Assembling the OTA

After printing both the top and bottom rings, the mid rings, and the mid plates I started putting together the optical tube assembly (OTA). First I found that the OpenBeam 2.1 was a little too wide for the printed parts. I had orignally tested these parts with an OpenBeam precut kit made up of version 1.0. So, if you are using the files on Thingiverse be sure to make test prints and adjust the size accordingly. Instead of reprinting all of these parts I made the holes larger with an X-Acto knife, files and sandpaper.  

Fig. 1 All of the OTA parts before sanding and general finishing.

Fig. 2 All of the OTA parts before final assembly. 

In figure 2 all of the OTA parts are organized on a table just before assembling them together.  To the bottom left there are all of the mirror cell parts; consisting of the round central hub and three triangular plates for the mirror to sit on.  The spider which holds the secondary mirror is just above that.  To the right in orange are the two main rings that will hold together the main structure of the OTA.  In the top ring is the encoder plate and mid-ring parts that also attach the OTA to the mount.  And to the far right is the rotor which hold half of the magnets in the axial flux motor and will also attach to the other side of the mount.

Below in figures 3 and 4 are up close looks at the top and bottom rings.  The bottom ring holds the mirror cell and the mirror and will have three adjustment knobs for colimation of the optics.  The top ring holds the spider which holds the secondary mirror. The secondary mirror will redirect the light from the primary mirror by 90 degrees to the side of the OTA and into the focuser and eyepiece for viewing the image.

Fig. 3 The bottom ring (orange) and the mirror cell (black) shown upside down after putting together all of the main parts.  The mirror cell here still needs the 3 triangular plates, springs and adjustment screws to hold the mirror. 

Fig. 4 The top ring and spider after assembly.  The secondary mirror will be held in place below the central hub.  

Below in figure 5 the OTA has been joined together by 6 pieces of OpenBeam aluminum extrusion.  The OpenBeam in this picture is 1 meter long and has not yet been cut to its final length.  The 9.75" mirror is intended to be f/3 but has not been finished, thus I will wait to cut the OpenBeam until the focal length of the mirror is set.  Also the focuser board was not placed on the OTA due to fit/finish issues and will be fixed at a later date.  The placement of the mid-plates/axis of rotation is not final because this placement will be chosen once the center of mass of the OTA is finalized.

Fig. 5 The optical tube assembly (OTA), with the rotor plate facing the camera and the encoder plate facing away.  The rotor plate will have 12 magnets placed inside for the altitude motor. 

Over all I am pleased with the look and feel of the OTA.  I will continue to develop the mount and hopefully have some of the parts put together soon.  It's looking like it is possible to develop a high quality 3D printed telescope.  Maybe I'll shoot to have most of this done in time for RTMC in late May of this year. 

Also, I am trying to put together a game plan for developing the hi-resolution absolute encoders for the mount.  In coming posts I will try and go into some more details about how the encoder will work and how I plan to test/develop the hardware and software for the telescope.  

Optical Tube Assembly Printing

I have been test printing parts for the 3D printed telescope since early October. Starting with the two main rings for the optical tube assembly (OTA). Early on I knew I would be limited by the size of the print volume of my Dremel Idea Builder, which is 230mm x 150mm x 140mm. This led me to break up the rings into 6 equal parts of a circle. I tried four different ways to connect the parts together and settled on an interlocking system held together by 3 M4 x 20mm socket head cap screws arranged in a triangular pattern.

Each piece of the ring has a slot for a ~2 foot long piece of OpenBeam connecting the bottom and top rings. These slots have four holes (two inside and two outside) for M3 x 6mm button head screws to hold the OpenBeam in place. For the bottom ring there are 12 other M4 holes arranged every 120 degrees for the mirror cell blocks that connect the central mirror cell to the OTA. On the top ring there are M4 holes arranged every 90 degrees for the spider blocks that attach the spider and secondary mirror assembly to the OTA.

Below are a few pictures of the top ring that I have printed and assembled. In the first photo you can see the interlocking system at each end of every piece. There are 18 M4 x 20mm bolts and 18 nuts that hold the ring together. All the hardware is countersunk making all of the surfaces clean looking. You can also see the vertical holes made for the spider blocks placed at a separation of 90 degrees. There are 4 holes when the spider block is near the center of the piece and 2 holes when the block is over one of the OpenBeam slots. The OpenBeam will extend about 1cm into the spider block that is over the slots to add stability.

Again you can find all of my current .stl files at http://www.thingiverse.com/thing:1112195