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.

Sunday, April 27, 2008

Creation of a Telescope

Since the beginning of the year, I have embarked on a new venture. Actually its a revival of an old hobby of mine known as Amateur Telescope Making. So a lot of what this blog is going to be about is what I have been doing out in my barn with regard to that.

A little background. In 1972 at 14, I got a kit to make a telescope, including the grinding and polishing of its objective, a 6 inch mirror. That was in East Haven, Connecticut. In late 1974, our family moved to Georgia, and I started another mirror as a highschool science fair project. I had half the focal length, and was a really rotten mirror. I loved it nonetheless, but life intervened, and it wasn't until now that I got back into it. I still have those old mirrors though!

I wanted a larger mirror this time around. At least a 10 inch, and finally I settled for a 12.5 inch. Back in 1972, that was the biggest you could find, and out of my price range. But nowadays its a good starting point for the hobby, particularly if you have done it before, and liked doing it.

Building a telescope requires a lot of thought and curiousity, and creativity. So many of the things that you need to do have never been tried before by anyone. And a lot of them fail, too. But you pull in a lot from a lot of different disciplines -- optics, woodworking, drafting, engineering, metalworking, electronics, building construction, mechanics, materials analysys, metalurgy, you name it. All of these were contributing factors in my decision to become an engineer back then. And new things are always coming along that add to it. Computers, obviously, and the internet are new inventions since my highschool days that have contributed greatly. I used to spend hours at the drafting board, which is so much easier now with CAD software.

All of this has an end, of course. To make a telescope of enough size to be able to see all those things I never could before. I want to see the icecaps on Mars, and the Cassini Division in the rings of Saturn. I want to roam through the Pleades and Hyades and count the stars in the Hercules cluster. I want to travel through space to the Orion Nebula and wink at Joves big red eye. Maybe I can see Pluto or discover a comet, or a supernova.

I'm luckier than most, I do this at the urging of my wife Susan. So I have designed this telescope to be as low to the ground as possible so that she won't have to stand on a box to use it.

When I first started this, I had nothing but a table saw, a broken router, a drill, and a dirty cluttered shed build in 1951. I had a lot of work to do before I could even start thinking of making a saw horse, much less a precise optical instrument. I had to clean the place up. It was filthy. Rats had gotten in, attracted by the grass seed, so there was rat crap and pee everywhere. Some of the clapboards and part of the sill had rotted and English Ivy had crept in from the woods out back and had made a home in some old potting soil. And then there were the lady bug infestations. Thousands of them -- just wierd.

So I cleaned the table saw which I had not used in years and fix the router that I broke. I cleaned up and refinished the workbench, added pegboard to the new replacement wall that my nephew Patrick and I put in last summer. And that only got me to the place where I could gain a sense of direction. I needed to know where I was, and I sure couldn't the way it was.

There were so many things I had to do. I needed to come up with a design for it. I needed tools to even come up with the design. So I got a CAD program, TurboCad. Its a 2D version, which I am upgrading to 3D. I scoured the internet and found out so many resources it was astounding. Many of the things I need to do with this project have to take place in sequence. I needed to make the tooling and test equipment for grinding the mirror, before I started grinding the mirror, because I am cramped for space in the barn. Its only 11X20. The last thing I wanted to do was to start grinding, and not have a place to do it, or a tester to test it. Mirror grinding is very picky about not contaminating progressively smaller steps in grinding grit. One ill placed grain of grit can ruin a mirror being ground, and you might have to go back several stages and start over. If I were to start, and then stop to make test equipment, and then start up again, then all the cleaning up before grinding might come to naught.

And then there is the process of getting the mirror blank, and tool. It doesn't come cheap. I was fortunate enough to lay my plea at the feet of the crowd at Astromart, and one guy gave me a good deal on a 12.5 inch mirror blank that he had started 40 years earlier and had not finished. He's from Kansas, and not too far away from a few of my cousins.

And then I had to start thinking about eyepieces. Should I buy a set, or get some lenses and roll my own? I broke down and bought a set. Almost new from a guy in Indiana.

And then the question arose about the diagonal mirror. Do I polish my own flat and then cut it? The finished ones are so expensive, new. But then it was another guy from California who sold me his extra one for cheap.

And how do I mount the mirror. Well, as it turns out, there is a guy in a town nearby that will cut the parts I need from 1/2 inch aircraft aluminum for $25. KI was really expecting it to be about $400 or so.

And then there was my friend Terry that I met back in the mid 1980's who sold me a cheap drill press for $20 and at the same time he turned me onto Harbor Freight tools. He calls it the "disposable tool store". Dirt cheap crappy Chinese tools that fall apart on you, and when they do, you just go and get another one. They generally last long enough to do the job. I'm thinking of getting a cutoff grinder there for $10 on sale. More fun than a candy store any day of the week.

Finally, I had to order the grit. That should arrive this week sometime, and I can get started before my 50th birthday party.

Here are some pictures of what I have been doing.

This is my mirror grinding stand. Its 37 inches high and 19/625 inches in diameter. On it is the vintage 1968 12.5 inch plate glass grinding tool. At the bottom are ballast weights that I made from 2 5o# bags of fast drying Quikrete. The molds are shown further down. This is a truss type of grinding stand. I got the idea from the Stellafane website. Its a design that comes from the Harford Connecticut ATMs. There were no real designs there, but here, there is a picture, and I will be posting the DCF CAD files for other to use.

The round top and bottom are 3/4 inch plywood from those 2' x 2' chunks that you can find at Lowes. I prefer them over gettin a whole sheet, because... well, I like my back.

The trusses are 2.5 inch by .75 inch yellow pine, very stiff. You can see two of the feet. I got them at our local hardware store (which I frequent more than the big box stores.) There are 7 little feet, and I have them attached to the bottom circle with 1/4 inch "T" nuts. (a wonderful invention!)

I bought a little orbital sander and lots of sandpaper and sanded it all down. I think it was important to get that very smooth. I took my time, and did it right. I filled in all of the screw holes with wood putty. Then I put 3 coats of polyurethane on the whole thing, and and couple extra on top. The cleats for the tool are made of oak. Everything is glued and screwed with deck screws. I had to predrill the yellow pine and oak for them, otherwise they would have split. It was a guessing game to get the angles cut right. But as it is, its "strong like bull". And its very portable and I can store it up in the rafters when I get done.

The reason I was so picky on this was that I'll have to clean this between grit sizes, and I want to have a s few places for the grit particals to hide as possible.

The last time I made one of these, it was made from 2 x 6's and was covered with a piece of visqueen. I got grit everywhere, and jewellers rouge too, and my mom was fit to be tied.

Here is a top view of the grinding stand. Notice that I labelled the cleats. They are actually removeable so that I can clean them. Under each is the corresponding number. That way there is no guessing as to which one goes where when it comes time to screw them back on. Also notice that the back of the tool is all marked up with the same labels and some lines. I did this so that I could make sure that the tool gets put back the same way (or differently!) each time. They are simply registration marks. You can't see it, but I spent about 4 hours bevelling the edge of this thing, and the mirror blank so that chips would not flake off and ruin things.

Here is one of the molds for the ballast weights. They are made out of scraps I had around, but with the dimentions I chose, 5.75 x 1.75 x 12.75, they were manageable. The two bags of concrete made 9 of them, plus two smaller ones that fit between the big cleats that the trusses attach to. I think I will have to coat them with something though. I don't like that they are just waiting down there to contaminate things.

Anyway, here it is resting on the stool I built (again to save my back!).

I hate to say it, but I'm anal enough to have labelled the molds too. I don't want to get into a cleaning mood someday and throw them away by accident or try to remember why I made them in the first place.

Each mold screws together with deck screws... I use the Spax type with the star shaped driver tool. I use my trusty Skil battery powered screwdriver, and I am rockin. If you don't have a battery powered screwdriver, then you really should take up another hobby.

I made two of these molds. I thought I would need more. But as it turns out, but the time you finish filling one, the other one has dried enough to unscrew and empty. I was able to do them all in about 4 hours total time.

Here is my new Pyrex baby. 12.5 inches around, 2.25 inches thick. Weighs in at about 28 pounds. Its on my workbench with some of that rubber mat stuff that Norm Abrams uses when he sands stuff on New Yankee Workshop.
Next to it are my two old mirrors from back in the 1970's. They are both aluminized, but one is pretty scratched up, and the other one is a bit spotty. One of these days I will regrind them both to make a pair of binoculars.

BTW, did I mention that I got first prise in my high school science fair and went onto the county fair?

The big mirror is already ground and polished to F8 (almost exactly, which says a lot about the original owner's abilities). I will bring that down to F5.5, which with a mirror of this diameter means a focal length of 68.75 inches.... ok, so maybe Susan will have to stand on a box sometimes... but not often.

Ok, thats enough for now. Next time I will show The Foucault knife edge tester, the tester stand and base and a couple of innovations I had with all of that.


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