From the prototype, the troop telescope needed to consist of the following parts: The ball (lower 1.tube) assembly, including the mirror and cell
2.The stand, with appropriate Teflon pads
3.The upper tube (focuser, finder, diagonal?lightweight but STURDY)
4.The rod, used to connect the ball with the upper tube
The troop's experience with the f7 Newtonian proved that it is impractical to transport an assembled OTA to a campout. The ball telescope would have to be assembled and aligned for every usage. I knew that this would be a challenge for the younger boys to do by themselves, but I had confidence that the older boys could do it especially those that had built their own 2.5 inch ball telescopes.
Stand
The first thing that had to be built was a stand for the ball. I had to have this in order to work on the ball itself! I wanted the stand to be as low-profile as possible, thus allowing short scouts to get to the eyepiece at zenith. I cut the triangular base from 3/4 inch MDF on a table saw, then cut a beveled circular hole in the center of the triangle with a scroll saw. The legs are 2x2's, dadoed to fit the base, with their tops cut at 45 degrees and fitted with 1/4 inch Teflon plugs. All of this was made from scrap I had lying around.
The spacing of the legs of the stand is tricky. As in a Dobsonian mount, the further apart the Teflon pads get, the stiffer the motion?but on the other hand, if the supports for the ball are too close together, the ball runs the risk of lifting out of the stand instead of rotating when trying to turn the scope! This was a problem noted with the prototype, which had the supports two close together. I opted to attempt to position the pads to contact the ball at 45 degrees from vertical.
Ball/Mirror cell
I purchased a 16 inch clear acrylic lighting diffuser from AZ Partsmaster for $25. Before cutting the ball, I decided to construct a mirror cell and support ring?this would tell me how big a hole I would need in the top of the ball.
I could not find a cheap mirror cell that would work adequately?I needed the mirror to sit as low as possible in the ball. I decided to fabricate the cell myself. It was made from 1 1/2 x 1/8 steel strap and a 3/4 inch slice of 3 inch round scrap steel, purchased from an industrial metal supply house for around $6. I bent the strap in my bench vise to the appropriate shape, hand cut and fitted the cell together. The 3 inch round stock at the back of the cell is heavy, and provides a decent counterweight. 5/16 hex bolts provide the collimation screws.
The cell was designed to just fit into the lower ring. This was made from two rings of 3/4 inch MDF glued together. Holes were bored in the sides of the lower ring to allow ventilation behind the mirror.
The size of the lower ring required cutting an 11-inch diameter hole in the globe. This I attempted with a coping saw, and things were going well until I went just a little too fast, when the acrylic melted and the sawblade stuck in the acrylic. I quickly found out first hand just how fast a crack can propagate in thin acrylic! Ugh! Rather than start over with a new ball (and another $25), I tried using CA glue (superglue) to repair the crack. This stuff is amazing! The crack is still visible, but the structural integrity of the sphere is completely restored. I tell the boys that the cracked ball gives the scope character!
The upper ring on the ball is fabricated out of three parts: a lower ring, beveled to fit inside the ball, slightly bigger than the hole, a middle ring the same thickness as the ball, and an upper ring on the outside of the ball that is larger than the hole.
The lower ring was eased into the ball (by warping the ball just a little), the other rings assembled in place, and all three rings screwed together. The middle ring has a beveled edge and acts as a light baffle. A circular cover was made of plywood that fits neatly in a recess in the upper ring.
Three 1x3 spacers were used to connect the upper and lower rings within the ball. These were initially trimmed for a firm friction fit, and eventually glued and screwed in from the top. Inexpensive pull handles were screwed into the upper ring.
Rod
The rod between ball and upper tube assemblies is made of 1/2 inch steel EMT conduit) which is very cheap, thus easy to replace if it gets damaged or lost. Prior to final assembly of the ball, a 3/4 inch hole was bored into the upper and lower rings. The conduit was placed in these holes, and a framing square used to alignment of the rings so that the rod was parallel to the axis of the telescope. A wooden split block was screwed into the bottom of the upper ring, and served to clamp the conduit in position.
Upper tube
The upper tube really needed to be lightweight but rugged! I was able to purchase a used helical focuser made of delrin on Astromart for $15. Everything else I could fabricate from scrap. The diagonal was made from a 5 inch diameter boy scout carmelcorn can. Note that this is not a full-diameter upper tube assembly? just think of it as a curved-vane spider! Placing the diagonal inside the can, opposite the focuser, puts it on the optical centerline and also protects it from inadvertent damage.
The can and focuser are screwed into a scrap of lightweight 2x4, which was suitably shaped to accommodate the curve of the can. A 3/4 inch hole was carefully bored in the 2x4 to accept the conduit rod. The wood was carefully cut, and a bolt used to provide a means of clamping the upper tube to the rod. The diagonal mirror mount was made of wooden closet rod, cut at 45 degrees and carefully screwed to the inside of the can. The diagonal was temporarily taped to its mount for assembly and alignment. After initial assembly, I ended up using epoxy to fix the diagonal to its mount.
A red-dot finder from www.chilonchina.com was added to the upper tube.
Once the upper tube was built, it was a simple matter to cut the rod to length. Aiming at a nearby hilltop with a low power eyepiece in the focuser, I slid the upper tube along the side of the rod until the image came into focus. I marked the rod, allowing for the distance the rod must protrude into the clamp of the upper tube, and cut it to length.
本文转自：China Industry News