My buddy Shane told me the basics of a mirror cell and tailgate:
- Tailgate is same diameter of the inside of the tube and mounts in three places to the tube with screws.
- It must provide adjustment for the mirror cell which is mounted on through bolts with springs between the tailgate and the cell.
- Must allow plenty of airflow for temperature stability of the primary mirror.
I cut a circle 7 7/16" in diameter for the tailgate. Laid out and cut the "bio-hazard" symbol to provide airflow. Pretty simple to layout, locate the center and the first line to the outer edge is your choice, with all other lines at 60 degrees from each other. Don't remember, but believe the center diameter left uncut was 1 1/2". All edges were routed with a 5/16" rounding bit.
#10-24 brass threaded inserts were used on each of the three legs for mounting. I installed the tailgate in it's location, 1" in from the end of the tube, and located the legs centered on a flat of the coopered tube and then drilled pilot holes through the tube into the tailgate to locate the holes. Dimension in from the end of the tube for the holes will be the distance from the end of tube to the back face of the tailgate plus half the thickness of the material used to make the tailgate. Devise a method of marking one leg of the tailgate to a location on the tube to make assembly easier in the event that your holes are not perfectly 120 degrees from each other.
The mirror cell was cut from 3/4" material as well, 6" in diameter to match my primary mirror. The same layout, 60 degrees apart like on the tailgate was laid out and then a circle drawn 1" in from the edge to provide additional strength to this piece as it is not mounted on the edges. See the photos. Inside triangles were cut with a jigsaw leaving a 1" support rim around the outside edge. Next I drew a circle with a compass 70% of the diameter on the tailgate to locate the adjustment screws. The holes were located 120 degrees apart, centered in each of the triangles. The mirror cell was centered and clamped to the tailgate and the cell was used as a template to drill the holes for the adjustment bolts to slide through. I then counter sunk the holes in the cell and epoxied brass flat head 5/6" x 18 machine screws in place. Assemble the cell to the tailgate to hold bolts square while epoxy is drying.
Lowes had a combo package of various springs, one size of which was conical in shape which allows the springs to compress smoothly and more completely when installed.
The tailgate and cell were painted black of course. NOTE: The first time I mounted my mirror to the cell it fell off because the adhesive for the rubber pads I used did not stick to the paint. I subsequently sanded off the paint in the mount locations on the cell and re-attached the pads.
The mirror was mounted using round adhesive backed clear rubber pads purchased at Lowe's on a circle 70% of the diameter. Be sure and sand the paint underneath the pad mount locations so adhesive attaches to bare wood. If you have been paying attention you are probably thinking the pads are adhered directly over the bolts. That probably would have worked fine but I rotated the layout slightly so they were adhered to the wood and not the epoxy of the screws. The factory adhesive was removed from the pads and they were epoxied in place. The mirror was mounted to the pads using a drop of 100% pure silicone adhesive on each pad. A strip of duct tape was cut and attached loosely but around the mirror and the cell for a safety catch in case it fell off again.
The scope has been out about 8 times at this writing and is still holding collimation very well.