Constructing a dob

I dont know if using steel parts for the mirror mounts is such a good idea- weight and thermal expansion/contraction characteristics would make me wary - unless you have thought about some sort of gasket/buffer to relieve any direct pressure on the mirror edges.

I'd hate to think of the effect of contraction of the steel against the mirror edges as the temperature falls on a very cold night!

The steel never touches the mirror. The mirror will be supported on pads, like this example: www.webstertelescopes.com/18_cell.jpg

Got the aluminium poles in B&Q today (thanks Rice!). Phoned all their stores around Dublin but they each only had a couple of poles. Luckily, their nearest store to me in Naas had just enough, so saved me a trip into Dublin. Got the following:
8 no. 6ft x 1.25inch aluminium tube (€12.50each)
1 no. 4ft x 1.0inch aluminium tube (€9.48) - for secondary cage
FWIW, these particular tubes look shiny so my immediate thought was that they were chrome steel. However, when I looked inside the tube, it had a matt silver finish. When I compared this to steel tubes, the steel had a matt black finish. The aluminium poles were also alot lighter than the steel ones.
Also picked up a mandrel and holesaw (64mm) to cut the hole for the focuser through the support board.

Getting there bit by bit...

Hey Michael,

Looking good!

Kydex - any suggestions for a source for this? I'll need a roll 55 inches long x 11 inches high.

I purchased some black felt for mine. It will be a bit heavier but it is supposed to be very very good at absorbing scattered light.

* Tube fasteners on secondary cage - ideas anyone?

I'm just using a bit of plywood that clamps by compression onto the UTA. They work very well, but I think I may replace them with ones made from Oak for durability. I based them on this www.efalk.org/Miatascope/Flavio/union.jpg
The plywood has a dowel driven in that will insert into a hole in the UTA to ensure proper placement.

* Tube fasteners on mirror box - ideas anyone? Ideally I'd like to mount them on the inside of the mirror box.

They also have 2ndry tube holders, a bit expensive though.
Have you tried split blocks? They look had to make but aren't difficult really. Another choice would be the Moonlight sockets from www.focuser.com/cgi-bin/dman.cgi?page=ca...e.cgi&category=5

By the way, I just finished my mirror box, cell, UTA and truss and stuck them all together a couple of nights ago. It's a 12.5" F6 with measely .75" chrome steel tubes so I didn't hold up much hope for it being rigid, but believe it or not a laser collimator will not move even a mm no matter what angle I place the assembly at (& this is with only the locking clamps holding the split blocks in place!). The Kriege truss design seems incredibly stable. An image of it here - very much based on Obsessions.



Also, if you are ordering from astrosystems take a look at their bargain bin. They have a few sets of very cheap and very nice baltic birch side bearings for sale. I may get some myself as their shipping is also very reasonable.

TS also sell Ebony Star and Teflon for a good price.

Cheers,

Trev

Trev,
Well done on building the Dob, its looks very good from here !.

Michael, it might be a bit late at this stage, to help you I took 2 pics of the cell of the 20" and put them below. I put in a tranparent 6" ruler so you can get a scale for things. The cell is deceptively simple and does an excellent job on supporting the mirror.

Trev,
Nice looking scope!

I found trying to get good quality wood to be difficult over the last couple of days. It all seems to be thin plywood, laminated plywood or laminated chipboard. Where would one get nice pieces of oak or cherry in sheets of 1/2" to 1" thick or joists a couple of inches thick?
In terms of the truss clamps on the secondary cage, I like the Obsession ones . Again, quite expensive but nice low profile to them and rock solid. I think I'll go for them.

For truss clamps in the mirror box, the ones as used by Obsession look difficult enough to make. Getting a piece of hardwood, has, for me at least, proven a little difficult. Perhaps I'm not looking in the right places.

In terms of the Moonlite ball joints, I wonder could someone use them turned at 45degrees on the inside corner of the mirror box - see hatched blocks in this drawing of my mirror cell to see what I'm talking about:



The advantage of them over hollow clamps is that there is a lip on them at the end so the poles would sit into each of them by a precise length. That *may* not be the case with the hollow clamps as they *might* allow the tube to go the whole way through, thereby having the tubes working at different lengths.

Thanks for the suggestions,

Dave,
Thanks for the photos!