Ok, it's been a while, so here is an update on my 18" Dobsonian.
Secondary Rings:
These were completed and ready for staining. Two rebates were routed for the primary focuser board, and a second, smaller board (I'll explain later why I needed to add a second board). I shall refer to this smaller board as the "Secondary focuser board".
Kydex Inner Tube:
Because I never had time to thoroughly source Kydex in enough quantity for this project, I decided to use 3mm 3-ply plywood. This was flexible enough to bend into a 19" diameter tube. It was then glued into place and trimmed.
See the Attached Thumbnail 1.
I manage to find a "Kydex-type" material at my local art and craft shop, in the form of A2 art folios. I cut out lengths to glue on the outer surface of the tube, between the rings. Attached Thumbnail 2.
Focuser Board:
I laminated three layers of 5mm 3-ply plywood using a bending jig. Unfortunately, when it was dry, it had a warp or twist to the board, so I fitted it to the rings, and trimmed it to a more even curve. Unfortunately this left a less than satisfactory surface for staining, so I decided to laminate the focuser board with 3mm 3-ply plywood strips. Look at the Attached Thumbnail 2, 3 & 4. These images show both boards with laminated 3mm 3-ply on their sides. I have yet to laminate the top surfaces of the boards.
This error simple came down to the lack of tools and a proper workshop. But that's okay. I'm not fussed with it.
Secondary Focuser Board:
I added a second board to mount any future electronics, finderscopes etc.
Secondary Truss Tubes:
As careful as I have been to keep the truss tube aluminium clean and scratch-free, it was hard to keep a nice brushed surface. So by pure luck I found "Contact-type" vinyl adhesive (similar to which is used to cover school books), with a brushed brass surface. I will cover the aluminium tubes with this material.
Well, it still looks a long way off from being completed, but with everything else happening in my life at the moment, I should still fulfill my completion date of December 2008.
A Note About Weight:
Weight. Some may say this is too becoming too heavy for a secondary cage. Too much timber. It needs to be lightweight. While I fully understand this argument, I do not see this telescope traveling very far from home. In fact, I will probably remain assembled when not in use.
Weight. Some may say this is too becoming too heavy for a secondary cage. Too much timber. It needs to be lightweight. While I fully understand this argument, I do not see this telescope traveling very far from home. In fact, I will probably remain assembled when not in use.
It's a bit of a furphy - there is no issue with weight. Keeping the weight down means that the pivot on the rocker box can be a little lower, but so what. It's only a pain once the build is complete and to add something really heavy up top and have to counter ballance the base, and that takes five times as much weight as the stuff added to the top.
Chech out the photos of http://www.sdmtelescopes.com.au/SDM018.html SDM018. This beauty has been made bottom heavy to accomodate some heavy scientific gear at the sharp end, and it is easier to keep perfectly ballanced this way.
Weight generally = thermal mass. Its good to try and find a balance.
I really can't see the thermal mass of the non optical components of a truss dob being a big issue - not unless you are using a night storage heater as a counterweight
I really can't see the thermal mass of the non optical components of a truss dob being a big issue - not unless you are using a night storage heater as a counterweight
The night storage heater is called the `plywood mirror box'
Thanks Stringscope, I plan to give it a medium colored stain, then 2-3 coats of a clear laquer to give it a gloss finish. I have aquired a compressor, and I'll buy a sprayer next week. I've never stained/coated anything like this before, so I'll do some testing on some scrap timber. I guess I want a finish similar to the SDM/ Obsession telescopes. A stain and clear coat which is dark enough to get away from the pale plywood appearance, but light enough to show the grain (whatever grain there is to show in plywood).I bought some Cabots Stain and Varnish all-in-one and used a "wipe-on" method on some scrap, but this left a blotchy surface. I need to experiment with my method, first...
Focuser Board:
I laminated both the focuser and Telrad boards with 3mm 3-ply plywood strips. I am very happy with the finished result.
See image 1.
Spider:
Significant work was done today on the secondary cage. Firstly, the holes were drilled in the struts and inner tube for the spider. After it was securely mounted, the centre point of the secondary mirror was calculated and this measurement was used to mark the hole in the focuser board for the focuser.
Finally the the spider and secondary mirror were installed for a test fit. The mounting screws were tightened and everything looks good so far.
See image 2.
Installing the focuser:
The first task was to cut a hole in the focuser board. A 70mm diameter hole was drilled. This was enough clearance for the focuser (see image 3).
Because I had purchased a flat mounting plate for the focuser, and constructed a curved focuser board, I needed to modify the mounting design to suit. Firstly, I constructed a flat board out of 3 x 3-ply sheets and glued them together. This gave me an extra flat base to mount the focuser on. Then I chiseled out a flat section in the focuser board to cater for this base (see images 4 & 5).
Once the leveling base was trimmed and ready for gluing, it was put into position and clamped down using the base plate from the focuser (see image 6).
And finally, I test fitted the focuser on its own base plate. Looks snazzy! (see image 7).
(p.s. can someone tell me how to code the text so the images are not seen as "Attached images" at the bottom of the post. I wish to add images at various locations amongst the post instead.)
Well, unfortunately a sad week this week. Owing to some family and other reasons I deeply regret having to halt the dob project. It wasn't a decision I made lightly, but I have decided not to continue. All the parts purchased have found new homes.
oooh, Astrojunk, I did put forward that proposal to the better half, but that would be stretching the friendship a little (sorry Peter R. I tried, hehe).
h0ughy, you gotta love the spouses, they put up with all the hobbies, don't they? I am happy to accept anything that is bought for me.
Well, unfortunately a sad week this week. Owing to some family and other reasons I deeply regret having to halt the dob project. It wasn't a decision I made lightly, but I have decided not to continue. All the parts purchased have found new homes.
I must of missed this post, Sorry to hear about your halt to works on your
18" dob. I can only imagine the cure for aperture fever almost healed with
the med's on the horizon, only to have them snatched away... It was John
Howards fault.
On a Serious note, I hope you are able to restart your project in the future.
And I look forward to when you do.
