Original+TRO+Testing+the+curve+for+accuracy

When the hogging out process was near completion great care was needed to ensure that we did not over run and cut too deep into the glass. During all the hogging out stage we had been keeping notes on the sagita reached at each stage. By now we where at an approximate 10mm sagita with only 0.5mm required to go for completion. We decided to take readings across the face of the mirror by drawing a dart board type pattern with a felt tip pen and then using a car feeler gauge. Then the wheel was backed off to a distance of 20 thou. from the mirror. Differences of distance between the wheel and the face of the mirror across the disc at the intersections of the pattern where taken. This gave us a profile of the surface to an accuracy of a few thou. Measurements confirmed variations in the disc of about 20 thou. These had to be reduced to 10 thou or less to have any chance of going onto the next stage. A number of very fine cuts where then done and readings taken at each stage. The last cut produced variations to within 10 thou across the disc with only some 5mm of the edge of the mirror remaining uncut. We decided to stop at this point rather than risk going too far. Remember, glass cannot be put back once removed. The next step would be to remove these irregularities by more traditional methods by hand. Above image shows the mirror after hoging out with a sagita of 10.5mm First we had to convert an old oil drum into a worktable. This was cleaned out, cut, welded, filled with sand bags and fixed to the garage floor. Above image shows the construction of the original mirror cell. This was used to complete the mirror by hand on the old oil drum. This would late be dumped for a slimmer mirror cell covered with a layer of crapet.

Above image shows Geoff putting more bevel on the mirror's edge. The dart board marks for measuring the rough grinding accuracy cam be seen on it's surface.

Original TRO Introduction and concept Original TRO Purchasing the blank and hoging out Original TRO Testing the curve for accuracy Original TRO Making the tools for grinding by hand Original TRO Evening out the irregularities left by rough grinding Original TRO Polishing and testing Original TRO Making the mount and tube assembly Original TRO Making the secondary flat Original TRO Aluminising the primary mirror Original TRO First light