The K-20 Project

The K-20 Project

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2/23/2007 - The Secondary Cage Part 1: The Rings

The secondary cage has two stringent requirements, it must be strong and more importantly, LIGHTWEIGHT.  Remember, this will be an f/5 telescope meaning for every pound of weight added to the upper end, FIVE pounds must be added to the mirror box to maintain that ever important balance that creates buttery smooth motion.  The Primary Mirror will weigh 50+ pounds and the cell and tailgate another 12 or so.  Even this weight combined with the mirror box itself will only do so much in balancing 12-15 lbs of secondary cage hanging 6+ feet off the other end of the see-saw.  Consequently, the heavier the secondary cage, the DEEPER the mirror box must be.  Deep mirror boxes are inherently LESS RIGID and create TALLER rocker boxes which also are inherently less rigid.  Long story short, the secondary cage must be built only large enough to provide adequate light baffling for reasonably dark skies.  In this case it will be 13.25 inches tall with the focuser mounted 8 inches below the top edge of the top ring.  When needed, an additional light baffle (in the form of lightweight Kydex) can be added with velcro dots opposite the focuser.  The secondary cage is simply two high grade plywood rings mounted 12 inches apart using 4 aluminum struts with threaded inserts.  Below the rings are cut from 5/8 inch multiply Baltic Birch plywood.  Not only is this wood strong but also cabinet grade so it will finish beautifully.

ENTRY PICTURES


The fullsize plot of my secondary cage AutoCAD drawing is used as a template to locate all strut seat holes, upper pole clamp holes and of course a center point for the router.	My brother using a router to cut the outer diameter of the rings. Note the plywood is on cardboard to avoid damaging the surface veneer while cutting the ring. 3 passes around the rings were done for both the inner and out diameters. This lessens any burning caused by an overheating router bit and keeps the bit speed up such that the edges of the cut are not jagged and will sand easily.

Outer edge of the ring cut, time for the inner diameter. Remember, if you're doing this yourself, you MUST measure to the OUTSIDE of the router bit for your inner radius and the INSIDE of the router bit for your outer radius.	The completed rings! All outer edge corners have a nice 1/8 radius on them which provides a finished look and there are no sharp corners where the cage will be handled to point the scope. How big are those? I can pass the ENTIRE secondary cage on my 13.1 inch scope through those rings.