16 inch/f4,8 Dobson

Mirror diameter: 16inch (406mm)
Focal length: 76.5inch (1945mm)
Mirror thickness: 1 5/8inch (41mm)
Material: Pyrex
Diameter secondary: 3.1inch (79mm)
Finders: Quickfinder + 3,5"/f4 Newton
Weight: 35kg

The finished telescope.

The secondary cage:

For me it is important to have the chance of using a binocular attachment with my telescopes. Because of this, the 16incher had to be build with a sled focuser:

Focus is adjusted by turning the knob on the right, which turns a worm gear that is connected to a shaft with pinions. All this works in the same way as it does in my Bino.

The secondary cage at the beginning of construction.

Instead of the tubes normally used as struts for connecting the rings of the secondary cage I made U- shaped profiles from plywood. This saves weight and the cage gets more rigid. As an additional benefit, the surfaces can be used for the mounting of the linear guidings.

The sled focuser during construction.

The guidings are called "DryLin", the worm gearbox is homemade with bought wheels.

For the use of the binoular attachment, the sled focuser has to move far down. To prevent doing this by turning the knob all way long, the shaft can be separated from the gearbox by a clutch. Then the guide slide can be shifted freely.

The spider is made from 1.5mm Aluminium sheet metal. In the middle, a piece of 20mm square Aluminium with a central bore is fastened by screws. This carries the secondary mirror.

At the bottom of the square Aluminium there is a plate fastened by screws that has also bores for the adjustment- screws. The plate connected to the cylindrical part of the secondary holder has got bores 5mm larger than the diameter of the screws (that´s the reason for the big washers), so the mirror can be shifted aside for adjustment.

Mirror box and rocker:

The mirror box is made from 12mm plywood. Rocker und side bearings are made from 30mm plywood. The side bearings have 315mm radius which keeps the rocker low.

The rocker and the basic frame made from 20mm square Aluminium tubes.

One of the three corners, on which the rocker turns. The rollers are made from polyamid and have a bearing made of brass tube in the center. The turn on a M8 scew. The latches on top of the rollers do have a gap of 1mm to the ground board of the rocker and prevent the frame from falling apart during transport.

The mirror cell:

18- point primary mirror cell

The parts of the cell, above top a complete mounted branch, below it the separate parts of another. (The tube below is not crooked in reality)

The cell is a frame made from 40x20mm rectangular Aluminium tubes which are connected by 6mm Aluminium sheet metall and screws. The bars connecting the triangles are positioned inside the tubes.

At thee points the frame is connected adjustable to the mirror box. The sling is connected to a bolt that is fastened to the frame from upside.

The mirror:

The mirror was made from a 16inch wide, 1 5/8inch thick Pyrex blank with a diamond generated /f5 curve. The tool has 330mm diameter and is made from two glass disks of 12mm thick floatglass and three disks of plywood. The curve of the tool was pre- grounded by a slow running drill equipped with a diamond cut head . The tool lies in a tub filled with water which is turned on a rotary table. The grinding underneath water prevents harmfull glas dust from flying all around the workspace.
The shape of the curve is checked with a radius- template from time to time. A tool made of glass can be pre- ground in that way close to a few 1/10mm of its ideal shape. Then it can be adapted to the mirror with grid#80 within half an hour.
Fine grinding with grid sizes 120-180-240-400-600-800-1200 went on without problems. I grind aproximately 30 wets of each gridsize.
Polishing was done with the 330mm tool covered with 28° pitch and took about 14h. After 2 hours of figuring a spherical shape sufficent to start parabolizing was reached.
Parabolizing was done with tool of 230, 150 und 100mm diameter and took about four weeks in total.
According to the calculation done with figureXP the wavefronterror is approximately Lambda/12, this should be sufficient.

The finder:

Because mirror making and telescope building is great fun, the 16incher got a matching finder:

The little one has got a 12mm thick Duran- mirror (self- ground, of course!) of diameter 93mm and 360mm focal length, resulting in f/3,9 and a large field of view.

As it shows a lot, he got a little rocker which makes him a real dobsonian.

The "focuser" comes from a hardware store and is originally a bulb socket. The insert was turned to 1 1/4" on a lathe.

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