8″ F/9 Planetscope


As you may have seen elsewhere on this site some years ago (15 to be exact) I ground a 8.25″ F10 mirror from scratch. Soon after I put the hobby on hold to parent two boys but now that they’re getting older I find I have some time to pursue the hobby again. Time to put that mirror in a telescope!
Optical Tube Assembly

The first thing I need to make in addition to optics is the optical tube assembly (OTA). This is the part of the telescope that consists of a upper tube assembly (UTA) that contains the secondary mirror and mount, spider, focuser, and finder scopes / telrad. Also part of the OTA is the lower tube assembly (LTA) containing the mirror mount and mirror. The design I’ve chosen for the OTA is a Serrurier Truss, named after CalTech engineer Mark Serrurier who developed this kind of OTA for the 200″ Hale Telescope at Palomar Observatory. An image of a similar OTA is depicted above except the LTA resembles the UTA with two rings and spacers so I can put a light baffle around the primary mirror to prevent stray light, and support a cover to protect it.

The Serrurier Truss has the interesting attribute that although it will sag due to gravity (as all tubes will) the front and back of the truss will sag equally so the optics will remain perfectly aligned. This is a huge benefit for a looooooong scope like the PlanetScope since with an 80″ focal length the tube will be about 72″ long.

Current status: UTA and LTA are complete, waiting on truss tubes to build truss.Mount

Once the OTA is complete, I will build a standard Dobsonian mount for the telescope. The scope will be driven using Meade Autostar computer control since I have a drawer full of Meade controllers and motors for some reason!

Current status: Awaiting completion of 16″ scope