My first Magazine Sky & Telescope - 02.2019

He decided to grind it back to f/2.6, which would leave him with a half- inch-thick edge. He did that in about a month of hand grinding, then ﬁ ne- ground it for another 70 hours. He had to stop at 20-micron grit, since the thin mirror ﬂ exed too much and scratched with ﬁ ner grits. There was no way Mel was going to polish and parabolize a mirror that size and that rough by hand. So he spent a year making a grinding and polishing machine. Because he eventually plans to make a 42-inch ultra-fast scope, Mel designed it to handle up to that size. He put the 25-inch on the machine, and 74 hours of polishing later, using pads on the grinding tool, there were no pits left. But polishing pads leave an irregular ﬁ gure, so Mel made a hydro- stone-and-pitch tool and polished for another 69 hours, eventually bringing it to spherical. Then came the hard part: parabo- lizing. I’ll spare you the gory details, other than to point out that Mel did the entire process with the matching Ronchi test and star testing, eventu- ally (after 181 hours) coming up with a parabola that provides (with a coma corrector) pinpoint stars from edge to edge of the ﬁ eld. The temptation to tweak it further was great, but as Mel says, “Somewhere in the intersection between personal skill, Ronchi match- ing test and star test results, and the risk of making the mirror’s proﬁ le worse p Mel’s grinding machine can handle up to 42-inch mirrors. Here it’s polishing the 25-inch mirror. and having to start over, the ‘observer’ seizes the mirror from the ‘maker’ and sends it off to be aluminized.” With the mirror done, Mel turned to the scope that would hold it. At f/2.6, the eyepiece would be below eye level at zenith, which meant he had some room to play around with a concept he had been considering while he polished: how to eliminate the dreaded Dob’s hole with an alt-az mount. The solution was simple, elegant, and apparently brand-new: add a second altitude axis! When the scope is raised toward the zenith, instead of twisting it in azimuth for sideways motion the way a regular Dobsonian mount requires, Dob’s Hole “Dob’s hole” is the gymbal lock experienced when observing near the zenith with a Dobsonian mount, although Mel points out that it’s “rather unfair to call this Dobson’s hole since John Dobson hardly wanted the telescope he invented to be called the Dobsonian in the ﬁrst place.” t An extra fl ex rocker provides a second alti- tude axis, completely eliminating “Dob’s hole.” just push it sideways on another axis 90° from the standard one. Mel built a second ﬂ ex rocker on top of the ﬁ rst, placed the OTA on it, and shoved. It worked beautifully! And thus the alt-alt- az mount was born. When aimed near the horizon, the scope behaves just like a standard Dobsonian: You raise and lower it for altitude, and you push and pull it around in a circle for azimuth. As you raise it up toward the zenith, however, the second altitude axis comes into play. By 60° eleva- tion or so, the scope gently starts to rock sideways as well as rotate when you push, p The mirror and the higher you’re box also folds pointed the more rock- up to protect ing and less rotation the primary you get. At the zenith during travel. it’s all rocking motion, no matter which direction you push. Mel designed the second altitude axis to have about 15° — one hour — of motion. He also gave himself an extra 7½° beyond vertical in the regular altitude direction. That lets him observe an object right on through the zenith without twisting the scope in azimuth. sk yandtele scope.com • FE B RUA RY 2 019 71

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