I took a little break from the 17.5" remake to try out a few techniques with making a dobbie mount for an 8" rich field scope. Peter also has a coma corrector which will be necessary with such a fast scope.
Peter (aka Hickny) and I purchased a new 8" f/4 OTA from Bintel this week that was lacking a mirror cell (cheap scope consequently). This is to be a grab-n-go scope which we shall share. You will see in the pics how small and 'cute' it looks,
We made a new cell for it, same design as the 17.5", with massive springs to double as collimation springs and locking mechanism with their brute strength. The pics show the cell just before we attached the lateral braces. The mirror sits on three silicone pads. The three white tabs seen in the pics are door stops we used to act a feet to rest the OTA on.
Today we installed the cell and made the mount.
We used 15mm C-D pine ply to reduce cost, and we like the knotty look of the material. The alt bearing is made out of a 100mm PVC plumbing flange like in the 4.5" dob mount I made last year.
We cut out a series of holes throughout the rocker box panels to reduce some weight and act as handles. I pay tribute to Galileo with all the scopes I build by making a pattern with holes which resembles the image Galileo saw of Saturn through his little telescopes, a 'ball with two ears, or lobes either side' on the front of the rocker box.
We used 3mm Teflon and cut it to size to follow the "15lb per sq inch" rule. Ended up using bugger all Teflon. The altitude bearing is silky smooth, and the azimuth bearing I'll be glueing the textured laminate to the under side of the rocker box tomorrow.
We designed this scope specifically to be as compact as possible. The rocker box is designed to accomodate the scopes brace wholly within it for easy stowing.
The height of the rocker box is also configured to set the scope at an easy chair sitting height for the observer. With the relatively small height travel between zenith and horizon of the scope, this was an important consideration.
This design also allows this scope to be eq. mounted should the need ever arise as the brace only gently squeezes the tube in place. Foam strips lie between the scope and the brace.
I've also included a pic of the 4.5" scope too.
It should be noted that the tools we used to make this mount are readily available: Cordless drill, jigsaw, power drill and circular saw. Nothing speciallised, and easy to make in as a weekend project with the kids.