Constructing an alt-azimuth telescope - page 2
By designing in stiffness I got by using very little wood in my top cage and the lack of rigidity of the lightweight mahogony plywood was overcome. The top cage was constructed with 3 rings, 18" o.d., 16" i.d. and 8- 11"x 1" uprights set at 45 degree intervals around the rings. the top and bottom rings were grooved to fit the uprights and the middle ring was notched to fit a notch in the middle of the uprights. The space between the bottom and middle ring was fitted with 8 panels to fill in the gaps and provide a place to mount the focuser and finders. This also increases the rigidity of the cage by about 500%. The cage was assembled using a 2 part epoxy with a one hour working period and fiished with an epoxy finishing resin. Both of these are available at your neighbourhood hobby shop. Because of the softness of the wood I faced the outside of the uprights with hardwood strips, these act as bump guards when moving the scope to your site, and also help anchor the screws that hold the spider.
The Mirror Box is a little more straight forward, a four sided box with internal braces at the corners and at the top and bottom along the sides, a top piece with a cutout for the mirror also holds the strut mounts. The bottom of the box is removable and has the mirror cell mounted on it with the collimation bolts sticking out the bottom. The bottom is a tight fit into the box and held in place with 4 bolts, this allows the mirror to be removed for cleaning or storeage.
The Struts are made of 1 1/4 x .065wall 6061 T6 aluminium tubing. these were cut and faced on a metal lathe to ensure accurate length. The ends were made of solid aluminium and press fit into the tubing and secured with screws. The strut mounts are made of 1.5 x 1.5 x 1/4 alum angle machined and drill to take a 3/8 x 1" shoulder bolt that the struts slip over and are secured with a wing nut. The mounts are attached to the mirror box and top cage with 1/4" ss shcs and ss washers. I tried to use stainless steel fasteners thoughout to reduce the worry over corrosion from all the dew that will form on the scope at night.
Mirror cell- Being a machinist I went a little overboard on the design of my mirror mount, but then that's what ATM's all about. I used an article from Sky and Tel to get the coodinates of the 9-point support. The three triangle were machined from 1/2" alum plate and mounted on an aluminium subplate. The adjuster screws were mounted in the bottom of the mirror box and screw into polyethylene nuts attached to the subplate. I didn't have any stainless steel available so I used titanium for the adjuster screws. Nice material to work with if you have the proper tools and equipment.
With the major components finished I needed to find the balance point before finishing the rocker box section. To do this I assembled the mirror box and top cage with all 8 struts, the mirror on the cell and the focuser, finders and secondary in the top cage. I layed the assembly horizontaly on a large table and placed a piece of dowel or a broom handle under the scope. Then I moved the scope back and forth until it balanced on the dowel. I marked this position and measured to the top of the mirror box, this becomes the pivot point of the scope and the centre of the alt-bearing. I finished the design of the rocker box and got the parts cut and assembled. The rocker box does not require the use of the light weight (and expensive) mahogany ply so I used 3/4 birch and sealed it with several coats of polyurethane varnish. the alt-bearings are also made from 2 thicknesses of birch ply and faced with Ebony Star formica on the bearing surface. The birch is quite a bit heavier that the mahogany and if lightening is required cutouts or recesses can be routed into the insides of the rocker box. Someday I'll redo the rocker box to lighten it a bit, right now it weighs in at about 62lb.
The bottom of the rocker box is covered with a sheet of ebony star and 3 - 2x2x1/8" pieces of teflon are secured with wood screws to the base plate. The base plate and rocker box are bolted together using the bolt as the pivot. I also drilled a 3/8" hole through both pieces in one corner and put a pin in to lock them together for transport. Then more coats of varnish, a lot of sanding and the scope is ready to take into the back yard.
I finshed the telescope at 3:30 in the morming, got up the next morning loaded it in the car and drove to the Nova East Starparty in Fundy National Park in New Brunswick, Canada. Nova East is put on annually by the
Halifax Centre of the Royal Astronomical Society of Canada.
Mirror and secondary - 15" F4.5 from Nova Optical Systems- po box 80062 Cornish, Ut 84308,
801 258 5699
Focuser- 2" Feathertouch focuser- Starlight Instruments- po box 68282, Schaumburg, Il 60168-0282,
847 985 9595
Spider and Secondary Holder- Kenneth Novak & Co. Box 69W, Ladysmith, Wi 54848,
715 532 5102
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