Tell me about grinding your own telescope lenses

[toadspittle]

I'm editing a column in which the author refers to the nearly vanished hobby of building your own telescope, including grinding your own lenses.

I'm not interested in the process, so much as the history of the hobby itself. Was it, in fact, once common? How much so? When? 1950s? 1940s? 1960s? How did people do it--did you buy complete kits, or did you have to send away for all the separate parts, find plans in a separate book, etc.?

[cornflakes]

I don't know about the past, but it's still going on. A friend came across two eighteen inch parabolic mirrors a while back (part of a vintage tool for making semiconductors) and gave me one. They have too short of a focal length though and will have to be reground and resilvered. It's a shame, since I am sure that the current surface is extremely accurate.

Neither one of us has ground the mirror yet. He's already active in amateur astronomy and will probably make his before I do.

[Fear Itself]

It was alive and well in the late 1960's when I was a teenager. I recall looking at the Edmund Scientific catalog which featured many kits for grinding your own mirrors for reflector telescopes. They also supplied everything else for telescope building; tubes, mounts, eyepieces, etc.

[CalMeacham]

For books, there's the three-part series of small-size bound books from Scientific American, amateur Telescope Making

http://www.amazon.com/Amateur-Telesc...6879963&sr=1-3

http://www.amazon.com/Amateur-Telesc...6879989&sr=1-5


Or you could get How to Make a Telescope by Jean Texereau, like I did. It was published in paperback by Natural History Press, so they sold it at the American Museum of Natural History:

http://www.amazon.com/How-Make-Teles...6880027&sr=1-1

Or you could get "Red" Strong's book Procedures in Experimental Physics.


Or there must be more modern books. And websites.


This was fairly common at least through the 1960s. I don't know how far off it's fallen since. The fact is that, with reasonable care, an amateur can grind a pretty good spherical mirror, since all that's required is patience and time. Random motions actually help make a good spherical mirror. If you want to parabolize, it's more work (see the three books above). But you can go pretty far with a straight spherical mirror. You typically sent it out to be vacuum-coated.

Nowadays I suspect most people have less patuience and more money, and big mirrors are cheap, so they just buy a telescope.



One of Heinlein's characters, I think, casually mentions spending a little money on some big round pieces of porthole glass to use as the basis for his telescope mirror. This would've been in the 1940s. Today I suspect you'd have to deal with a specialty shop (or sale through magazines or the internet) or go directly to manufacturers to get glass blanks and grinding grit.

[Squink]

Originally Posted by CalMeacham

Or you could get How to Make a Telescope by Jean Texereau, like I did.

It's still on my bookshelf!
The Foucault test apparatus described on page 95 worked nicely. I wonder what ever happened to it?

[Xema]

Originally Posted by toadspittle

I'm not interested in the process, so much as the history of the hobby itself. Was it, in fact, once common? How much so? When? 1950s? 1940s? 1960s?

It goes quite a ways back. This short Wiki article says:

But it wasn't until 49 years later in 1721, when mathematician John Hadley showed how, in practice rather than in theory, to produce much larger paraboloidal mirrors (initially about 6 inches in diameter), that Newtonian reflectors began to proliferate.

It's worth noting that it those days no commercially produced telescope existed - if you wanted one, you built it yourself.

[toadspittle]

Originally Posted by Xema

It goes quite a ways back. This short Wiki article says:

It's worth noting that it those days no commercially produced telescope existed - if you wanted one, you built it yourself.

Cute. I was referring more to the 20th century, when it was a widespread hobby. (Unlike in earlier times, when science itself was still a hobby, rather than a profession.)

Thanks for the general perspective, everyone.

[Phlosphr]

Here is a great link to building your own 10 inch Dobsonian Telescope. Great pics on a sort of hokey website. But good down to earth writing.

Here is another one [PDF] with much more information and a lot of commentary.

I believe both links have news on grinding your own lenses...

[Hari Seldon]

If you were in Philadelphia during the 50s, there was a telescope making club at the Franklin Institute. A HS friend was a member. You paid for supplies and, I suppose a membership fee, but in return you not only got to use equipment but also advice from a professional astronomer. Maybe it still exists.

[bethpage89]

Lens making is done by some. Sets of refracting lens blanks are sold by Newport Glass Works. The telescope making workshops of Washington, DC and Hartford, Conn. have been centers of such activity.

Sets of lenses for eyepieces are sold by Surplus Shed, and are used in homemade telescopes.

Telescope making--including making the main mirror--is still done. There are annual gatherings in Europe and the United States. The Stellafane website lists some telescope making groups.

[njtt]

Galileo did it. I am not sure if that counts as a hobby. Maybe it does, his job was supposed to be teaching math.

[kunilou]

A couple years ago we went to the funeral of one of the original homeowners in this neighborhood. For three generations he taught all the local kids about astronomy and, yes, how to build your own telescope and grind your own lenses.

I don't know if it was ever "common" but there were definitely people who did it.

[Francis Vaughan]

Yeah, the three books CalMeacham links to were the bible of telescope making. But for enthusiastic kids making a 6 inch Newtonian was not all that hard. When I was a member of the local astronomical society in the 70's there was a large and regularly meeting group devoted to telescope building. A 6" Newtonian could be made almost from scrap parts. A mirror blank could be purchased cheaply, but getting a circular piece of thick enough plate glass locally was a viable option. Pyrex was preferred due to better thermal properties, but for a 6" it mattered less, and the piece didn't need to be all that thick (half an inch I think was fine). Actually you needed two, one as the mirror blank, the other as the tool.

As mentioned above, simple grinding gets you a spherical mirror, although actually you need to carefully figure your mirror in conjunction with testing to even get that. A simple interference test with a light source and a knife edge and a lot of care can get you a prefect spheroid, and then with a little more tweaking of the interference pattern a paraboloid is made. The fact that you could make a test rig from about a dollar's work of bits that was an optical interference test that could let you figure a mirror's surface to fraction of a wavelength of light was probably the single most important part of the process. It was this figuring that allowed an amateur to create a mirror that was as good as any commercially purchased item.

The magazine Sky and Telescope used to devote pretty much the back half of each issue to telescope making. Telescopes featured were quite jaw dropping. Like most hobbies, things could get out of hand.

A few things probably conspired in the demise of the hobby. Kids of today (get off my lawn) have a vastly wider range of possible pursuits. Even the very geeky. The skies are much more polluted with light, and it just isn't as exciting to look up there from most cities. For those that do want a telescope (and I am very glad to say many still do) there are many affordable telescopes commercially available. A modern Shmidt-Cassegrain with automatic servo driven pointing is beyond anything imaginable a few decades ago.

But you can't not mention John Dobson and the San Francisco Side Walk Astronomers. John changed the face of amateur telescopes. Rather than try to make a mini professional telescope he made simple light buckets that were good for one thing only - actually looking at the stars. The design was intended for amateur manufacture, and it was probably a bit of a surprise when some became commercially available.

[the_diego]

Even the Book of Popular Science advised kids to grind their own mirrors. Same principal applicable to making a one-sided lens.

Take a round disc of annealed glass 4 inches dia and at least 1 inch thick. Lay it on the ground or a steady table on top of a few sheets of wet paper and cardboard to hold it steady and avoid scratching. Get an abrasive tool like carborandum and grind the glass forward-back along different directions but always along the full diameter. The pressure and shape of the glass will cause a natural convexing. Use progessively fine medium. And you should have a template (could be made of anything) to ensure correct convexing. If you convex too much, just reverse things. Hold the glass and abrade it against a stationary stone. Then polish with 1 micron grit.

[VernWinterbottom]

I remember my dad grinding lenses for telescopes in the late '60s. Edmund Scientific was the best catalog we got at our house back then. (Well, maybe the Johnson-Smith catalog. . .)

[Kevbo]

Source for lens and mirror grinding supplies.

[Dr. Woo]

My husband and I made our own Dobsonian telescope in 1990. We used a salvaged porthole for the primary mirror and built the tube out of redwood strips around a frame (similar to a canoe). It's a big sucker - 12 inch mirror in a 7-foot tube. We did the grinding with the blank installed on an old oil barrel filled with water for stability.

The coolest thing is the postage stamp in the exact middle of the primary mirror; John Dobson himself did the final figuring and placed the stamp.

We attended the Riverside (California) Telescope Makers Conference a couple of times in the '90s, and there were plenty of people with homemade scopes.

[ducati]

I introduced my BIL to astronomy a few years ago. What a mistake.

As a small-town preacher, he's poorer than a church mouse, so he barters and builds everything himself. He decided he needed a 10 inch mirror, but these things take time. He would force me into hard labor every time I'd visit, and I'd have to sit and grind on the damn thing for hours while he turned lawnmower parts or plumbing supplies into eyepieces, stands, cases, struts, and more.

More work than I like, and I rebuild cars as a hobby.

[Whammo72]

Originally Posted by Squink

It's still on my bookshelf!
The Foucault test apparatus described on page 95 worked nicely. I wonder what ever happened to it?

From what I understand, it's still on your shelf.