Original+TRO+Evening+out+the+irregularities+left+by+rough+grinding

Rough grinding began on Saturday the 19/4/97 using the ceramic tiled tool. We began by covering the cell with plastic sheet to add additional protection against water and then placed a disc of small cell bubble pack between the cell and the blank. This would take out small variations and provide damping of the mirror and help combat vibration. 60-grade carberundum was used to start. The mirror was wetted down and 60 grade sprinkled on the surface. Normal stokes with a 2 inch overhand where used to start moving the tool back and forth for a few strokes, then turning the tool and taking a step to the right. Geoff and I took turns to have a go and hopefully give some randomness to the stoke. Reading information of the Internet and other books we did some full 'W' stokes to also give some variation and try and smooth the whole surface at once rather than deepen the centre even more.



The glaze on the tiles wore off after a few 'wets' and the tool bed into the curve. But it was not to long before we realised that the tool did not seem to be doing its job. The carberundum was collecting near the middle and the tool was breaking down making a light coloured sludge on the surface. The tool seemed to be riding over this and not doing much work. Adding more carberundum did not work and we seemed to be throwing it down the grid when we cleaned up between wets. Work was stopped and I decided to redesign the tool. This was done by covering the surface with metal washers, (see, Making the tools for grinding by hand) an idea off the internet which said they worked quicker than ceramic tiles but must only be used on pre-generated curves. This was done and the mirror and cell cleaned down It was while doing this that I noticed that the cell had swelled at many separate points on its surface. Water had got in under the plastic sheet and penetrated the varnish on the surface causing the surface to swell. It looked at first that the cell was ruined and another would have to be made. I left the cell to dry out for a few days and the finely planed out the damaged areas. The cell was then painted with 1 undercoat and 3 gloss coats then left to dry. All nooks and crannies where treated to ensure water could not get in. The cell was then covered with two sheets of plastic to make sure water would not get through again.

Grinding was resumed on Saturday the 19/4/97 with the new tool and 60 grade carberundum. A number of grooves from hogging out remained on the mirror and we watched these disappear over a few hours, gradually moving out towards the edge of the mirror. The 'W' stroke was used mostly with a few normal strokes to ensure we had even coverage. Direction and length of stroke where also changed. The 60 grade ran out and we changed to 80 grade. This seemed to work faster than the 60 and after 3 hours work there were only a few very faint irregularities on the very outer edge. Curiosity got the better of us and we decided to test the mirror. The mirror was removed from the cell and carefully propped up against a tree in my garden. We wet the surface with clean water and stood near the focus point. As we varied position relative to the focus we could see the roof of the house, chimney pots and TV aerials.

Over the next week I completed the 80 grade and moved onto the 120 grade. This was well on the way by Saturday the 3/5/97. We completed the 120 grade and moved onto 240 grade after cleaning up and replacing the bubble pack, plastic sheets and cleaning tools (sponge etc) to ensure no contamination. Geoff changed his shirt from a 120 to a 240. Care had to be taken when touching surfaces to ensure no cross contamination. The supprising thing that was apparent about grinding by hand was how quickly it seemed to be going. This could be partially attributed to the fact that the mirror was plate glass and probably softer and easier to work than harder Pyrex or other low expansion materials. The table below shows the times of total work done for each grade. At the end of work on the 3/5/97 it was near 9:30pm and was going dark. We removed the mirror from its cell and propped it up against the tree at the end of the garden and wet the surface. A very bright image of the inside of my garage could be seen with details of light fittings and the reflected light off the roof. The imaged was diffused a bit because of the water not being a perfect reflecting and smooth surface but these images where very encouraging and showed that the mirror was well on the way and would give a reasonable image if maybe not a good one. Only time would tell and we planned on starting polishing within a few weeks.

Grade used Total time of work done Roughing out 60 80 120 240 320 400 20t (500) 12t (600)

9.0 Fine grinding

I went to Orion optics on the 9/5/97 to pick up some fine grades of 20t and 12t, two pounds of each. These would be used in the last stages of fine grinding before polishing. Orion optics stated that there was no point in going further than 600 grade as scrtching of the surface would be inevitable, and in any case it is not necessary. Polishing may be started from this point. This was confirmed by Andrew bates of our society (he makes mirrors also).

The 20t was mixed 6 heaped teaspoons to 3/4 full sauce bottle and shaken well. A spiral was added to the mirror and both tool and mirror misted with water from a hand spray bottle. This was all used up and the mirror took on a very smooth surface with the pits so small they could hardly be seen in the microscope. Great care had to be taken in moving the tool on and off the mirror and cleanliness during grinding. This grade was completed in one session.

We then moved onto 12t on the 31/5/97. This proved to be a problem. It appeared that the tool was beginning to scratch the surface. The grinding powder was too fine for the tool and scritching sounds could be heard. We stopped work, cleaned down and inspected the surface. Some fine buff scratches could be seen. We decided that no more fine grinding be attempted and that we go onto polishing. Picture above is a close-up of Dave Owen (His hands anyway!) using a sub-diameter tool during fine grinding. You can see the 'sweep' marks of the washers from the tool on the surface of the mirror.

Picture above shows Steve Southern 'having a go'. Many Liverpool Astronomical members helped out. See the credits on the main Home page.

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