Constructing a dob

Looking great Mike, a host of clubs will be queueing up to take advantage of you now...

Good to hear you got first light Trev! Well done!
Let me know how you get on with sorting out the little issues that will inevitably crop up at the start.
For the feet I was just planning on using a few pieces of timber.

Hopefully, I'll be able to complete mine this weekend. I have the bearings cut, just to treat them. I have the holes drilled in them for attachment directly to the mirror cell. The mirror cell is also finished as is the rocker box.

Found it very difficult to get formica. Seems it can only be got in specialised stores in Dublin. Problem is that they aren't open at the weekend. So, I just bit the bullet and ordered it from Telescope Service along with some Teflon. Should arrive tomorrow. Oh, can't wait to get it finished!

Hey guys,

First light achieved

1/ While movement is quite smooth the damping times are not good. I know my rocker and mirror box are rigid, so it's a combination of my pole clamps and ground board I think. The ground board is undersized as I ran out of Ebony Star (D'Oh!). Vibrations become an annoyance at powers above 150x. I must take a look at the rocker arcs again and make sure they are properly aligned.

Great stuff on achieving first light. !!
I'll try to help using experiences with my own dob although I did buy it so I didn't build it., I'm a tiny bit confused about this damping you mention, when I move my scope towards the zenith or down to the horizon and then stop, it just stops, do you mean there is a wobble for a few seconds afterwards, I dont see how the pole clamps come into it unless the UTA is loose on the truss poles, is this what you mean?, if so then this isn't a damping problem.

2/ I think I need to revisit the upper cage clamps. The plywood is just not strong enough to hold the poles stiff without suffering damage. I may cut a new bottom ring from a sheet of hardwood, maybe Oak, if I can find it.
Has anyone any suggestions on how to improve damping times? What do ye use as feet for ye're groundboards?

The obsession clamps work fairly well and are made out of metal, I can see there is alot of stress in this part of the structure, so the materials there need to be very strong. I'd leave the bottom O ring as is and get metal clamps.
The foot on the 20" is has 3 legs with teflon on them and the box has very smooth formica like material. I'm confused by your use of the term damping in this case, when I move it and then stop, it just stops, can you explain exactly what problem you are seeing.

do you mean there is a wobble for a few seconds afterwards?

Yep, exactly that Dave. It must be caused by the ground board section and/or pivot bolt. Actually, just thinking about that I remember I mad a last minute change to the ground board after reading a paragraph in the Kriege & Berry book. they recommended putting an extra teflon block in the centre. At first I thought this odd as surely three pads arranged at 120 degrees apart must be more stable than the same arrangement with an extra pad in the middle. Must reread that section again.

Makes me think that the middle pad is a fraction too high. I'm going to take the middle pad out and see how smooth the movement is.

Cheers,

Trev

That's interesting Trev, the 20" doesn't have a centre Teflon pad, but the 3 pads that are there are as far from the centre as physically possible, I guess this help prevent rocking of the base abit. If the centre bolt is too loose, you'll get the same rocking effect, that centre bolt rides a fine line between too loose and binding.

Yep, that was it. I removed the centre teflon block and it's much stiffer now. Only problem is it's not as smooth moving in azimuth, but moving the pads slightly should fix that

I checked the Kriege book again and the centre block was recommended for "the big ones". I should have been more careful in my reading. :oops: