This one will be an 18" f/4.5 Obsession-style Dob of modified design. I decided on this one to try building it as inexpensively as possible, especially since I have little personal money to contribute. Since I will be using it as an educational outreach tool in local schools, I am looking for a lot of community support.
So far, I've had several contributors. One company gave me some architectural-grade Baltic Birch plywood for the secondary cage rings and rocker box. Another cut the cost of some aluminum scrap tubing, and another has volunteered miscellaneous hardware.
At this point, the rings and tubes on the secondary cage are mostly completed and I'm looking for a focuser and secondary mirror assembly. I also thing I can get some laminate for the light baffle pretty cheap and am looking to have the aluminum parts anodized for free.
One point to notice is that I requested the donation of 8 1.25" tube inserts from a certain unnamed Dobsonian builder who unfortunately couldn't help (though he generously faxed me part of a book I was missing), so I decided to make my own tube inserts. To wit, I designed my own at half the cost from materials I had around.
- First, I cut the 1.125" hardwood dowels to fit inside the 1.25" O.D. x 0.58 wall x 11" long aluminum tubing of the secondary cage.
- Next, I marked the center of the dowel ends and recessed them 1/8" with a 3/4" Forstner bit so as to create a recessed seat to fit a 1/4-20 T-nut.
- I then drilled the center hole out with a 1/4" bit to a depth of 1.5 inches.
- After cleaning the dowel off with a can of compressed air, I then coated the underside of the T-nut in 4-6 minute epoxy and hammered the T-nut lightly into the end of the dowl until it was flush with the end of the dowel and let it dry. I repeated this step 8 times (twice per dowel - one on each end).
- Then, I sanded the dowels down and cleaned them with compressed air and a slightly damp cloth.
- Lastly, I stained the completed dowels with Minwax stain color "Colonial Maple 223" and let it dry overnight.
- Then, I bolted the rings and tubes together to form the secondary cage frame.
I have included some pictures below, with more to follow.
