Click Here
Homepage
  Original Observatory build Split Ring Mount Steel Telescope pier 12Inch Cassegrain AE Mount Refurb New Observatory build Home
Homepage

 

Building the Skybadger 12 ” Cassegrain

This page is about designing and building the Skybadger 12” Cassegrain telescope. It draws heavily on advice from both Texeraux and Van Rooij regarding how to design a Cassegrain tgelescope and the sizing of components for best resolution, coma and field illumination behaviour. I purchased the mirror set for the Cassegrain telescope from Crawley Astrosoc in September 2010. The mirrors came already mounted in plywood frames (for the primary) and a single-arm metal holder for the secondary. Their original telescope design can be seen in these two pictures.. There was nothing left of the tube but rotting bits when the telescope was cleared from an inheritance clearout request to Crawley. So all I have to work with is the pictures and some designs that are rumoured to exist within Crawley Astrosoc papers (but a copy is promised if it surfaces). The primary is inscribed on the back, fortunately, with: G Hole and Son Limited Brighton 1974. Focal length 88cm, focal ratio 2.77, paraboloid. This mirror is 315mm in diameter and 32mm thick with a 72mm central hole in the primary. The secondary therefore must be a hyperboloid (to make a cassegrain scope) and provide a magnification factor of at least 4, to achieve a nominal f/ratio of 10 or greater. The secondary is measured at 92mm across the diameter. Using basic geometry, the secondary would be placed at 92*2.77=254mm from the primary. Using calculations from Van Rooj for a fully illuminated secondary, this suggests a working focal ratio of f/8 to f/10 since the seconday mirror is so large. The obstruction ration is 92/315 or 29%, suggesting not a terribly good telescope for high contrast viewing.