10" Twin Tube Dobsonian - by Mike Caine - September 2011
Last February, I heard on the internet that it was very easy to locate Uranus. On a clear Joburg evening, I took out my trusty 6’’ Dobs, and pointed it the indicated direction. Uranus was supposed to be really close to Jupiter. In the pale blue milky sky of the northern suburbs, I could barely see Jupiter. I decided I needed a bigger scope.
Julian from the ATM class managed to find me two 10’’ blanks and with 80 grit I was off like a bull in a china (glass) shop. Within a week I was down to saggita of 2.5 for an f 6.2 scope. However, I discovered that I had just dug a hole in the centre, and had to spend many more hours getting it spherical. I had decided to make a twin tube ( 2 strut) scope, and the focal length was selected to suit my height, so that I could peer through the eyepiece when it was vertical.
The rest of the polishing and figuring went through the usual frustrating TDE, TUE... But mainly a mountain kept growing in the middle, and the surface was rough. Johan nagged me constantly to slow down, and he eventually got through. The final test showed a nice parabola, but with a small (2mm) turned down edge. Since I am moving to Durban in October, I do not have time to re-figure, so the ATM experts agreed that I could simply blank out the outer 2mm. Rodney did a fine job of aluminising the mirror, and by mid September I could assemble all the optics into the mechanical bits.
On 24 September I took it to the ATM class and got help collimating the optics, and we all had a good clear look at the usual target buildings. I got some useful advice from Dave on stiffening up the base. Unfortunately my new scope had to be packed and sent to Durban the next Monday, so I have not yet had an opportunity to use it to check its ability to unlock the Universe. At least the skies in Pinetown are 100% clearer than over Sandton.
I will let you know if I ever find Uranus.
Thanks to Chris, Johan, Julian and Dave and the other willing guides at the ATM class.