I’ve stressed before that most of what’s needed to build a telescope can be bought at a standard DIY shop, but just like the grinding tool a whole lot of effort goes into putting those parts together. The last few weeks have been spent scrounging supplies and ‘MacGyvering’ together a few last items in preparation for grinding.
Mirror grinding involves regularly changing the orientation of the tool and the Pyrex so that the resulting surface is an even sphere with no areas that have been ground down too much or too little (more on this in the next post). To achieve this the tool needs to be attached to a surface that can rotate, or set on a pedestal that the worker can walk around. As I’m doing this in my tiny London kitchen I’ll be making a turntable. I Picked up some old shelves and pieces of a dresser that had been discarded on my street. Garbage picking for the win! I drilled holes in the boards, bolted them together, and put furniture skid protectors between the boards to keep some resistance while turning. I also put some skid protectors on the bottom to keep the whole thing from sliding around, and screwed in some stacks of even more skid protectors on the top of the turntable to hold the tool in place. So there it is, a cheap and cheerful turntable!
Another important part of grinding is knowing how much Pyrex has been removed, and to do this means measuring the depth of the ground out hole in the centre of the Pyrex. Here’s the catch: it needs to be measured to within 0.025 mm! That’s small, really really small. For comparison, human hair is about 3 times as thick as what needs to be measured. A device that can easily measure these small lengths is a dial indicator, and these are actually common workshop and machining tools. I bought one that is accurate to 0.01 mm for only £10. In order to measure the depth at the centre of the Pyrex compared to the edges I fastened the dial indicator to a spirit level. This Frankenstein measuring tool looks a bit of a beast, but it should do nicely.
To use it I just need to place the spirit level on a flat surface and read the value on the dial indicator (this becomes my ‘zero’), and then place the level on my mirror and take another reading. The difference between the two readings will be the depth. Trying this out on the fresh Pyrex has revealed an uneven surface with a deep point of almost 2 mm (nearly a Grand Canyon!). This should not be a problem as my mirror will need to be deeper than 2 mm, and so all of these hills and valleys will be ground away.
Now I finally have all the bits and pieces I need to start grinding, so no more procrastinating!