Rebuild to a lowrider

Although the 17.5 inch telescope I build in 2008 was a great telescope changes in my personal live ( two kids) dictated that I needed a more compact telescope. So I decided to rebuild the entire telescope.


  • The footprint of the new telescope needed to be smaller. The groundring for a flexrocker is a large part to transport. I wanted to decrease the diameter from 80 cm to 60 cm.
  • I wanted to make the mirrorbox more compact, so I could carry it more easily. In this version the mirrorbox and sidebearing would be 8 cm narrower than the previous version.
  • I wanted a more precisely manufactured telescope to create the most compact en lightest goto controlled 17.5 inch possible.
  • I also wanted to eliminate the necessity for stairs when observing around the zenith. By angling the secondary mirror more steeply than usual to create a socalled “lowrider”. In my final design, the secondary mirror was build at an angle of 64 instead of 45 degrees. This allows me to move the focuser 22 cm down the OTA.
  • All motors, the computer and the battery would be placed on the rocker so that no wires would be on the ground around the telescope.
  • The telescope would also have a adjustable secondary mirror holder. On previous versions I would collimate by adjusting the strings and poles, this would work OK, but not great.


To achieve these objectives, I had to design differently. During the construction of the first versions, I made drawings by hand. For this new telescope I used Google Sketchup to create a extensive 3D model of the telescope. I also had the plywood parts CNCed directly from a computer file. The plywood parts were then all sandwiched in carbon fabric. This way I could reduce the thickness of all the parts and still retain stiffness in the design. For instance the thickness of the side bearings was reduced from 30 mm to 10 mm. The mirror box is an asymmetric octagon consisting of a bottom plate of 12 mm plywood, carbon tube material and a top plate of 9 mm plywood, all finished in carbon fiber. The tube material is generally used for the fabrication of hockey sticks and is extremely strong and light.

Secondary ring

The secondary ring is a sandwich of 4 mm plywood and polyurethane foam board. Carbon fiber was used to strengthen the ring. Because I wanted to place the secondary ring on the mirrorbox for transport I had to be able to remove the focuser. This was a tricky part to make. It consists of a 3mm thick aluminum plate that was bend and coated with 8 layers of carbon fabric. This resulted in a 6 mm thick plate, which was then strengthened by adding some pieces of carbon to the sides. Some round magnets ensure the correct positioning of the focuserplate relative to the secondary mirror.

Bearings & mirrorcell

The 10 mm thick side bearings connected by a frame which is in turn connected to the mirrorbox. The carbon tubes (hockey stick) in the side bearings serve to strengthen the side bearings and provide an easy way to lift the telescope. The three pieces (box, side bearings, frame) can be transported separately, but are usually transported as a whole. The main mirror has the same holder as the old telescope. It’s a 9 points cell calculated with Plop. However, the aluminum triangular plates are now resting on round magnets that in turn are resting on the head of a bolt. By turning these bolts one can tilt the mirror for collimation. It is a very simple system which has the added advantage that the cell requires very little depth. The mirror is almost touching the bottom of the mirrorbox. Due to spatial constraints the lateral support needed to be different. The lateral support of the mirror consists of an L-section which is sawed in.

Rebuild to a lowrider (2)