Zeroing in on Hyperboloid Heaven

A week ago we met at Steve Follett’s shop and shared the progress on the mirror and the azimuth drive, the mount fabrication and the design drawings.  In that meeting, Steve pointed out that he was at an interesting point in crafting the curve on the secondary mirror. You’ll remember that the secondary is convex and hyperboloid in shape.  This allows us to build an effective F/14 40-inch telescope without having to build a tunnel from the west wing to the east wing of the observatory to stow it (14 x 40″ = 46 feet).

It’s an interesting moment because, just as we are beginning work on the secondary cage, its distance from the primary, the related distance to the third mirror, and the distance from the optical center of the telescope out to the focal plane all converge on the shape of that secondary curve.  At any given combination of those distances, there is a particular version of the curve that gives a perfect image (or as perfect as we can get it!). If we end up making small adjustments in the secondary mount or on the third mirror mount, we will be moving the focal plane (probably why we are doing this adjusting), but we will also be moving the image away from perfect. Within a reasonable adjustment distance, the image will still be acceptable in a way that we can quantify.  But we should make sure that our base design is specified well enough so that Steve can choose the matching curve and do his best to put it on the glass of the secondary mirror.

Happily, the design drawings have also arrived at a state of completeness that allow us to begin to answer the question Steve posed.  With the new, improved secondary cage designed to lighten that end of the scope so we won’t need counterweights, and with the addition of the third mirror with its spider and anchors (now a 3-inch minor axis elliptical), and, finally, with the addition of the Astrophysics 4-inch focuser with a 2.2-inch extender inserted in the path, we have a position for the focal plane.

The official dimensions are:

Primary to Secondary: 109.7″ (2786.9 mm)

Secondary to Tertiary: 87.3″ (2218.2 mm)

Tertiary to Focal Plane: 39.25″ (997.0 mm)

This assumes we use the full length of the truss tubes that we have in our inventory of parts – 96-inches.  The next post will contain the curve of the hyperboloid from Steve’s calculations, given those dimensions. I really hope they work!

Of course, Mark isn’t happy with this yet. It would be better to push the focuser out even further to make it less likely that our visitors will whack their knees or stub their toes on the telescope infrastructure while they are viewing. I haven’t figured out how to do that yet, but if your creative juices come up with an idea, send me an email or a sketch and it will be used either as-is or to come up with something even better. To give you some feel for what it looks like so far, check out these PDFs (3D style). One is with the scope pointing to the zenith, and one at about 45 degrees in altitude.

Project 40 Telescope Assembly.8

Project 40 Telescope Assembly.9

While I’m waiting for your inspiration to meet Mark’s requirements, I’m on the prowl for belt drive components for the altitude drive. If you are belt drive expert, ping me – the last belt drive I had was on a turntable!


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