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Building an 8" Open Tube Reflector
Mirror Cell and Saddle
The Mirror cell
The mirror cell is an integral part of the external tube structure and functions both as a mirror holder as well as an end cap. Rather than use the traditional wing nut and 1/4-20 bolts, the collimation bolts are 3/8" x 20 pitch thread counter sunk, flat head, Allen wrench bolts that are recessed into the back of the cell. This give a sleek and finished appearance much as you would find at the back end of a Cassegrain telescope. The fine threads give noticeably more control over collimation and the whole works very smoothly. Also, the larger size bolts lend a sense of rigidity and stability. Note the round rubber feet located near to the collimation bolts. This allows the telescope to be conveniently rested or stored on its end. The cell itself is held to the end of the tube by six flat head counter sunk Allen screws. Using bolts of this type and in this arrangement give the back end of the telescope an attractive appearance, something Newtonians have rarely had.
The end cap is cut from 3/4" thick good quality plywood or MDF. The above plan shows the holes cut into the end cap for ventilation. In the prototype I constructed I used the arrangement to the left, but in retrospect I would likely use the plan to the right inasmuch as it allows for the removal of the central bolt that holds the mirror in place without disassembling the tip-tilt plate and its associated washers and springs.
The tip-tilt plate is simply a triangle cut out of 3/4" thick high quality plywood or MDF. The straight triangle sides can be curved inward as shown at the right. The spade drill shown is being used to cut a recess hole for the mirror mounting bolt. The collimating bolt holes have been previously drilled as shown below to insure proper alignment.
After the end rings have been glued into the tube the cap is centered and taped into position and the six mounting holes drilled. At this time the tip-tilt plate can also be taped into position and drilled for collimating holes as shown at right. The reader is referred to the section on tube and strut construction for further details connected with making the cell parts that are part of tube construction.
The tip-tilt plates shown above have been drilled and coated with polyester resin. The resin will encapsulate the hygroscopic wood and prevent swelling or shrinking. The plate at the left shows the back side and the recess where the mirror mounting bolt is attached and passes through. At the right is seen the front side where the mirror will be mounted. The ring at the center is the artifact of a false start with the spade drill - wrong side. At the ends of the arms are seen the recesses for the collimation mounting bolt nuts. These are drilled out to receive the nuts and held in place with RTV as shown below. The RTV has a little give and accommodation, which is nice. Apply RTV and then immediately assemble cell and allow to dry in assembled position to insure proper alignment of nuts.
This shows the finished and painted cell, with the mirror attached, removed from the tube. The cell can be placed face down on a piece of Viva paper towel. This is sufficient to avoid scratching the coating as long as you don't slide it around.
This shows the end of the telescope after the cell has been removed. Note the six inserts. These metal inserts screw into the wood and serve as a means of receiving the six 10-32 cell attachment bolts. After eveything is in place and properly adjusted one can hit the inserts with a few drops of cyanoacrylate glue to lock things in place.
Here is the tip-tilt plate with the end cap removed. At the left the mirror attachment bolt as well as the collimation washers and springs are in place while at the right they have been removed and are sitting at the side.
At the left the tip-tilt plate is removed to show the mirror back. The mirror mounting bolt consists of a 3/8" x 16 pitch bolt cut off just long enough to pass through the plate and the washers and screws and enter the mounting hub by about 1/2 to 3/4". Between the bolt head and the tip-tilt plate is a 3/8" washer and two spring washers which I supply with the mirror. And remember, never sock any of these bolts down hard. A little tight with an Allen wrench is tight enough except for the mirror mounting bolt which should only be hand tightened until the mirror feels snug - nothing more
This is the a general plan for the saddle. There is a lower part, the actual saddle, and an upper part, or ring clamps. The entire assembly allows the tube to rotate. The inside of the saddle and ring clamps are covered with 1/8" thick X 1" wide adhesive backed felt. This felt lining bears against 2" wide Teflon tape wrapped around the central tube section. The upper and lower sections are held together with 1/4-20 threaded rods that passes through holes drilled as indicated by the line A.
Saddle parts shown coated with resin and sanded. The parts are made of two thicknesses of 3/4" plywood glued together. The assembly of the lower saddle sections to the base section was done entirely with polyester resin. The parts were individually painted with resin and sanded and finally properly positioned with the joints painted with resin. The resultant structure has no screws or bolts holding it together. At right is the adhesive backed felt obtainable from McMaster-Carr.
|Unpainted and fitted parts ready for painting.|
At left is a finished and painted upper ring clamp. Note through hole for 1/4-20 clamping rod. Felt was masked off using paper tape during painting. At right is the center tube section showing 2" wide Teflon tape near ends.
Finished saddle assembly. Note use of felt on upper front end of saddle. This bears against the upper inside of the center tube ring and forms a sliding friction surface during rotation. When rotating the telescope tube, grab the center tube section - not the struts - and gently lift up a little. The tube will rotate easily. Upper tube clamps should be tightened just enough to hold telescope in place but not tight enough to make it difficult to rotate tube.