Things have gotten quite busy at work – servers have crashed, and I’ve been putting in a lot of late nights putting out fires, rebuilding services and restoring backups. This has left me no time at all for mirror grinding, so yesterday afternoon at the Class was the first bit of work I’ve managed since last time. I made a near-fatal error, however, which almost ended the project right there…
Johan was there with an interesting toy: a vintage naval gun-sight (Correction: Not a gun sight, but a moonscope – see the note at the end of the page) converted with the usual ATM flair into a compact little 5 inch refractor. The objective lens was quite heavily masked to hide the many large chips (some the size of a thumbnail) which had presumably been caused by decades of rough treatment. The original tube remained intact (a heavy metal tapered tube, shaped to approximately follow the shape of the light cone), and had a diagonal with helical focuser stuck to the end. What made it particularly interesting to use, though, was the altazimuth mount: The altitude axis was centered on the optical axis of the eyepice, meaning that the observer does not have to move his head as he tracks across the sky. Obviously such an arrangement puts the entire weight of the telescope to one side of the axis, so it was balanced with a heavy weight projecting some distance from the axis. It sounds pretty clumsy and I’ll admit that it looks that way in the flesh as well. But it was surprisingly smooth and fun to play with. Besides, telescopes are for looking through, not at.
Having seen the new scope, it was time to work. I put in about an hour’s work with the #500 grit (not very nice to work with – some moisture must have entered the salt shaker I use as a dispenser, so that it formed clumps and would not pour. I eventually took to removing the top and pouring unavoidably huge piles directly from the bottle. Wasteful, but at least it worked), and check with the loupe.
This testing for pits business is getting to be more and more time consuming. The pits left at this level of grit are approaching the limit of what can be seen with a simple magnifier. Still, I managed to identify a lot of the features I’d discovered the last time. To check whether they were actually pits, or just microbubbles and other flaws beneath the surface, I shone a bright torch across the surface while inspecting the same areas, lighting them up from different angles. My conclusion at the end of this: NO pits left. Fantastic!
So, the usual clean-up procedure began, although with a little more care – previous sessions had seemed to create a fair number of new pits, which made me worry about contaminants and scratching the surface. Not wanting to put the glass anywhere where it might pick up old grit, I rested them back-to-back in the water bucket. I then rolled up the wet newspaper I’d used to line the workspace, carefully packed away the saltshaker full of grit and the sponge I’d been using to wipe the glass clean between wets in a ziplock bag, and scrubbed my hands thoroughly. I then opened a packet of #800 grit (Such a light grey, and so very fine that it actually raised a little dust cloud every time it was disturbed!) and poured it out into a brand new salt shaker. I moved the bucket and spray bottle to the floor, and laid out clean newspaper. Only two tasks remained: Wash the bucket and spray bottle as thoroughly as possible, and clean my hands again. I took the bucket outside, and threw the water into the bushes. A sickening glass-on-glass sound rang through the air. I had forgotten to remove the mirror and tool. I had just hurled 18 month’s of work into a garden dotted with heavy bushes and rocks.
I died a little on the inside.
After a little thought, I reasoned that, since I hadn’t heard glass shattering, it might be worth trying to salvage the situation. I further concluded that, since the pieces of glass had already come to rest, they could be left to lie where they were until I’d decided what to do next. So I went ahead with clean-up, scrubbing out the bucket and rinsing off the spray bottle, wetting the new newspaper, arranging everything where it needed to be, and only then ventured into the garden to find my mirror.
I found the tool first – it was lying on a bed of loam, between the garden’s concrete edging, and a spiky bush. I rinsed off the dirt, and put it in place at my workstation. I then went back to the garden and searched for the mirror. And searched. And searched. Several times I stopped, wondering if perhaps I’d removed it from the bucket after all… but it eventually turned up, wedged between the branches of a particular dense bush, suspended about 20cm from the ground.
To cut a long story short, the mirror and tool both survived, with no chips or cracks. Which is why, for just a few hours, I was the luckiest man alive. Incidentally, I was too nervous to inspect for scratches, so just dove straight in with the #800 grit. Everything I said about the #500 grit applies doubly here. It takes a good minute of each wet working back and forth before the film of water is thin enough for the abrasive to even touch sides and actual work to begin. And it’s such a subtle feel, that the only way to tell when a wet is done is to watch for when the slurry changes colour from light grey to very light grey (indicating that it’s now composed more of ground glass than carborundum). But it’s very very fast – less than half an hour after beginning, it looks almost complete with just a few pits near the edge. If I can only get in a session or two at home, I might even be able to start polishing next time I make it to class. And I seriously genuinely cannot wait!
If you’ve ever made a telescope, I’m sure you’ll have a few horror stories of your own. Share them in the comments below.
Update: Chris Stewart wrote on the Urban Astronomer facebook page to say “The scope Johan had is not a gunsight, it is a moonscope – a 5″ achromat refractor with a roof prism. Many of these things were distributed around the world in the early stages of the space program, when teams of volunteers would track and time satellites and send the data to the US for analysis. This allowed scientists to deternine orbital element, ascertain the degree of atmospheric drag on orbital devcay, and so forth. It is a lovely little scope with a long and illustrious history.”