My ultralight Dobsonian

Mirror diameter 402 mm (16") Focal length 1522 mm (about 60") Mirror out of float glass, thickness 22 mm (about 7/8")

Weight of the scope 25 kg (about 55 pounds) Mirror cage and secondary cage welded out of square steel tube 10x10x1 mm (about 3/8" x 3/8" x 0.04"). The mirror cage has 8 steel pins, two in each corner. The aluminum tubes of the truss fall over these pins, under the secondary cage the truss tubes rest in U-formed holders. The secondary cage is pulled against the mirror cage by two steel cables (cables of a bicycle brake).

Dobsonkl40cm_schachter_sept02.jpg (102400 bytes)

The scope fits in this wooden box for storage and transport. Dimensions 58x58x46 cm (23"x23"x18"). The truss tubes are stored in the cardboard tube.

The altitude bearings are attached to the mirror cage and resting on the rocker.

The mirror is protected by a aluminum shield. Finder and baffle for the focuser are stored on this shield.

The aluminum tubes fit under the secondary cage in holders. A nut was welded to these holders horizontaly. A hacksaw was used to cut the nut verticaly, so the steel cable can pass. The soldered end of the cable falls into the nut. The underside of the secondary cage is not flat but was bended a little bit (about 1 mm or 0.04"). When the cables are put under tension, first the upper and lower truss are compressed.

The tensioner for attaching the cable to the mirror cage. The cable is attached to a aluminum disk, using a couple of setscrews. A aluminum bar is attached to the disk, this bar is used to tension the cable by turning the disk. Disk and bar have been drilled to receive a bolt M6 (about 1/4"). At the right a aluminum wingnut can be seen, sawed out of aluminum bar and tapped M6. The wingnut is used to lock disk and bar in a tensioned position.

.	The head of the bolt M6 fits in a oval hole in the mirror cage, the cable is tensioned by turning the aluminum bar. Both alumnium bar and disk are locked in position by fastening the wingnut. The altitude bearings fit with two pins in the mirror cage and are attached with a bolt M6.

Here both tensioners have been locked and the shield has been removed to clear the mirror. Finder and baffles have to be installed. After collimation the scope is ready for use.

The mirror rests on a classic 18 point flotation. Plop was used to calculate its dimensions. Side support by two nylon blocks, separated by 60 degrees and turning freely around bolts M8 (5/16"). Four nylon clamps avoid that the mirror can fall out of its cell.

Due to the light weight of the scope movements are very easy. The altitude bearings use aluminum on teflon. The rocker uses 9 mm plywood (about 3/8") treated with a water resistant and slippery material for pouring concrete. The teflon pads glide directly against this layer.

Further plans: for normal use the scope has enough stability. However wind can be a problem. I am planning to cut the tubes in two and connecting them with a sleeve, this will make the tubes stiffer in the middle and enable to store them in the wooden box. Perhaps it will be necessary to bend the secondary cage a little bit more, to enhance stability in horizontal direction.

Also I am planning fans to speed the cooling of the mirror. Using the secondary shadow in the startest is a nice indication of the speed of cooling, now I need several hours before the shadow appears equally fast on both sides of focus. And I hope to have GoTo possibilities in the future, using the Bartels system.