At the 2002 Riverside Telescope Makers Conference I bandied about the idea of making a big scope for myself with friend and fellow telescope maker, Dave Rowe of Los Angeles. I asked if he knew of any 30" or larger mirror blanks kicking around that could be had for a less-than-astronomical price. Dave told me about a 1-meter mirror blank at Wangsness Optics in Tucson, Arizona. Wangsness Optics specializes in very light-weight mirror blanks, and large spin-cast blanks.

The big blank could be had for a fraction of the usual price because in the process of spin casting it, several of the mould's cores came loose. As a result, a section of the mirror has a non-uniform rib and faceplate thickness. While the customer rejected the blank, it was still deemed usable in a telescope for a budget-conscious individual.

But there was one problem: It was spin-cast to generate an f/1.5 curve!

There was no way I would attempt a 1m f/1.5 aspheric mirror - life is too short! There had to be another way to convert the blank into a useable telescope. During the entire weekend at Riverside, one question kept bouncing around in my mind:

"How can I turn a 1 meter f/1.5 spherical mirror into a telescope?"

The key was correcting the spherical aberration of course, but how to do it without nasty, huge corrector lenses? The answer came to me during the drive down the mountain to LAX. Once the idea of applying the Offner null concept to the problem came to mind, the rest was down hill. As we drove along the interstate, I could picture in my mind how replacing the Offner relay lens with a mirror would be ideal. The system would be layed out to throw the final focus behind the primary (effectively a Gregorian design) and the small corrective lenses could be constrained in size to fit within the centrally obscured beam. Upon arriving at LAX, I could not wait to fire up my computer to see if the concept would work. Sitting in the Air Canada lounge with a beer, it was amazing how fast the design converged toward a practical solution after only being roughly laid out. I can still remember my excitement when I looked up at Doug, "Holly s**t, it works!"

At first I had Zemax software use the asphericity of the secondary as a degree of freedom, thinking that an all-spherical design was too much to ask for. After exploring solution space for a while, I was able to converge on solutions employing all spherical optics.

Initially, it seemed too good to be true that such a simple layout of four tiny lenses could correct the huge aberration of a big, fast mirror. Only until I sent the design to other optical designers for confirmation did I accept the idea as having real potential. One of the thoughts gnawing at me was why such a simple layout had not been devised before. Apparently the Gregorian configuration has few fans because of the added length of the tube. The only designer to say that he had applied a similar layout to a telescope was master optical designer Dick Buchroeder, of Tucson Arizona. Interestingly, Dick used the same overall configuration I had arrived at, not to correct a huge amount of spherical aberration, but to solve an entirely different problem.

About ten years ago Dick was contracted to design a Cassegrain field corrector for an existing 6 meter class telescope. The aim was to achieve correction over a wide field without resorting to big lenses close to the Cassegrain focal plane. Dick's solution was to use a Gregorian layout with an aspheric secondary, and place several relatively tiny meniscus lenses around the intermediate focus. While the concept would have worked very well, the astronomer balked at the added length imposed by the Gregorian layout. The design was not put to use, and Dick never published his work.

Peter Ceravolo