The HAS 12" Truss-Tube Dobsonian Project
PART IV: The Mirror Cell: Fitting the mirror
Now that all the parts of the mirror cell have been cut, three 9mm holes are drilled in the cell body to take collimation bolts, which also keep the floating supports in position. Three more holes are drilled to take M6 bolts, which are used to fix the mirror brackets to the cell body.
On the photograph you can see the collimation bolts fitted to the cell body. These bolts are made from M8 threaded rod cut to 100mm long bolts. Once the mirror cell is in place, it can be collimated by tightening the wing nuts against the springs. These springs are used valve springs from a diesel engine.
On the other side of the cell body, the collimation bolts double as pivots for the floating supports. For that purpose, a shallow 9mm hole is drilled in the geometric centre of each of the supports. The supports are simply stuck on top of the ends of the collimation bolts. Once the mirror is in place, the supports will press against the back of the mirror in 9 places.
Because the supports can freely pivot on the collimation bolts, the weight of the mirror will be spread evenly among all of the pressure points. This way, the mirror cannot sag or bend under its own weight.
A 'tailgate' is made from an old 'T'-shaped aluminium brace which is no longer in use. The tailgate will be fitted to the bottom of the mirror box, holding the mirror cell and mirror in place. The brace, which is made of 2" x 1" x 1.6mm aluminium box section, is cut to the correct size and three 9mm holes drilled to take the collimation bolts.
On the photograph you can see the mirror cell body fitted to the tailgate by means of the collimation bolts. Because of the high tension of the springs, it is not necessary to use locking bolts, making collimation of the main mirror an easy job.
Finally, the mirror brackets are made, and these are three L-shaped brackets made out of 6mm aluminium strip. The long end is 100mm, the short end 45mm. Slots are drilled into these brackets to be able to fully adjust to the exact diameter of the mirror, as well as to be able to tighten the small alloy blocks that will hold the mirror in place.
Now that all parts have been machined, all surfaces of the aluminium are sanded. Here you can see Antony sanding the cell body. The purpose of this is to remove some of the thin oxide layer and give it a rough finish for the paint to adhere to.
Black Hammerite spray paint is used to dull the metal down in order to prevent stray light scattering and causing phantom images. The Hammerite also stops the aluminium oxidising further so that it doesn't give off annoying black dust. Just before the Hammerite is applied, all parts are thoroughly cleaned using methylated spirit.
