been set to their maximum height from factory on the lower tube assembly.

Thanks Alex :thumbsup:

[paracheirodon]

Hi mbaddah,
I'm also a little disappointed with the handling of this scope. Have you made any modifications already?
I found a post about using a bearing in the base (http://www.axminster.co.uk/triangle-lazy-susan-bearing-prod20991/) Lazy Susan Bearing. Also 3 pads are necessary to add friction as just the bearings makes the base too easy to turn.
The post URL http://stargazerslounge.com/diy-astronomer/106781-skywatcher-200p-dob-base.html.

I'm about to first balance the OTA to see how the altitude movements change before I start other modifications.

[overlord]

LOL yeah! I guess u have an f/5. 100x is the max I will use for planets with my f/5. The rack and pinion focuser can't handle any more!

I used to use 300x with a barlow an on f/5 (very briefly) but the detail is so low it's not worth it and it's hard to find the planet too. :P

My advice is get a computer controlled scope and you choose the object and let it move it for you. At least that's what I would do lol... and that's what I am going to do with my future MAK.

[mbaddah]

I finally got around to ordering some ebony star laminate and virgin teflon pads from scopestuff (http://www.scopestuff.com/ss_ldxx.htm).

Hopefully I'll be able to find some time during the week to put the thing together. Will report back once I do :)

[mbaddah]

After slaving away for 2hrs this morning, I've managed to attach the ebony laminate and teflon pads to my SW base :D

If the sky is clear tonight i'll give it a thorough test (tracking 200x) to see whether it was worth it or not. So far it does seem pretty smooth, but still requires some force to push it.

[Rod]

Good to read your progress. If it's still too stiff you can try making a washer up of plastic milk bottle material. Place it around the central pivot so it lifts the pressure slightly off the Teflon pads.

Rod

[mental4astro]

I agree with Rod. I've PMed you on this too.

You can also back off the pivot nut too. It could well be that you've put the nut on a little too tight. If not then there is too much flex in either the ground board or the rocker box together, so the assembly flexes and instead of getting a nice even spread of the load over the Teflon pads, the load is being distributed to an edge, or even a point, of the pads.

Adding a "washer" around the pivot will help with this flex.

Let me know if you're stuck with getting your hands on some Teflon or laminate for this.

Also, where are the feet of the ground boards placed? They should ideally be directly underneath the Teflon pads. If not, this can make any flex issues worse.

One last thing, you didn't overtighten the screws holding the Teflon pads, did you? This can distort the pads, changing the shape of the contact surface. Even if the pads are loose on the ground board, rotating a little around the screw, it doesn't matter, they aren't going anywhere. As long at the screw heads are out of the way, no problem.

Oh, did you wax the laminate too?

It's all little details, I know, but it all helps if you go to this trouble to improve your scope.

[Barrykgerdes]

Hi
This mod is probably a bit too sophisticated for what you want but it is what I did to an old Astro Optical dob mount kits some 15 years ago to make it into an accurate Alt/azm mount to use with a DOB 11 driver,

However it may give you some ideas.

Barry

[mbaddah]

Thanks Rod, i'll definitely give this a go.




I made sure to place the Teflon pads directly over the feet of the board, as is mentioned in the good book :) I may have over tightened the screws, will have to double check. I didn't apply the wax :(

Looks like I'm going to have to disassemble it again lol. Thanks all for the help.

[mbaddah]

Thanks Barry for the file but you are right, it's a little too sophisticated for me. I barely know how to use a hammer :rofl:

[Satchmo]

A lot of the stiffness on the AZ for a small scope can come from having too large a bearing circle- giving a very low leverage factor with the fairly short distance between the top of the tube and the balance point.

Whilst ebony Star is great for big heavy telescopes, if you don't have the cut-out in the ebony to play with smaller bearing diameters i wouldn't hesitate to use the old Dobson standard - a 12" LP record as bearing and move your pads in to fit. I used to have a 16" on this type and it was silky smooth in Azimuth.

Packing the center up is fiddly and there may be a critical balance between easing it up and loosing stability on the outside bearings.

[GWING]

Hello new here. I have the same 10" S/W scope bought last Dec. I'm also looking to make the azimuth motion a bit smoother...So I've read through this post and I'm wondering what the steps might be (i.e. 1st, 2nd, 3rd, etc.)
Cheers! -Greg

[mental4astro]

Hi Greg,

The first thing to do to improve the movement of your scope is to clean all the bearing surfaces, regardless if it's just the altitude or azimuth bearings that are bugging you. Dobs often are neglected in this aspect and end up sticky and hard to handle.

And it doesn't matter if the bearing surfaces are laminate or aluminium, they all get soiled.

*Use something like methylated spirits (denatured ethanol) or isopropyl alcohol and with a soft cloth clean well these surfaces - these don't leave a greasy residue. You'll be surprised at how much dirt you'll remove.

* Wipe over these surfaces with as good a high quality car wax as you can get. If you don't have a car, you'll have enough wax to last til your grandkids retire, ;)

* The Teflon pads only wipe over with the alcohol. DONOT wipe them with the wax! If the azimuth bearing material on the ground board is of the roller type, take your time to clean as best as you can with the alcohol.

Is the bearing material on the ground board Teflon or the roller type?

Reassemble your scope and you'll notice straight away an improvement.

Clean these components first and let us know how you go. It is far cheaper and easier than choping and changing components, and the improvement in action is really, really significant.

I clean all the bearing surfaces and wax the laminates once a year on my 3 dobs, and I am ALWAYS surprised at the improvement in their action. Cleaning is all the maintanence that is required really. I look after my scopes, yet the bearing still need a little attention.

If you are still unhappy with the azimuth movement after cleaning, if the ground board bearing is of the roller type, you may like to change this for some virgin Teflon pads. The only other modification to be done after that is the Ebony Star laminate on the base of the rocker box. This last one is a last resort.

[alistairsam]

Hi,

One thing to remember if using laminate is that it needs to be as coarse and dimpled as possible so there is less surface area in contact with the teflon pads and so less friction. motion becomes very smooth.

I couldn't get ebony star, so I went with a lazy susan bearing for my home built mount which makes it too smooth.
so I sandwiched glide eazy pads that you get to put under chairs and tables and a layer of felt on top of the pad between the two azimuth boards in 3 places.
The thickness of the sandwich exceeds the bearing thickness by 1mm so in effect the felt presses against the upper board and acts as a brake.
I also used an M12 bolt in the centre with a nyloc nut.
so with just a little pressure with the centre bolt and nut, my movement is now controlled and very smooth at the same time.
Thanks to Alex for his tips.

[bojan]

This thread just gave the idea :thumbsup:.. my dob is not rigid enough at vertical axle (I used the shaft and bearings from front loader washing machine for this.. but there is still significant and annoying amount of wobble visible, especially if windy).
I was thinking the lazy susan might be a good way to achieve mechanical rigidity and consistency.. which I need for my bartelised GoTo dob.. And too smooth movement for you folks without tracking can be dealt with by a layer of felt.

[alistairsam]

yes, the lazy susan mod is good after adding the brake.
I've uploaded a small video to show the smoothness. just requires a light push, but I have done some observing at high magnification and it stays steady. just adjust the central bolt tension or felt height, not too much though.

these are the furniture protectors I used. it comes with a nail on the other end, and I installed the lazy susan in the middle. I used the 8-10" bearing.
hope this helps.

http://www.youtube.com/watch?v=hq0b7fyeOLs&feature=youtu.be

[GWING]

Thanks Alexander! I will give your idea a try first. Yes, they are Teflon on the base board according to Celstron who sells SkyWatcher here in the USA, but they don't know if it is the better Virgin material. Cheers! -Greg

[GWING]

"Thanks Alexander! I will give your idea a try first. Yes, they are Teflon on the base board according to Celstron who sells SkyWatcher here in the USA, but they don't know if it is the better Virgin material. Cheers! -Greg"

Alexander,
I gave your suggestions a try today and the improvement by just cleaning and waxing was remarkable (No wax on the teflon). The Alt. is buttery smooth now and the az much better. The telflon pads on the base board are 1"x3/4" by 1/8" thick. I may still replace them with Virgin PTFE Pads from Scopestuff http://www.scopestuff.com/ss_tp41.htm I did notice the original pads are not beveled and so the base borad must be shaving off little tiny pieces making teflon dust. That was likely slowing things from smooth movement. Do you think I could step up to the 1"X1" by 1/8" size pads or should I stick with the origianl size?
Thanks again! -Greg

[Amunous]

I replaced the teflon bearings as i was having heaps of trouble with the up and down motion of the scope.. going up or down it would "stick" and then suddenly jump over what it was stuck on. it was very hard to accurately point to anything. so i replaced them with skateboard ball bearings.

smooth as butter now. you do have to tighten the handles a little more now as its a bit too smooth. but once it wears in and you get used to it. no dramas at all.

[mental4astro]

The old adage: "if it ain't broke, don't fix it"

Leave it alone is my thought. Mohammed went the whole hog and got the virgin Teflon and Ebony Star, and the result was magic for him. If you replace the whole lot like he did, you'll gain something, but the balance between cost/benifit is what you need to deal with for yourself.

If your scope was a big beasty, say 16" or more, then reworking the mount may be a more viable option - better quality laminates, virgin Teflon, the 15lb/sq inch rule, etc. The stiffness requirements of these large dobs is less problematic in smaller apertures.

The 15lb/sq inch rule is the size of Teflon pads for the weight of the item they are carrying. The altitude pads will be smaller than the azimuth ones as the alt. will just be carrying the OTA. The az. pads have the OTA and the rocker box.

As an example, if the OTA weighs 45lb (45 pounds, or approx 20.5kg), then the total area of the Telfon used will be 3sq inches. The is then divided into four for each of the four pads used to carry the OTA. Remember that you'll need to take into account the area lost to the counter sinking of the screw heads that hold the pads in place.

Now, the rocker box adds another 15lb to the OTA total, so the area of Teflon used on the ground board totals 4sq inches. BUT, there are FOUR pads here on the ground board, not three - the fourth is a 'doughnought' shaped ring that goes around the pivot bolt to stop the bolt from crushing in the ground board and rocker box base toghether.

Michael, sounds like there may have been some other problem with your altitude bearings to change the whole lot for skate board bearings. I bet it's smooth as now!

[Amunous]

yeah the teflon bearings just weren't cutting it. its soo smooth now. i too looked at making/changing to a different base. but this solved all my problems.