6 March 2011 – Two hundred and twenty
There’s something marvelous about moving on to the next finer grade of grit. The long work building up to it means there’s a huge sense of reward. The careful clean-up process gives it a sense of occasion. The different feel and sound of the grinding gives it novelty. I love it!
So I recently got tired of the relentless grinds which seemed to make no progress. I was producing a beautifully smooth, spherical surface, but not gaining any depth. My mirror was sitting stubbornly at f/7.5, so I threw caution to the wind and attacked the glass with long half-diameter strokes, moving in a broad ‘W’. I used as much pressure as I could sustain for the 90 minutes I had available, and was astonished to find that I’d actually overshot the mark slightly! In my previous post, I thought I’d reached exactly my target ROC (Radius of Curvature), but I’d forgotten to take into account the thickness of the glass. My tape measure butts against the wall, as does the mirror, but the ROC is measured from the front of the mirror – where all the work has been done. I don’t see this as a big problem, though. A shorter focal ratio does make for more difficulty figuring (when I get to that stage), but I also get a shorter and more portable instrument at the end of the day.
The only problem with pursuing the shorter focal length so aggressively is that I sacrificed the spherical shape of the curve. So Saturday evening I sat down and returned to my strictly 1/3 center-over-centre strokes to restore the sphere. The work started okay, grinding normally but tending to grab and stick more and more as each charge of grit broke down, indicating that the curves did not mate properly, and that the shape of the surface was therefore not spherical. But after only 90 minutes, the surface was back to it’s earlier spherical shape. As I ground, the mirror would move smoothly and evenly in all directions at all times. My mirror had a short enough focal length, somewhere close to f/6.5 and the surface was as close to spherical as I could reach with the #120 grit.
So yesterday I began the ritual of moving to the next finest grit. I emptied my bucket of water and mud (thick, smooth, sticky stuff, like a very wet clay, made of broken down silicon carbide grit, and ground glass) into the garden, leaving a large patch of grass dull and grey, and spent a few minutes filling it with clean water, swirling, wiping with the sponge, and emptying. It took a while, but eventually the bucket was absolutely free of evidence that it had ever been used to rinse and wet mirrors. No grit, no dust, not even underneath or in the corners of the handles.
The spray bottle got an equally thorough washdown, along with my hands. While this stuff was drying, I took every bit of newspaper I’d used and re-used to cover the work-surface, and wet it, folded it up, and threw it away. Then I took all the cannisters in which #120 grit had been stored, and packed them into a ziplock bag, along with the sponge and the saltshaker. Then I washed my hands again. Then, and only then, I opened the bottle of #220 grit and emptied it into a brand new salt shaker (Cheaply made generic tupperware clone, from the Crazy Store – perfect for the job at hand), lay down fresh newspaper on top of the rubber mat, filled the bucket with fresh water, unpacked a brand new sponge from its wrapping, unpacked and wet the mirror and tool, and began grinding.
If all that sounds like an incredibly fussy procedure, well it is, but for good reason. It is very important that not one single grain of the old grit ever touch the glass again, because if it does and I then start grinding, it will dig a huge scratch into my nice and pristine surface. While this won’t stop the telescope from working, it will cause light to be scattered which reduces contrast so that the mirror will not work as well as it should. Also, scratches are as ugly as sin.
So what’s happening now? First, the finer grit is so very quiet! It makes a gentle scrubbing sound, compared to the steady scrape of the #120, or the harsh loud scratch of the #80. And as the grit breaks down, the tone steadily softens and gets higher in pitch till it sounds a bit like a very distant whistle. It doesn’t feel like it’s doing very much at all, yet after only one single wet the glass was very definitely smoother to the touch of a fingertip. I ground for about 90 minutes and then took the mirror to my surface testing station, which consists of an ordinary bedside lamp and a 28mm eyepiece. To examine the surface, I rest the glass on top of the lampshade so that the light shines brightly through the glass. I then hold the eyepiece upside down against the surface of the mirror, looking down through its field lens. The result is not so different to using a jeweller’s loupe, and the pits and scratches from the grinding process are hugely magnified and stand out strongly from the backlighting. So how does my mirror look? Unfortunately I haven’t figured out how to capture this on film, else I would attach an image, but I’ll do my best to give a description: Through the eyepiece, the surface looks a bit like rough cardboard – coarse but uniform. However, scattered across the surface (especially near the edges) are hundreds of pits and holes, all roughly the same size. These ‘pits’ are the deeper holes dug by the #120 grit. My goal now is to grind away, removing microns of glass till all these pits have vanished. The job is made harder by the occasional presence of much bigger and deeper pits. I would expect these to be left over from the #80 grit, but Johan assures me that the #120 grit will create them as well. I guess that, still being a quite coarse grade of grit, it’s not very pure and is contaminated with some coarser material. Fair enough. I’m excited enough by the new level that I’m feeling sufficiently patient to work away till every one of these huge pits has been smoothed over. So now we which lasts longer: The pits, or my supply of #220 grit!