Some how, I managed to nick my pinky, and it blead all over the grit as I was grinding the back of my mirror. I was cool out there in the shed, and I was doing this in a sweater, but I don't think you can actually say there was sweat... or tears ... yet.
Still grinding the back of the mirror. Went for two more 45 minute whets. That's about all I can do in an evening before its time to come in for din-din. That includes clean up for the next day.
One thing I notice as I do a whet is that the first one on a clean tool lasts about twice around the barrel, but later ones sometimes only go half way around before I have to re-charge the grit. In a 45 minute stint, I recharged the grit 8 or 9 times. After that point I rinsed the mirror and tool and swapped them so that the tool was on top for another 45 minutes. When I had the tool on top, I did a wv pattern, that is, 5 times across from left to right, then turn the tool ccw a little, and then the same pattern back, then step cw and turn the tool ccw again. The amount that I turn the tool is about the same as I step, about 20 to 25 degrees. I have 4 cleats around the perifery, and it takes 4 to 5 steps per quarter.
At the end, at least the marker lines on the mirror were gone, and all the concentric molding marks except for one rough area. I'll give it another couple of hours tomorrow.
One thing I noticed, that even with as little as I did yesterday and today, (3 hours total) the back of the mirror is already concave, and the tool convex. I don't know how that happened, since I gave them equal time on the barrel. The sagitta is about the same as half the thickness of a box cutter blade. I need to find out from someone how to get that flat.
More next time
-Bill
Tuesday, April 29, 2008
Monday, April 28, 2008
I Got Grits.
My order of grits came today, smothered with saran wrap and covered with starch peanuts. Thanks Tom!
I pulled out the 120 grit and gave it about an hour and a half on the back of the mirror to smoothe it. Its going to need twice that before I'm happy with the results. Pictures soon.
-Bill
I pulled out the 120 grit and gave it about an hour and a half on the back of the mirror to smoothe it. Its going to need twice that before I'm happy with the results. Pictures soon.
-Bill
Mirror Testing
As part of my new Telescope venture, I had to make a device to test it. The amazing thing is that this device, known a Foucault knife edge tester can measure the figure of a mirror to well within 1/10th of a wavelength of an average green colored light. One wavelength of green light is around 500 nanometers - that's 500 billionths of an inch or 0.0000005 inches, and this device made of wood and metal can measure 1/10 of that or 0.00000005 inches. Pretty nifty, actually. And it was invented by a guy named Leon Foucault back in the 1830's, and it hasn't changed a lot since then.
Here is the idea... a concave mirror is gound to the shape of a part of a sphere. If you have a point of light at the center of that sphere aimed right at the mirror, then it should bounce right back to the center of the sphere. (even if there is no aluminum on it yet, its still shiny enough) If you move that point to the right a little bit then it should bounce back a little to the left. and if you put your eye there, the mirror will appear to be all lit up. That distance is known as the radius of curvature.
If you cut the returning cone with the edge of a knife, then it will appear to evenly darken from light to grey to black. In practice, its green, because we use a green LED for the light source.
Well this would be great, if all we wanted was to see the dot of light, but we really want to see the stars, which are infinitely far away by comparison. So when the light source is way out in space, the focus falls somewhere close to half the radius of curvature. Its commonly called the focal length of the mirror.
Well, its close, but it doesn't focus very well. Only a parabola will focus all the rays from infinity to one point. For very long focal lengths, it doesn't matter much, but for normal ones 8 times the mirror diameter or less, it matters a whole lot. And until old Hugo came along, telescopes were very very long. William Hershel, for example had telescopes with focal lengths 20 times the diameter and it was quite limiting for him. At the focal lengths I am using here, the difference between the shape of a sphere and the shape of a parabola is very slight. if you dig the center of the mirror a little deeper than a sphere, and you flatten out the edge a little, what you are left over with is a paraboloidal shape. Testing it at the average Radius of Curvature (ROC) with this tester, it becomes really appearent that the outside of the mirror is flattened out a little and that the center is dug a little deeper. You can actually see that the focus at the edge is a little longer than it is in the center and other places in between. If you measure this within a few ten-thousandths of a inch, you can get a really accurate picture of what the shape of the mirror is. And measurements like that are quite possible to do with tools you can make yourself.
With a design that I found in several places, most notably the Stellafane website, I constructed a design of my own. But you not only need the tester, you also need
a way to safely hold the mirror while you are testing it. So there is a matching test stand for that.
One thing I want to avoid is screwing my back up again, so the last thing I wanted was to have to bend over with my head right near the top of my workbench staring at a mirror reflection. So I changed the design someone. I raised it on top of a box. And since its a hollow box, I put a lid on it with a hinge, so that the moving parts can be removed and placed inside for storage.
The photo to the left shows the box with the testing stage atop it. This is looking at it from where the mirror is. You can see the battery pack that lights the LED to its right, and further down and to the right is the switch to turn it on and off. right above the LED is the knife edge (a razor blade), which is mounted in a photographic slide I found at a vintage camera shop in Decatur. To the right is the knob that pivots the whole stage back and forth to cut the bundled cone of light rays bouncing back from the mirror. One thing here is that the hinge is not attached yet in this photo. Again, I took my time with these things, made sure they were sanded well and coated with at least three coats of polyurethane.
Here is a side view of it. In use, the mirror is to the left, and my head would be to the right. By turning the knob in, it raises the near side of the platform, and pivots it around the aluminum bar in the background. To the right, along the vertical back face, you can see a slot about two inches high, and wide enough to slip the
slid with the knife edge through it into the rails behind the slot. The slide can be interchanged with one that contains a Ronchi grating (parallel lines) that are used for a slightly different kind of test.
Here is a close up of the pivot knob. Its really just a 1/4-20 carriage bolt. one turn is 1/20 of an inch, and at about 6 inches from the pivot, that translates to about 1/40 of an inch per turn at the knife edge, so the amount of control is tremendous. The whole thing goes left and right on that aluminum bar,
and so the head of the bolt rides on top of a little teflon furnature slider I found at the hardware store.
Here is a close up of the knife edge in the plastic photographic slide resting in two little rails. Next to it is the LED socket, and below it is where the wires for the LED get routed through to the back. Also note the battery pack. You can't see it, but its stuck there with some velcro.
I found a half a pack at Lowes that someone had pilfered, so the gal at the checkout line gave it to me for 90% off, since it was half missing. You can see my finger pointing out the slot for inserting the various slides for the knife edge and/or the Ronchi grating. You can make a Ronchi Grating with a good printer. It has to be at least 1200 dpi to work, and it has its wierdness. A better one can be made with a photographic reduction method. I'm thinking of going down to that vintage camera shop to see if they can do that for me. They do all kinds of actual black and white film in their actual photo lab. You don't see that much anymore, eh?
Here is the back side of the tester. I'm pointing to the slot again for the knife edge slide. To the right is the window with the knife edge and the LED mounted in its socket on a little stick of leftover pine scrap.
Below my finger is the little switch to turn the LED on and off. I should probably also have a little dimmer there too, but I forgot. To the right is the homemade micrometer. Its divided up into 500 parts, and each turn is 1/20 of an inch, so each mark is .0001 inch. In practice, its a little shakey. But I bought a dial indicator at the disposable tool store to replace it. Haven't done that part yet.
This next picture shows the micrometer better.
The head of the carriage bolt presses against the stage riding on the aluminum bar. The stage is held against it with a small spring visible underneath at an angle.
Again, from the other side. You can see here that it is almost all the way screwed in. On the left side of that plywood bracket is a "T" nut. The wheel of the micrometer is hardboard
.
Here is the back side of the testing stage showing the LED in its socket and the resistor for it mounted on a little piece of scrap pine.
The stand is adjustable. I can test any standard mirror diameter from 12.5 down to 4.25 with it. It sits on a shelf atop a low base and its tipped back at 5 degrees so that the big mirror won't accidentally fall forward. The top of the box has to be aimed parallel to the axis of the mirror. I do that by proping the lid at a 5 degree angle too. So when testing the mirror, the tester is looking down at the mirror by 5 degrees, and the mirror is looking up at the tester at the same angle.
I'll show pictures of that later on when I edit this again.
Here is the idea... a concave mirror is gound to the shape of a part of a sphere. If you have a point of light at the center of that sphere aimed right at the mirror, then it should bounce right back to the center of the sphere. (even if there is no aluminum on it yet, its still shiny enough) If you move that point to the right a little bit then it should bounce back a little to the left. and if you put your eye there, the mirror will appear to be all lit up. That distance is known as the radius of curvature.
If you cut the returning cone with the edge of a knife, then it will appear to evenly darken from light to grey to black. In practice, its green, because we use a green LED for the light source.
Well this would be great, if all we wanted was to see the dot of light, but we really want to see the stars, which are infinitely far away by comparison. So when the light source is way out in space, the focus falls somewhere close to half the radius of curvature. Its commonly called the focal length of the mirror.
Well, its close, but it doesn't focus very well. Only a parabola will focus all the rays from infinity to one point. For very long focal lengths, it doesn't matter much, but for normal ones 8 times the mirror diameter or less, it matters a whole lot. And until old Hugo came along, telescopes were very very long. William Hershel, for example had telescopes with focal lengths 20 times the diameter and it was quite limiting for him. At the focal lengths I am using here, the difference between the shape of a sphere and the shape of a parabola is very slight. if you dig the center of the mirror a little deeper than a sphere, and you flatten out the edge a little, what you are left over with is a paraboloidal shape. Testing it at the average Radius of Curvature (ROC) with this tester, it becomes really appearent that the outside of the mirror is flattened out a little and that the center is dug a little deeper. You can actually see that the focus at the edge is a little longer than it is in the center and other places in between. If you measure this within a few ten-thousandths of a inch, you can get a really accurate picture of what the shape of the mirror is. And measurements like that are quite possible to do with tools you can make yourself.
With a design that I found in several places, most notably the Stellafane website, I constructed a design of my own. But you not only need the tester, you also need
a way to safely hold the mirror while you are testing it. So there is a matching test stand for that.One thing I want to avoid is screwing my back up again, so the last thing I wanted was to have to bend over with my head right near the top of my workbench staring at a mirror reflection. So I changed the design someone. I raised it on top of a box. And since its a hollow box, I put a lid on it with a hinge, so that the moving parts can be removed and placed inside for storage.
The photo to the left shows the box with the testing stage atop it. This is looking at it from where the mirror is. You can see the battery pack that lights the LED to its right, and further down and to the right is the switch to turn it on and off. right above the LED is the knife edge (a razor blade), which is mounted in a photographic slide I found at a vintage camera shop in Decatur. To the right is the knob that pivots the whole stage back and forth to cut the bundled cone of light rays bouncing back from the mirror. One thing here is that the hinge is not attached yet in this photo. Again, I took my time with these things, made sure they were sanded well and coated with at least three coats of polyurethane.
Here is a side view of it. In use, the mirror is to the left, and my head would be to the right. By turning the knob in, it raises the near side of the platform, and pivots it around the aluminum bar in the background. To the right, along the vertical back face, you can see a slot about two inches high, and wide enough to slip the
slid with the knife edge through it into the rails behind the slot. The slide can be interchanged with one that contains a Ronchi grating (parallel lines) that are used for a slightly different kind of test.Here is a close up of the pivot knob. Its really just a 1/4-20 carriage bolt. one turn is 1/20 of an inch, and at about 6 inches from the pivot, that translates to about 1/40 of an inch per turn at the knife edge, so the amount of control is tremendous. The whole thing goes left and right on that aluminum bar,
and so the head of the bolt rides on top of a little teflon furnature slider I found at the hardware store.Here is a close up of the knife edge in the plastic photographic slide resting in two little rails. Next to it is the LED socket, and below it is where the wires for the LED get routed through to the back. Also note the battery pack. You can't see it, but its stuck there with some velcro.
I found a half a pack at Lowes that someone had pilfered, so the gal at the checkout line gave it to me for 90% off, since it was half missing. You can see my finger pointing out the slot for inserting the various slides for the knife edge and/or the Ronchi grating. You can make a Ronchi Grating with a good printer. It has to be at least 1200 dpi to work, and it has its wierdness. A better one can be made with a photographic reduction method. I'm thinking of going down to that vintage camera shop to see if they can do that for me. They do all kinds of actual black and white film in their actual photo lab. You don't see that much anymore, eh?Here is the back side of the tester. I'm pointing to the slot again for the knife edge slide. To the right is the window with the knife edge and the LED mounted in its socket on a little stick of leftover pine scrap.
Below my finger is the little switch to turn the LED on and off. I should probably also have a little dimmer there too, but I forgot. To the right is the homemade micrometer. Its divided up into 500 parts, and each turn is 1/20 of an inch, so each mark is .0001 inch. In practice, its a little shakey. But I bought a dial indicator at the disposable tool store to replace it. Haven't done that part yet.This next picture shows the micrometer better.
The head of the carriage bolt presses against the stage riding on the aluminum bar. The stage is held against it with a small spring visible underneath at an angle.Again, from the other side. You can see here that it is almost all the way screwed in. On the left side of that plywood bracket is a "T" nut. The wheel of the micrometer is hardboard
.Here is the back side of the testing stage showing the LED in its socket and the resistor for it mounted on a little piece of scrap pine.
The stand is adjustable. I can test any standard mirror diameter from 12.5 down to 4.25 with it. It sits on a shelf atop a low base and its tipped back at 5 degrees so that the big mirror won't accidentally fall forward. The top of the box has to be aimed parallel to the axis of the mirror. I do that by proping the lid at a 5 degree angle too. So when testing the mirror, the tester is looking down at the mirror by 5 degrees, and the mirror is looking up at the tester at the same angle.
I'll show pictures of that later on when I edit this again.
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.
Anyway, here it is resting on the stool I built (again to save my back!).
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.
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.
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.
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.
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.
-Bill
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