This blog is mainly about Telescope making, and some things about my politics. At last we finally have a President that can say "Fool me once; shame on you. Fool me twice; shame on me." instead of mixing up with an old Who song.

Saturday, May 03, 2008

A hogging we will go

I've always loved hardware stores from the time I was 8, going down to Katz's hardware store with my brother to get glass to replace windows he broke while throwing shoes at me. No ordinary glass, mind you. This was 250 year old hand blown glass in our 250 year old house in Connecticut. My mother was in tears every time one broke, poor old girl. But what did we know? Years later Mr. Katz bought a parsel from the people we sold the house to and built a house behind it. I sure wish I still lived there -- full stone foundation basement.

Well, I got all the final questions I had answered on the newsgroup, about making my mirror a little deeper. Today is Saturday, and I started it out by making coffee, doing the dishes, washing the water bottles with the closable lids I've been saving, and filling them with the grits that I got from I put everything except for the #80 and the #60 grit into these little squirt bottles. Here is a tip... The really fine WAO grits pack down so hard in the paper funnel I made, that they simply would not make their way into the bottles. So do this, use a shish-ka-bob skewer. I have metal ones, but the bamboo ones would work too. Push it down into the paper funnel into the bottle and twirl it around to re-create the opening through the funnel. Keep doing that until it all goes into the bottle, and then rattle the skewer around in the funnel to get any remainder. Then add the water to the bottle. I did this from my tap which is filtered. I made sure to let the filter run a little bit to make sure that it didn't have any extra grit of its own that it wanted to add. There were several bottles. The ones I'm not going to use for a while I put up in a shopping bag near my beer making supplies in the house where its a lot cleaner than the shed. I figure that when I need them I'll take them out, one by one. There will be a lot less chance of contamination that way. After walking our gods Dave and Max, and smelling the first of what is promising to be a spectacular rose season, I set about my grind -- "hogging out". It's called that because after a while in the heat, you smell like a hog... either that, or you have to be as big as a hog to get the job done. It's a lot of hard sweaty work to scrape all that glass away.

Before I started, I measured the sagitta with my nifty new digital dial indicator that I got at the disposable tool store, and it said 0.097 inches. That is about as close to a 101" f/8 as I would even hope for if I were to try it. Paul, the guy I bought it from did that, 40 years ago this year. And that is where I started from. I really hated to mess up the work he did. Honestly, one of the reasons it has taken so long for me to start was that I wanted to make sure I was going to do it. A number of times I hesitated and thought, "Do I really want do this to a perfectly good f/8 mirror?" But in the end, I did. Plop down a teaspoon of #80, dribble some water from an old milk jug, and start rubbing... crunch, crunch crunch. What a noise!

Yesterday I bought a 2 inch pipe cap to grind with. To make the end cap fit better on the glass, I ran it past my belt sander to make it flat. That took most of the zink galvinizing off of it. I used that to grind with for about 25 minutes and measured the sagitta. It was at .1005 inches, and I realized that it was going to take me forever with the pipe cap. I took Mel Bartels tip about a 3" pipe flange, so it was off to Intown Ace Hardware. It seems that pipe fittings, like every other cheap thing is made in China these days, so I got two, and a 3/4" close nipple to go between them, so I would have something to hang onto. A spray bottle finished up the array of things I needed (I told the hardware gal it was for cat herding, and she completely understood, and had suggestions of her own. I kept having thoughts about "Cat People"). The thing about pipe flanges is that some are flat on the bottom, and others are ribbed. I got the 3" one with the 3/4" thread because it was flat on the bottom, and it was just about 1/4 the size of my mirror. I took the belt sander to it too, and restared grinding.

18 minutes with three re-charges, then 9 with 2, then 20 minutes with 3 more. Sweat dripping from my brow now, because the shed was in full by this time, and the rain clouds had cleared. I cleaned up and measured the sagitta again. It was 0.113 inches, and I started thinking, "Hey! those guys were right, I might actually get done with this in one weekend (were it not for the honey doos). So I took my nice new mirror testing stand out into the yard and put the cleaned up mirror on it and doused it with some water from the cat torture device, and presto, an image of the sun on my shed. I measured it out, and the focal length was now sitting at 76 inches.

Way cool! All the work I was doing was going in the right direction! I went from 101" to 76" in a little over 73 minutes. Sheesh, at that rate, I better start paying closer attention to how much I am grinding, since I am aiming at 68.75 inches. So, as far as I can tell, my target sagitta is 0.141 or so, so I have .028 inches to go, and that's not all that much. Another hour, and I should be pretty close to the point where I have to make the tile tool. BTW, in the middle of all this I made a little radius template using a large compass I made. I had a 1/2" x 1' x 8' piece of plywood a attached to a short 2x4 with a nail as a pivot. The plywood was too short, so I clamped a longer 2x4 scrap to it. To that, I clamped a 1/4 inch square iron rod I sharpened into a knife at the radius. Then I took some aluminum flashing I had laying about and nailed it to a board with some matt board in between, and scored the flashing at the radius I wanted. Then simply bending it at that radius broke the aluminum at the point I wanted, and now I have a little radius tester tool for rough checking.

Anyway, I wasn't so sure I was doing this whole "Hogging Out" thing right, so I decided to make a little movie with my camera. 'sideways because I don't really have a tripod, so I had it clamped to my drill press table. So have a look, but you will have to turn your head to the side. It's my first attempt at uploading to YouTube, so I'll try to do better next time. Anyway, it's here:


No comments: