Let’s try this again
29 months after I first put grit to glass, I’ve put my mirror on the test stand for the first time. It’s not as good as I’d hoped.
To recap: I last touched this project 10 months ago. Since then, the mirror and the polishing lap have been sealed into plastic containers, and left to rot in the loft while life overwhelmed me. But today, I got in my car and hauled everything down to Joburg and the ATM class. When I unpacked everything, I found the mirror caked with dry cerium, and the pitch lap had slumped down into a very flat mushroom shape, and gone rock hard (Also, some mildew had taken root in the pieces of damp fabric that lined the containers to provide padding and keep everything from drying out).
The first order of the day was to check my progress on the mirror. Last time I checked, I was pretty sure I had successfully finished polishing the mirror, so I wanted to test it to see what the next step would be. I washed it, rinsed it, dried it, left it to cool to ambient temperature, and popped it on the test stand. I then spent about a half hour struggling to get the foucault tester to work… turns out there’s an art to it, and that one does not simply apply book knowledge to do a quick Foucault Test. It takes a bit of skill and finesse.
There was another tester set up, this one with a CCD camera up against the knife edge. I brought my mirror there, and let one of the resident experts set it up for me, and there on the monitor was my mirror. It was obviously not a sphere (In the Foucault Test, a perfectly spherical mirror will show up as a smooth featureless white disk. A fully corrected mirror, with a paraboloidal figure, appears to have a shape closer to a donut. Or a very softly rounded impact crater), which was slightly disappointing. More experts came to have a look, until Chris Stewart gave his diagnosis: “Your mirror is filthy. You must have wiped it after washing it. Never do this. Also, you have a turned down edge. Also, it’s an oblate spheroid”.
What does that mean? Well a turned down edge is exactly what it sounds like: the outer edge of the mirror is flattened, and does not follow the rest of the curve. To fix this, the entire surface of the mirror needs to be polished away slightly, sinking the curve a little deeper until it extends all the way to the edge. This is my first task, and the stroke to correct it is short (1/4 diameter) center-over-center strokes with the mirror on top. This is to be done briefly, and the mirror should then be tested again. Repeat until the edge is fixed.
After this, we have to correct the overall shape. But what is an oblate spheroid? Take a sphere, and spin it hard enough for centrifugal forces to distort it. You now have an oblate spheroid. Planet Earth is an oblate spheroid, because it spins around its axis. Slice off a thin sliver, from around one of the poles, and you have the shape of my mirror. How do we deal with this? Put simply, certain changes have to be made to a spherical mirror to turn it into a paraboloid. If the exact opposite of those changes were to be applied, you get an oblate spheroid. So all I have to do is apply the standard parabolising stroke, and my mirror will gradually acquire a spherical shape, and then reach the desired paraboloid. Easy! (It’s a third of a diameter wide ‘W’ stroke, in case you’re wondering)
But before I could do any of this, I had to create a new lap. Step one is to remove the petrified pitch from the glass base (Which, you’ll remember, began life as the grinding tool). This was done by violently chopping at it with the edge of a putty knife. This causes the pitch to chip away in shards, like very soft glass. Pro tip: Do this at arms length, while holding your work deep inside a rubbish bin. You should probably wear safety goggles too, although squinting slightly seemed to work just as well… As I type this, my hands are still speckled with tiny crumbs of pitch that will probably stay with me for a few days. It’s messy stuff.
But once most of the pitch had been knocked off, leaving just a sticky film and a few black specks, I took it back inside and (with the kind assistance of Dave Hughes – thank you Dave!) made a new lap. This new lap, incidentally, sings. Every few strokes, it emits a soft whistling squeak, like a very boring birdsong! I’m loath to add more cerium, in case this lovely sound goes away!
So I’m going to have to finish building my own tester if I’m to make any progress, or get any work done at home. And once it’s built, learn how to actually work the thing. But I’ll be back to my usual schedule of attending the class every second weekend until this telescope is done. And for the third year running, I hope to have it ready for ScopeX!