Construction of a Newtonian 6" Telescope
After having paused with amateur astronomy for 15 years, the clear and dark sky in the Spanish suburb of La Coruña where I live, inspired me 1995 to take up my hobby again. Unfortunately my former "firstscope", a 60mm Refractor was still in Germany, so I had to find a way of getting a new telescope.
Looking through some ads from Celestron and Meade convinced me very fast that a good commercial telescope would cost far more than I was willing to spend at that time. So I started investigating possibilities to build a telescope myself. Soon I found the ATM Mailing list , where I received a lot of support encouragement and information.
I selected a 6" Newtonian Telescope as my first project. Since I did not know any ATMs close to the place where I live, I wanted to start small.
After deciding that I would grind the mirror myself I ordered a 6" low expansion glass kit from the German astronomical association VDS. I wanted to spare the second glass disk and made a Tile on Plaster tool .
Grinding, polishing and figuring the 6" mirror took roughly 40 hours and was completed by testing the mirror with a self-made Slitless Foucault Tester.
I felt quite uncertain about the focuser and spider design, since I had never worked with a reflector before. This uncertainty led me to constructing them myself, because I did not want to spend money for something, what later on might not fulfil the requirements. In the end I decided to construct a
motorized sliding focuser
and a two vane fully adjustable spider/secondary holder.
Both are almost fully made of wood.
The next step was to design the mount. To avoid complications, I decided to construct Richard Berry's 6" Dobsonian mount. I liked the look and it seemed easy to construct. I finally ended up with many modifications, but the basic design stayed the same.
I bought one 122x250x10mm and one 122x250x20mm plywood sheet. Half of each I used for the mount.
As side bearings, I initially used two 18 cm PVC rings that alternately ride on ball bearings or Teflon pads , depending on whether I want to use the telescope manually or guided by computer.
Eventually, however I decided to build a new mount as can be seen in the images to follow.
Gallery of the original design and modifications :
- Original mount
- Modified mount
- Motorised mount V1
- Motorised mount V2
- A close-up of the baffling rings of the main tube. The baffle diameters were calculated with Dale Keller's Newt program.
- I used ordinary ball-bearings in combination with screws and plywood supports to construct the bearing surfaces for the ground board and side bearings.
In order to keep a tight fit between the supporting screw and the bearing, I increased the diameter of the screw head with Teflon tape to fit the inside diameter of the ball bearings.
That system worked reasonably well considering the low weight of the telescope.
- As counterweight I added a plywood disk to the top end of the telescope. Counterweight can be easily incremented by adding additional plywood elements.
Later on I also added another counterweight to the centre of the telescope tube in order to be able to shift the centre of gravity.
- A very simple accessory but nonetheless very useful is this small tray that accommodates eyepieces, filters, pencils etc.
- One of the most useful additions to Richard Berry's design was a mirror trapdoor that not only allows for quick cooling of the mirror, but also for some intermediate cleaning sessions without having to open the whole tube.
- For the second version of the telescope mount I changed the way the ball bearings. are mounted. Since I no longer needed the possibility to convert from motorised operation to manual operation, I eliminated the optional Teflon pads. Therefore I was able to support the ball bearings. in a better and more stable way as you can see on the photographs.
- Since the telescope is driven by stepper motors, I opted for oversized side bearings which are rimmed with aluminium.
The telescope tube is supported in a housing that at the same time serves as support for the two side bearings. The advantage being, that the telescope tube can be moved in longitudinal direction for changing the balance point of the whole assembly.
- The plywood baseboard is covered as well with 3 mm aluminium sheet to provide a hard contact surface for the ball bearings and azimuth drive roller.
Balancing of the tube:
- To be able to balance the telescope properly, I added spring tensioning to the ALT-axis. This is necessary, as the weight of the tube assembly is not enough to provide sufficient downward force on the drive roller.
- At the longitudinal center of gravity a counterweight was added that allows to modify the vertical location of the center of gravity. This allows for even movement and no slippage irrespective of where the scope is pointing.
Motorised mount 1. version:
- Initially, the motors were attached to the amount with simple plywood brackets and rigid shaft couplings to attach the final roller.
I also replaced the original small PVC side bearing with a large plywood disk that had been rimmed with 3 mm aluminium.
- This photograph shows the assembled altitude drive. Due to the low weight of the tube assembly, I added a spring tensioner to increase friction between the side bearing and the roller drive.
You can also see a camera mount on the altitude axis to attach a piggyback camera. Later on I developed a dedicated computerised piggyback mount for that telescope.
Motorised mount 2. version:
- ALT-motor mount:
- For the second version of the motorized mount, I used Vexta motors which I mounted together with the gearboxes on plywoodbrackets.
- A lot of attention was given to decouple the motor mount accustically from the telescope mount in order to avoid transmission of vibrations and motor stalling due to accustic feedback. Materials used for isolation are cork, neoprene and mousepad rubber.
- The drive roller is connected to the gearbox via flexible shaft coupling.
- AZ-motor mount:
- The AZ-motor assembly is mounted on a plywood board that is attached to the baseboard via 2 screws that allow it to autoadjust to the angle of the cone-shaped drive roller.
- The connector boxes for the cable connection include a simple current limiting circuit.
- The drive shaft is connected to the drive roller via flexible shaft coupling.