Dob Bearing Material

[PeterStudz]

When I made the base for my Dob I designed it in such a way that, if I wanted, I could experiment with bearing materials relatively easily. Personally I prefer the traditional laminate/sheet running over PTFE pads. For the AZ bearing I initially settled on an ABS textured sheet running over the pads. This worked well with (being very critical) just a small amount of stiction at very high magnification (somewhere where I don’t often go).  Mind, I can have a tendency to be overly critical.

However, far too many wet, cloudy days/nights and coming across this material by pure chance changed my mind. I do like to experiment anyway - as long as it doesn’t cost much! Haven’t a clue if anyone has tried it before but the material looks the part and is often called “stucco embossed aluminium sheet”. I purchased a 500mm x 500mm sheet (my Dob ground board is 520mm so near enough) from a local supplier which of course meant no postage. It’s thin at .4mm thick and anodised gold. The cost was £13.49 from…

https://hardware-warehouse.co.uk/Embossed-Textured-Anodised-Aluminium-500mm-500mm-0.4mm

But similar is widely available and  not expensive, Eg eBay…

https://www.ebay.co.uk/itm/302206497370?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=yc6KhDB0Qom&sssrc=2349624&ssuid=I8jCpZsmREe&var=601049597017&widget_ver=artemis&media=COPY

I chose this type simply because it’s local. The sheet is thin, easy to cut with tin snips (old scissors would do the job) and could be shaped to altitude trunnions by hand.

The picture is after I cut mine to shape. I just tacked it onto my AZ base with small wood screws. I’ve tested it several times now and even at 375x (it’s a rare night when I can go to that power with my 200p) on Venus and initial impressions are great. Starts smooth, has a nice weight, no roughness, moves smooth and immediately stays where you put it - eg no overshoot. It’s fab! Will see how it performs over time. Eg previously, with other materials, there would be a slight change in performance when very cold, lots of dew etc. Will the anodised surface wear? If it does will it make a difference? We shall see.

[NGC 1502]

Over many years I’ve made several Dob mounts and modified others.  I think experimenting is key because it’s hard to predict how well it will work in practice, especially (as mentioned) at high power.

There’s so many factors involved and not just in the choice of materials.  I once made a Dob mount for a Skywatcher 10” Newtonian tube assembly bought secondhand. It was ok at low power but iffy at over 100x. After lots of frustrating DIY eventually the problem was revealed. The plywood base of the mount was not completely flat.  As it turned on the ground board at times it was ok but at other times it was not, like trying to push something uphill or downhill.

Let me explain further “pushing uphill” :-  As the mount is rotated you are also trying to lift the rocker box and tube assembly a tiny bit higher from the ground.  At other times the opposite occurs.  Stiction will vary greatly from no stiction to obvious and frustrating stiction.  

I’m hoping the above makes sense.  Although the construction of a Dob mount looks simple in design don’t allow that to make you casual. Make everything exactly square and flat, rigid as possible.  Make it less of a hassle to take apart if necessary by not using glue in the joints, just screws.

The “stucco embossed aluminium sheet” certainly looks the part- and as it works so well rejoice👍

Ed.

[markse68]

Excellent find Peter. My hunch is the anodising is harder than the melamine surface of formica but like moonlight focusers, the softer foundation material (alu) will lead to eventual wear esp with abrasive dust. BUT it’s cheap enough to replace and easier to work with so looks to be most excellent! 👍 If I do ever get round to building my 12” for which i have a beautiful mirror in a box, I’ll give this a go- thank you. Nice bit of bling too

Mark

[PeterStudz]

On 30/05/2023 at 06:49, NGC 1502 said:

Over many years I’ve made several Dob mounts and modified others.  I think experimenting is key because it’s hard to predict how well it will work in practice, especially (as mentioned) at high power.

There’s so many factors involved and not just in the choice of materials.  I once made a Dob mount for a Skywatcher 10” Newtonian tube assembly bought secondhand. It was ok at low power but iffy at over 100x. After lots of frustrating DIY eventually the problem was revealed. The plywood base of the mount was not completely flat.  As it turned on the ground board at times it was ok but at other times it was not, like trying to push something uphill or downhill.

Let me explain further “pushing uphill” :-  As the mount is rotated you are also trying to lift the rocker box and tube assembly a tiny bit higher from the ground.  At other times the opposite occurs.  Stiction will vary greatly from no stiction to obvious and frustrating stiction.  

I’m hoping the above makes sense.  Although the construction of a Dob mount looks simple in design don’t allow that to make you casual. Make everything exactly square and flat, rigid as possible.  Make it less of a hassle to take apart if necessary by not using glue in the joints, just screws.

The “stucco embossed aluminium sheet” certainly looks the part- and as it works so well rejoice👍

Ed.

I agree that experimenting is key. And I enjoy experimenting, so it adds to the fun. And for me it’s something to do during the summer months too, when it’s obviously not completely dark at night and visual opportunities limited.

Getting plywood, or any sheet wood, nice and flat without warps can be a task. It’s better with the more expensive wood but I’ve found that even then there can be warps. One reason that when I made my Dob base I went to the supplier and inspected each section of 18mm plywood that they had. Then only purchased sections that were warp free. Mind, I think that they thought that I had a case of OCD  But a few sheets were getting to a banana like stage. Buying online must be pot luck and something to avoid.

[PeterStudz]

On 30/05/2023 at 07:43, markse68 said:

Excellent find Peter. My hunch is the anodising is harder than the melamine surface of formica but like moonlight focusers, the softer foundation material (alu) will lead to eventual wear esp with abrasive dust. BUT it’s cheap enough to replace and easier to work with so looks to be most excellent! 👍 If I do ever get round to building my 12” for which i have a beautiful mirror in a box, I’ll give this a go- thank you. Nice bit of bling too

Mark

I’ve used my Dob with this for a few sessions now and it definitely works well. Stiction, and by that I mean the holding force (static friction) that must be overcome before the bearing begins to move, is excellent. Eg - it’s enough to hold on target when changing eyepiece etc. But when I want to move the OTA it moves smoothly without any, or very minimal, jerk or overshot - even at high magnification and/or when the Dob has been stationary for a while. 

I’m doing to add some of this to my altitude bearings which still have Teflon pads on a textured ABS sheet. This works OK, but there’s some stiction, especially at high powers. I also want the feel in all directions to feel identical as possible.

As you point out the aluminium/anodised surface will wear (everything does) overtime. Will wait and see how long this takes and when it makes a noticeable difference. When using other materials I have found that temperature, especially cold, can make a difference to stiction. But I haven’t yet had a cold night to try this. 

[NGC 1502]

Please update when you can, will watch with interest👍

Edited May 31, 2023 by NGC 1502

[Piero]

For my 16" dob, I used embossed FRP and teflon for both altitude and azimuth bearings.

Every 6 months or so I lubricate the FRP with a wet bar of soap to make the telescope as smooth as desired.

[PeterStudz]

5 hours ago, Piero said:

For my 16" dob, I used embossed FRP and teflon for both altitude and azimuth bearings.

Every 6 months or so I lubricate the FRP with a wet bar of soap to make the telescope as smooth as desired.

Where did you buy yours from? I think that like me you are in the UK. I looked for embossed FRP but the places I found didn’t offer small-ish sections like and/or charged a lot for postage. Eg here…

https://www.whitecladding.co.uk/product/white-embossed-frp-fibreglass-wall-protection-panel/

The smallest size is £55.99. But add VAT and “economy” postage of £30 the total comes to £103.

[Piero]

40 minutes ago, PeterStudz said:

Where did you buy yours from? I think that like me you are in the UK. I looked for embossed FRP but the places I found didn’t offer small-ish sections like and/or charged a lot for postage. Eg here…

https://www.whitecladding.co.uk/product/white-embossed-frp-fibreglass-wall-protection-panel/

The smallest size is £55.99. But add VAT and “economy” postage of £30 the total comes to £103.

 

I bought it from http://www.beplas.com/ and it was a sheet larger than 500x500mm. The extra material has been used in other projects. Considering the cost of building a dobson + mirror, this felt rather cheap. It is also something I don't want to compromise.