Ep. 328: Telescope Making, Part 2: Serious Gear

Some astronomers are control freaks. It’s not enough to buy a telescope, they want to craft every part of the experience with their own hands. If you’re ready, and willing to get your hands dirty (and covered in glass dust), you can join thousands of amateur telescope makers and build your own telescope from scratch.

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This episode is sponsored by: Swinburne Astronomy Online, 8th Light, Cleancoders.com

Show Notes

  • Amateur Telescope Makers of Boston
  • Stellafane telescope makers in Vermont
  • Telescope Maker’s Workshop at the Chabot Science Center
  • Hayden Planetarium
  • Detailed info about making your own telescope — Stellafane
  • Making a Reflective Telescope — UNAWE
  • Mirror grinding tutorial 
  • Extreme amateur telescope: a 70-inch reflector
  • Gary Gonnella on G+
  • Mike Phillips on G+
  • Transcript

    Transcription services provided by: GMR Transcription

    Announcer: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the world’s longest-running online astronomy degree program. Visit astronomy.swin.edu.au for more information.

    [Recorded music] [00:00:34 – 00:00:40]

    Fraser Cain: Astronomy Cast episode 328: Telescope Making Part 2. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos where we help you understand not only what we know but how we know what we know. My name is Fraser Cain. I’m the publisher of Universe Today and with me is Dr. Pamela Gay, a professor at Southern Illinois University, Edwardsville and the director of CosmoQuest. Hey, Pamela – how are you doing?

    Dr. Pamela Gay: I’m doing well. How are you doing, Fraser?

    Interviewer: Good, good – so this is the last show that we’re recording for 2013, which was a fantastic year in space and astronomy and now on to 2014.

    Dr. Pamela Gay: Yeah – that was perhaps the fastest year ever. It’s one of those weird things; when you’re a kid time never seems to pass and when you’re an adult you take as many leaps as you can and the world still gets ahead of you.

    Fraser Cain: So what was your favorite thing in 2013? We don’t want to do a long intro, but what’s the big thing do you think?

    Dr Pamela Gay: Oh – I think the most awesome thing I did was watch the sunrise with a fur seal in New Zealand which was the whole Southern Hemisphere thing so that was all kinds of awesome. And my favorite science thing was my curiosity keeping on with a whole series of; yes, I’m roving along a river bed. Yes, here’s more evidence I’m roving along a river bed. Yes, this is a river bed. That was kind of awesome.

    Fraser Cain: For me I got to tour Space X. I think that was the big highlight for me for the year. That was amazing and I think my favorite thing was Voyager – actually Voyager leaving the solar system was pretty awesome.

    Dr. Pamela Gay: Or did it?

    Fraser Cain: Well, it left the heliosphere, so it’s into interstellar space but obviously clearly it’s still in the gravitational field of the sun. But yeah – I thought it was great that we fired a spacecraft that far out and it’s still communicating. Go Voyager.

    Dr. Pamela Gay: Go Voyager. I agree with that.

    Fraser Cain: All right – well, let’s rock.

    Announcer: This episode of Astronomy Cast is brought to you by 8th Light Inc. 8th Light is an agile software development company. They craft beautiful applications that are durable and reliable. Eight Light provides disciplined software leadership on demand and shares its expertise to make your project better. For more information visit them online at www.8thLight.com. Just remember: that’s www dot the digit 8, T-H; L-I-G-H-T dot com. Drop them a note. 8th Light – software is their craft.

    Fraser Cain: Some astronomers are control freaks. It’s not enough to buy a telescope. They want to craft every part of the experience with their own hands. If you’re ready and willing to get your hands dirty and covered in glass dust, you can join thousands of amateur telescope makers and build your own telescope from scratch. Now, this sounds like madness to me, so why on earth would a person build their own telescope?

    Dr. Pamela Gay: Why would someone knit their own sweater; why would someone build their own car; why would someone build their own anything? It’s because; A. you want the experience of knowing how it’s done, and because you want to make it yours and there’s just the bragging rights of; this one’s mine – I made it from scratch. There are also ergonomic issues some people get into. They have specialized photography they want to do.

    One of the coolest things I saw that someone built was a pair of – I want to say – 12 inch telescopes turned into binocular –

    Fraser Cain: Oh, man – that was something.

    Dr. Pamela Gay: And so that was – it was custom done so that you could adjust the separation on the eyes by rotating things and it was just this really crazy, off-axis design that made it work kind of like your eyeballs were in prime focus.

    Fraser Cain: So, you’re telling me that people may want to build their own telescope so that they can learn about the science and enjoyment of astronomy more, that they can customize their own telescope to exactly what they want so that they can just spend time building a thing, carving it out of nothingness, which has value on its own. But I think one of the important reasons is not to save money.

    Dr. Pamela Gay: No – that’s –

    Fraser Cain: Do not build a telescope if you’re hoping that that’s going to be the inexpensive way to get a telescope.

    Dr. Pamela Gay: And this is where I have to keep going back to the knitting analogy as I look – I have knitting and glass apparently tangled in a mess on my desk – if you look at how much it costs by the yarn to knit a sweater you should just go out and buy the most expensive hand-knit wool sweater from Ireland that you can find and it will be cheaper.

    Fraser Cain: And better.

    Dr. Pamela Gay: Well, not necessarily. It depends on how good a knitter you are.

    Fraser Cain: Okay – all right.

    Dr. Pamela Gay: And the same is true with telescopes. It may or may not be better, but when you’re hand-building it, it’s completely bespoke; it’s completely made to your design. Now, that doesn’t mean you’re not going to screw something up, but since it is completely bespoke every single thing has an individual cost. There’s no bulk discount that gets passed down to the buyer, so you’re crafting your own focusing mechanism or buying it; you’re crafting your own tubes; you’re crafting your own supports; you’re buying the raw materials and you have to factor in your own salary at a certain level.

    One of the things that I had a colleague once point out to me was; if it’s going to cost you five hours to do a task and you can hire someone to do it who’s better than you in three hours and it’s going to cost them – or cost you to pay them – less than you would earn in three hours, pay them. You’ll earn more money that way. And that’s kind of a crazy thing to think about but when you grind a mirror you’re looking at putting in potentially hundreds of hours into getting the surface exactly right, re-doing it, testing it – doing it some more, testing it – figuring out how to silver or aluminize it; shipping it back and forth, testing it some more, installing it – Yeah.

    Fraser Cain: And a good telescope is going to last you for years and years and years.

    Dr. Pamela Gay: Oh, a lifetime –

    Fraser Cain: Yeah, a lifetime if you do it right so this is a one-lifetime job. Now, of course it’s a total bottomless rabbit-hole of hobby time and you’ll finish that one and all you’re going to do is you’re going to want to think about the next one that you’re going to want to build, but this is not going to be something you’re going to want to regularly do.

    I think another really valuable thing is just that you understand at a very deep level how the telescope works and so if you buy a telescope you could modify it, customize it, extend it, and make it more of what you want. So, there’s always value in going down this road and building things with your own hands and understanding it. It’s always time well spent.

    Dr. Pamela Gay: And the really good point you’re making here is you’re not learning astronomy. You’re not going to learn anything about Jupiter building your own telescope, but you’re going to learn so much about optics, learn so much about how light works, how mirrors work, how lenses work and so you’re beginning an optical engineer instead of becoming an astronomer. This is showing the other side that there is to how we understand our universe, because it takes engineers on the computer side, on the optical side, and the astronomers all working together.

    Fraser Cain: Yeah – so then all right; a person has decided that they want to jump into this field of building their own telescope. Where do they start? How do you begin this process?

    Dr. Pamela Gay: Well, luckily there’s a ton of books and a ton of clubs out there just waiting to help you. If you happen to live in New England that could perhaps be the absolute best environment for being an amateur telescope maker. And for those of you outside the United States, New England is the strange regional name that we give to the part of America that is Massachusetts, New Hampshire, Vermont, Maine, and Connecticut.

    So, outside of Boston and my hometown of Westford, Massachusetts is the meeting house for the Boston Telescope Maker’s Association. They meet in Westford, Massachusetts on the grounds of Haystack Observatory and this is a club of people who are constantly finding new ways to innovate building telescopes. There’s the Stellafane Club up in Vermont and these two organizations have both been around since before World War II and they’re a great resource of humans.

    If you’re out on the West Coast, then the Cabot Science Center has its own amateur telescope making society. The Hayden Planetarium in New York City has off and on throughout the years had astronomy clubs that focus on making telescopes. In all of these organizations you’re going to find someone who probably has the gear to loan you to get you started and the expertise to teach you so how do I start? The first stage is always grinding your own mirror and that’s the hard part and the easy part in a lot of different ways.

    Fraser Cain: So then – so this is obviously going to be a Newtonian telescope here that people are building. They’re not building refractors; they’re going to be building a reflector.

    Dr. Pamela Gay: So, when you’re getting started you want to give yourself as few pieces to screw up – sure you could go ahead and start by making a refracting telescope and grind all four surfaces of the lenses and curse yourself over and over again, but if you start with a Newtonian telescope the only surface you have to grind is the surface of the primary mirror. The nice thing about grinding mirrors is if you screw it up and you start with a thick enough mirror blank you just keep grinding until it’s the right shape. You can’t do that with a lens.

    Fraser Cain: So, what does a mirror blank cost? So if I want to buy a four-inch, a six-inch, and eight-inch; do you know roughly what a mirror blank costs?

    Dr. Pamela Gay: It’s actually not that expensive. All a mirror blank is is a round disc of glass that’s fairly thick and you can get a 14-inch one for around $80.00 when you factor in shipping. So there you are, looking to build your 14-inch telescope and the mirror is like $80.00.

    Fraser Cain: That would be like a $3,000.00 telescope or more.

    Dr. Pamela Gay: That would be about a $2,000.00 telescope if it has all of the gears and everything else. If you’re looking at a Dob you’re looking at a $500.00 telescope if it has no motors. So it’s looking like its going to save you money so now it’s –

    Fraser Cain: Oh – it’s a trap.

    Dr. Pamela Gay: It’s always a trap; it’s always a trap. So, once you have your chunk of glass you need a surface that you can adhere the mirror to so that it doesn’t move while you’re grinding it. This is usually some fairly generic flat surface and something – either it is tree sap or something resembling tree sap in consistency – that you can essentially glue your mirror non-permanently to that surface.

    Fraser Cain: Right – and get it back off again.

    Dr. Pamela Gay: Exactly – then you have to buy a grinding stone, so this is a surface that is more solid than that piece of glass is that you can use to grind away at the surface. You need grits of a variety of different thicknesses just like you have sandpaper with a variety of different coarsenesses when you’re making furniture and that’s just to make your mirror.

    Fraser Cain: But how – I mean I’m imagining – so you do this by hand?

    Dr. Pamela Gay: Yes. And the kind of awesome thing is, the way nature works is if you just kind of randomly rub a rock across the surface of a piece of glass it’s going to naturally tend toward a spherical hole. This just has to do with the fact that all of those random movements add up to cross the center most often. You cross just out of the center a little bit less often and so on and so forth – it’s a just a probability function; essentially gaussing the probability of where you’re going to hit the surface with the rock as you randomly rub it across the surface.

    Fraser Cain: But how do you know when to stop? How do you know when you’ve done it right?

    Dr. Pamela Gay: You grind on it and as you go you periodically stand it up and you use what’s called a knife-edge test to reflect light off the surface. And you know the curvature you’re aiming for and it usually matches the curvature of the grinding stone that you’re using and you use this knife-edge test to look at the pattern made when you pass a pinhole of light across that knife edge onto the mirror. And if you get nice, perfectly even illumination that means you have a nice, perfectly spherical mirror. If you don’t, well the way that you don’t have a nice, evenly-illuminated surface reflects quite literally all of the errors that you have in your mirror-grinding.

    Fraser Cain: And so you can reflect this light; you can see the reflection, the mistakes – and then you grind it and polish it a little bit more in an area and those mistakes will start to go away and the optics will start to come online and match what you’re looking for?

    Dr. Pamela Gay: Exactly.

    Fraser Cain: I think there’s also like a – like you can put like a form into it right and you can turn it and make sure that you’ve got roughly the right spherical shape.

    Dr. Pamela Gay: There are forms you can use. It’s one of those things where every one of those old experts who’s been doing this for a couple of decades has a whole laundry list of tricks of the trade that they can teach you that use everything from different types of diffraction gradings to lasers, to pinholes, to you name it – and there’s no one right way. There are wrong ways, but there are many different right ways to measure the surface of your mirror.

    Fraser Cain: And so if you sit and just grind on this mirror how long do you think you’ll be looking at?

    Dr. Pamela Gay: It varies depending on what focal length you want. So, the longer your focal length; the larger the number of your telescope – that means that it’s a segment of a bigger, and a bigger, and a bigger circle. For a spherical mirror the focal length is one-half the radius of the sphere that that mirror would be a segment off of. So if you’re imagining you have something with a 300 millimeter focal length, well you don’t have to grind too much to get that as compared to something that is, say 100 millimeter focal length compared to a 50 millimeter length would be ludicrous, but as you change these numbers you’re changing how deep into the mirror you have to grind.

    Something with a very, very long focal length you’re going to need longer exposures; you’re going to have a smaller field of view, but you don’t have to grind as long so it may be a matter of a few weeks to – if you’re doing something with a really short focal length – months and months and months.

    Fraser Cain: Really? Wow – all right, so I’ve taken this time. I’ve ground the mirror. I’ve done all the tests. It’s perfect. I’ve polished it and it’s smooth as silk. It’s a beautiful piece of glass. I’ve only broken three of them and now have had to triple the price of the mirror blanks, but now I’ve got this nicely-ground mirror. So what do I do with it?

    Dr. Pamela Gay: Well, so it’s still not a mirror actually. Right now you just have a big old chunk of glass that you’ve made nice and perfectly smooth. Once you have that chunk of glass the shape that you want it, you have to find someone probably with an aluminizing tank and what they’ll do is mount your mirror on one end of this vacuum chamber and escape all of the air out of the chamber and then use a coil to heat up and melt aluminum that will then – thanks to a charge that you put on the mirror – go flying through the chamber and adhere quite nicely and hopefully in a nice couple of atom thick layer all across your piece of glass, making it into a mirror.

    Once you’ve aluminized it you overcoat with something to protect that surface. You can also use silver, but the problem is – as anyone with actual silver knows – silver oxidizes. It tarnishes, it changes color, so for a lot of purposes just go ahead and use aluminum.

    Fraser Cain: Gold is nice too, isn’t it?

    Dr. Pamela Gay: I don’t know the spectral characteristics of gold. In general people make mirrors out of silver or aluminum.

    Fraser Cain: Yeah – but I know like with the James Webb, it’s gold.

    Dr. Pamela Gay: Yeah, but it works in the infrared so we’re not viewing that from the surface of the earth.

    Fraser Cain: No. Okay, great – so then I’ve got my mirror. Now I have my actual mirror back and really the hard work is done.

    Dr. Pamela Gay: That’s the annoying part, so now you need a flat mirror to make your secondary, you need a tube or a set of supports – so this is the stage where you can go the; I’m just going to use a bunch of either carbon fiber tubes or PVC pipe is often used, to build a support for your mirror, build the tube coming up, build a spider across the top to – well, not across the exact top for a Newtonian; inside where the top is – for where your secondary mirror is going to be. Shoot that light out the side into an eyepiece – except now you’re in the land of; oh God – I have to arrange all of this so all of the surfaces are perfectly aligned.

    Because if you don’t, well you run into problems of all sorts of different aberrations where your images can end up looking like comets instead of like stars and that’s kind of annoying.

    Fraser Cain: And so if things aren’t lined up perfectly again back to lasers, back to shining light, line it all up – you’re going to get chromatic aberration, you’re going to get things are –

    Dr. Pamela Gay: Not chromatic – we’re talking about mirrors.

    Fraser Cain: Right – you’re going to get blobs, you’re going to get – things are just not going to feel nice and crisp and it will never focus.

    Dr. Pamela Gay: And you have to make sure all your distances are correct because otherwise you’ll never be able to get your telescope in focus and that is what one might call a very annoying problem.

    Fraser Cain: And that I guess is going to be tough because you’re going to drill a hole on the side of your carbon fiber tube and then mount your focuser there and if you got the measurements wrong, it’s time to drill another hole or get a new tube.

    Dr. Pamela Gay: One of the nice things is that some people will actually instead of using a solid tube that’s the same diameter as your mirror you just build a trestle out of multiple, small-diameter tubes, but you’re still trying to figure out how to mount everything on the side, screw it in the right place, wrap all of that with a piece of canvas to act as a light block – but yeah, it’s kind of annoying when you don’t get it right. And since you’re folding the light you have to take into consideration the thickness of the mirror you’re using as your secondary and all that sort of stuff as you go.

    Fraser Cain: Now, what about the mount? I mean you’re not going to want to try to build your own mount.

    Dr. Pamela Gay: Why not?

    Fraser Cain: Okay – build your own mount.

    Dr. Pamela Gay: Just build a Dobsonian.

    Fraser Cain: Yes, yes – with plywood and wheels and –
    Dr. Pamela Gay: Casters and –

    Fraser Cain: And set the whole thing up – sure if you do want a Dobsonian. I think one of the reasons that didn’t mention at the beginning of the show is one of the reasons to build your own telescope is so that you can have the biggest telescope, right? If you’re going to a buy a 20 inch telescope, a 22 inch telescope, a 24 inch telescope –

    Dr. Pamela Gay: You can’t afford that easily.

    Fraser Cain: You can’t afford that – no. But you can grind it and you can build it and it’s going to be 20 feet long, 12 feet long; it’s going to be a gigantic Dobsonian telescope. Like, have you looked through a 22 inch –?

    Dr. Pamela Gay: One of the Obsessions – I’ve looked through a 30 inch Dob.

    Fraser Cain: Like a homemade 30 inch Dob?

    Dr. Pamela Gay: Yeah.

    Fraser Cain: Right – and so that’s a colossal amount of light-gathering power and you’re not going to buy one off the shelves. Someone’s going to have to sit down and grind that –

    Dr. Pamela Gay: Well, you can – they’re called the Obsessions. There are a few companies that make 30 inch Dobs, but you can make one for yourself. These are actually works of art. They’re beautifully stained. They’re made out of beautiful pieces of wood in some cases. I’ve seen people that have made telescopes that – they make the best carpentry in a furniture store go to shame because they’ve lined up the grains just right – different types of dye to bring out different colors – stunning works of art.

    And they move smoothly so you’re worrying about it has to be able to tilt and tip and you have to be able to rotate it about the base. They’re gorgeous. There was actually a project for a while called Group 70 out in California, which was a group of amateur astronomers who were trying to build the world’s largest amateur telescope. Eventually they gave up. They ran into funding issues, but it’s been one of those things where people have realized you can go bigger than a mirror when you’re making it yourself. Cut the costs and still build just a really solid instrument.

    Fraser Cain: Yeah. Now, so I think as you said you build a Dobsonian and that way you handle the mount, but if you do it right you can bolt this onto a nicer mount – bolt it onto a pier and use it as a real – the nice thing about having a mount is that you’re going to be able to track the sky; you’re going to be able to do some astral photography, some CCD – so if that’s your goal just buy a telescope. But if that’s your goal mount it to a serious mount that will track the sky.

    Dr. Pamela Gay: And there are people out there who have figured out how to build tracking systems for Dobsonians.

    Fraser Cain: Core-Schmidts?

    Dr. Pamela Gay: Yeah. They’re basically – its clock gear systems. The technology has been around for well over 100 years. Again, it’s really hard to get everything right and here you start having to worry about things like perfect balance, about slacking your gears, about being able to consistently move when the center of mass is straight up versus when it’s a right angle to the system. All of these things come into play. Telescopes flex. That’s really annoying when you’re trying to point. All of the stuff adds up to more and more time in the machine shop.

    Fraser Cain: Now, we talked about building a reflecting telescope. Is there any value at all in attempting to build a refractor or is that just a whole other level? Is that the super-Ninja skills, those people?

    Dr. Pamela Gay: That falls under the category of super-Ninja skills. There are people out there who have decided I don’t like any of the stuff available for sale, so I’m going to figure out how to design optics that do exactly what I what and then get them custom-manufactured. That’s where Al Nagler started, is he was an optical engineer working in industry. He wanted the best dang eyepieces on the planet and out of his desire to do something awesome we now have Tel Vue telescopes, which I desperately want to own one of them someday.

    Fraser Cain: Like looking into one of those is just like falling into space.

    Dr. Pamela Gay: Yeah. He has eyepieces that have like 180 degree fields of view in terms of how the image is spread across what your eye sees when you look into the eyepiece and just the fact that it fills your peripheral vision is really magnificent.

    Fraser Cain: Yeah – and so that’s the same thing. I guess someone who knows their optics can say; I would like more field of view please. Well, I can’t buy it? Then I will make it.

    Dr. Pamela Gay: And you also now have people who are now custom-making their cameras, which is really cool. People who want to be able to do more high-speed imaging, so they’re figuring out how to set up systems that have the shutters working in different ways so when you do the really short exposures there isn’t actually a radical difference between how long the center and the edge are exposed; you have people figuring out how to build systems that you can mount at the prime focus.

    Gary Ganela is one of our Star Party folks who’s figured out how to hack his telescope so his camera is up at the prime focus part of the telescope where it was never designed to be but he gets so much of a wider field of view because of that.

    Fraser Cain: Yeah – and so one person who also helps out with the Virtual Star Party is Mike Phillips and he has built his own telescope. It’s Akulay I think or Akula – I’m trying to remember how he pronounces it – but it’s a 14 inch Newtonian telescope. It’s just a classic example and it’s phenomenal for planetary work and so he’s gone into; I want to build a telescope that’s going to be really good for planetary work. And you see some of the pictures he’s taken of Europa and you can see the colors – and Mars and the Martian moons and stuff – so, if you really know what you’re looking for you almost have to build your own telescope.

    Dr. Pamela Gay: Well, I’m not sure nowadays if it’s so much of a build it from scratch as a customize-it. I think we’re moving with telescope technology into the era we were at with cars in the 1960s where back then you’d buy yourself a muscle car and fine-tune it and change this and change that until it did exactly what you wanted and was as fast as you wanted and was as loud as you wanted. Well now people are buying telescopes so that it can get images as fast as they want, as detailed as they want, with the field of view they want. You buy it, you customize it; you get what you want but you don’t start from scratch.

    Fraser Cain: Do you think you would ever build a telescope?

    Dr. Pamela Gay: I don’t think my eyes are good enough to justify it. I’m very much a; let me write software to make the telescope do exactly what I want – that is something that I did in grad school. I got fed up with having to by-hand move the telescope in a specific pattern to move it around the galaxy clusters I was imaging, so I just wrote software.

    Fraser Cain: Yeah – you’re a coder.

    Dr. Pamela Gay: Yeah – I’m a hacker.

    Fraser Cain: I don’t know if I would have the patience for it. I don’t know. Probably not, but I sure appreciate when people have built their telescopes, so if you build a telescope send us an email. We’d love to see some pictures of the telescopes that you’ve built.

    Dr. Pamela Gay: There are some amazing designs out there.

    Fraser Cain: Yeah, and I would love to – we run stories on universities all the time where there are people who have done this. So, I’m a huge fan, but I don’t think I’ll ever do it myself. Cool – well, thanks Pamela. Next week we’re going to talk about building your own space telescope, which is going to be super-fun.

    Dr. Pamela Gay: It’s a whole new level.

    Fraser Cain: Yeah – cool. Well, thanks Pamela. We’ll see you next week.

    Dr. Pamela Gay: Keep your face up –

    Fraser Cain: And next year –

    Dr. Pamela Gay: Yes, and next year – Happy New Year.

    Announcer: Thanks for listening to Astronomy Cast, a non-profit resource provided by Astrosphere New Media Association, Fraser Cain and Dr. Pamela Gay. You can find show notes and transcripts for every episode at AstronomyCast.com. You can email us at info@AstronomyCast.com. Tweet us at AstronomyCast. Like us on Facebook or circle us on Google Plus. We record our show life on Google Plus every Monday at 12:00 p.m. Pacific, 3:00 p.m. Eastern or 2000 Greenwich Mean Time.

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    [Recorded music] [00:30:21 – 00:30:55]

    [End of Audio]

    Duration: 31 minutes

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    2 Responses to Ep. 328: Telescope Making, Part 2: Serious Gear

    1. Bob Blinn January 19, 2014 at 4:18 pm #

      Thank you so much for episode 328. To me, mirror grinding is based on a very old and universal principle of hand craftsmanship which was swept away by interchangeable parts and mass production. The principle is that parts will fit well if they are ground together. We can imagine how this might have been applied in many ways in pre-industrial eras. When two glass discs are ground together while constantly changing their relative rotational orientations, the only surfaces that can result will be two corresponding spherical surfaces that fit perfectly. The same is true for polishing, except, for polishing, the convex tool is surfaced with pitch to carry a polishing compound. The last step is to deform the near perfect, polished concave sphere into an aspeheric conic section, which for an amateur like me is usually a parabola. The difficulty of this parabolizing step increases inversely and geometrically with the selected focal length of the mirror. To me, the charm and wonder of amateur mirror making is that something so extremely precise, even within a few millionths of an inch, which has the power to bring to an observer astonishing cosmic wonders, can be fashioned from a crude chunk of glass (even a surplus maritime porthole glass) with nothing more than a pair of muscle powered human hands, some abrasives, a lump of pitch, a lamp, a pin hole, and a knife edge.

    2. AmyD. January 28, 2014 at 8:57 pm #

      I started grinding my own mirror a few years ago. It’s not done yet – I had to put it on hold for a few years due to illness. Hearing this podcast has re-inspired me to get it finished!
      Thank you!

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