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Dobson on Dobsonians
RTMC 1983 


Since I am to talk on Dobsonians, I have to make my old complaint. Long before they were called Dobsonians, I used to complain that telescopes

should not be called by people's names. (Laughter) We used to say, "Maksutov Schmaksutov, Ccsseyrain Schmassegrain, Schmidt Schmidt."

(Laughter) Now it's Dobsonian Schmobsonian.


Before I start talking about telescopes, I should tell you something about the Sidewalk Astronomers. We wanted to go to Canada this

summer. There's a lady there who runs a roving planetarium through Alberta, and when we were there with our telescopes last summer, she met

us at Banf and invited us to come this summer. But they couldn't get it funded in time; so, as of now, our summer tour this summer is cancelled

because some of us are going to India in SeptPmber. Jim Jacobson, of Coulter Optical Company, has been kind enough to donate two Odessy I's

and they've already been shipped to Chicago. Chicago will get them to India, and we'll be in India for about four months. There we will not

be running out of eyes; so we wanted telescopes to which you can quickly walk up and quickly walk away. There's no way that we want a twentyfour

inch telescope with a twelve foot ladder in the night. (Laughter) But, as of now, the summer tour for this summer is cancelled, unless we suddenly

get some donations Which we're not expecting. We've postponed it till fall. But the Canadian people expect to get us funded to go to

Alberta next summer. They said that it has been approved in principle, and in their country that's the important thing. If they've approved

it in principle, they'll find the cash.


I think most of you know the Sidewalk Astronomers and what we do. There's a sun telescope just out there, and in front of it there's some

literature about us, including some diagrams of the sun telescope. Those of you who are interested in making a sun telescope, please help



Richard Berry heard that I was coming down to talk on Dobsonians and he asked me if I would please send him the pictures from my slide

show, and the text of my talk, so that he could get it into some kind of anniversary issue of Telescope Making Magazine; so I had to write back

that the pictures I'm going to use are the pictures that were long, long ago designed to go in a book called Sidewalk Telescopes. That's what we

were going to call our book. Meanwhile we have been scooped by Richard Berry, and there's already a book on How to Build a Dobsonian Telescope.

I propositioned him already that if he wants, in a future issue, he could get all these pictures, and our comments. We wanted a telescope

book with pictures clear enough so that someone who couldn't read could get the job done. We also wanted the text clear enough so a blind person

could get the job done; (Laughter) so it could be translated into Braile.


Those of you who think that these pictures ought to be published can bug Richard Berry. He might get them published. We're probably not

ever going to get out a book on telescope making because it is no longer necessary. Somebody else has already done the job, but I've been working

on it for the last fifteen years. (Laughter) A lot of the text was already written fifteen years ago. Some of you were just little kids.

I've been old a long time. (Loud laughter and applause) Anyway, I should tell you how these pictures were made. I went down to Palo Alto,

and one of our hard-core Sidewalk Astronomers, Molly Lusignan, got out her camera and, right in front of her camera, I put together the front

end and the back end of one of these telescopes. Then the pictures have been taken off to line drawings by Earl Junghans, over a five year

period, so that they could be cheaply published in the book. Meanwhile the book's been published by Richard Berry. (Laughter)


Starting the slides

Here we are measuring the tube. It's a ten inch tube for an eight and a half inch mirror. Here we're peeling out the plastic from inside

the tube. It comes out easily.  Here we're peeling off the outside. It comes off with difficulty,

but if you leave it on, you can run a hose over your telescope and get off scot free.


This is the diagonal mirror block. We put shingles into these slots, and the mirror is put onto the 45° cut, mounted on three blocks

of leather, about half inch on a side. It's put on with white glue. If you're not independently poor, (Laughter) like we are, you can use silicone

rubber. If you do, put three blobs on there and let them set up. Then put blobs on top of your blobs, and put your mirror down.


Here we're cutting the shingles to go into the block. We don't glue the shingles into the block, because what happens to them is that

some kid, with ice cream on his hands, reaches into the front end and breaks them off. Then, if you have them glued in, you need a table saw

to re-cut your grooves. If not, you can pick the pieces out of the grooves with a nail, put in another shingle, and you're back in business.

Our telescopes are made of crude materials, and the repair tools are also crude. (Laughter)


Here we're going to measure the distance from the center of the block out to where the shingles get cut off. You put your compass at

the center of the block and put those little lines out there and saw the shingles off. We're sawing them off with a hack saw. Then we'll cut

the corners off, otherwise, when you slide them into the tube, the shingles tend to split.


Years ago, when I lived in a monastery up in Sacramento, I had to make these things on a shoestring. It took me several years of selling

Green Stamps and Blue Chip Stamps to collect enough money to aluminize our first eighteen incher, which is now called The Little One. In those

days we used shingles that were blown off the roofs in a storm. (Laughter) We picked them up off the ground, already black from the sun, and

you don't have to paint them. (More laughter) Now we do it this way, but we still use shingles. If I designed something to hold the spider

mount, it would be a cedar shingle. Actually we use shim stock. They are already cut an inch and a half wide. If you use shingles, you have

to split them.


Here we have the little pads of leather, and we're putting them on with white glue. The mirror's lying down there. We'll put white glue

on the leather, and put the mirror on top of the white glue.


There's the mirror on the block. Now we-stand it up on the bar that goes away from the direction the mirror faces, and rest the other

two bars against something that keeps it level while the glue sets up. See, we lean it up against a can. Isn't that jazzy? (Laughter) and if

it slips, you put your pliers behind this bar and it keeps it as level as you like.


Here we're beveling the inside of the cardboard tube that the plumbing piece goes in. We put our eyepieces into chrome plated brass

tubing (1 1/2° o.d.) from a wash basin drain line trap. You just have to go to the plumber, and he has a whole bunch of them in the scrap bin,

and if you're a lady, he'll give you a handful. (Laughter) Years ago, up in Sacramento, we used to make twelve inchers out of the cores of

hose reels. They were 22 inches long, 12 inches inside diameter, about A quarter inch wall, rigid cardboard tubes with these big plywood wheels

on the ends. I was in the monastery in those days, and I used to go in there and get those tubes. But the kids in Sacramento, a whole lot of

them, took to making telescopes and we needed a whole lot of those things; so I turned the job of getting them over to the mother of one

of those kids, and she went down with a pan of brownies (Laughter) and she got a whole lot of tubes, and after that they wouldn't give them to

me. (Much laughter and applause)


So we use this chrome plated brass tubing, sliding against cardboard. It's about as good a focusing fit as you can manufacture, and

it's cheap. It doesn't cost anything.


Here we're drilling the eyepiece hole. If your tube is as long as your focal length, then you measure back from the font end of the

tube by the radius of the tube, and cut your eyepiece hole. We're cutting it with a fly cutter, run around backwards, by hand.


(One of the pictures failed to show on the screen.) Never mind, We've got plenty of pictures. We wanted it so you didn't have to read

anything. I showed these pictures to some kids in Palo Alto once and asked them, "Could you make this telescope if you didn't have any text?"

and one of those little girls said, "The're a few places where I'd need words." (Laughter)


The eyepiece tube fits through the main tube from inside through the hole. Eventually we'll put glue on the little tube where it fits

into the big tube, and on the edges of the masonite where it runs into the wall of the big tube, and that's where it'll stay. These things

don't catch flue. Eventually they get knocked off on door jams, (Laughter) and when they do, you get out your tube of glue and put it back.

Bones are much harder to mend. (Laughter) I have some more funny stories. I might as well tell one while this slide burns. (Laughter) We

were here in 1978, and we had a twentyfour incher and a twentytwo. Gerard had his twentytwo. It had a focal length as long as the twentyfour

but it was a little skinnier, and in those days it was called Long Eye. He took a prize here for "good workmanship", but on the way to the

Grand Canyon the tongue of his trailer sheared off and he lost the telescope at 55mph on the freeway, and it crashed into a post. Gerard was

fit to tie. He wasn't even going back to pick up the garbage, but one of the other Sidewalk Astronomers and I persuaded him that we'd have to

go back and get that stuff, and in two days we had it back on the sky. (Gasps) In two days, with government plywood, we rebuilt it and put it

back on the sky. Then it was called The Phoenix because it arose from its own ash. (Much laughter and applause)


Here we're painting the inside. Only the region visible through the eyepiece tube needs to be painted, and it needn't be black. I said

in my classes for years that it doesn't need to be black, only dark. You can put murals down the inside. Finally somebody did it. They put

planets and things all down the inside in a deep blue sky.


Here we're putting in the diagonal mirror. The parts of the diagonal mount that face the eyepiece tube should be dark.


I'm cutting a piece of cardboard to put under the back bar so the spider will be tight. Matches boxes are okay, those things you carry

in your pocket to light up "The Red Man's Revenge." (Laughter) The back bar goes away from the eyepiece tube, so that, in the final adjustment

of the diagonal, we can move the back bar wherever we want the image of the objective to go as seen through the eyepiece tube.


Here we're glueing in the spider and the eyepiece tube. We do that after our mirror is aluminized and our optics are all lined up.

This isn't a real telescope. This is just to show you how to do it. 


Here we're putting the 7x50 binocular eyepiece into the brass tube. I think I should tell you another funny story. About twelve years ago

I bought a few hundred of these eyepieces from a man in Seattle at about $4.50 apiece, and I sold them for $5.00 apiece, and we had eyepieces for

quite a long time. I still have two or three to replace the eyepieces in the twentyfour and so forth, just two or three. But I found out from

the man who sold them to me that he had sold several thousand to Edmund Scientific Company for 75 cents apiece. They went on sale for $6.75.

Then it went up to $9.75. The 75 cents has always been in there. (Laughter)


When we put the eyepiece into the tube, we take off the eye cup.  Now the reason that we take off the eye cup, and the reason that we

Sidewalk Astronomers use long focal length optics and long focal length eyepieces, is that we are astronomical entertainers by appointment to

her majesty the American public, and she has to get in there with her eyeglasses on. (Murmurs) And if you have a half inch focal length eyepiece,

or one of those quarter inch jobs which are like tear immersion lenses, you cannot get her in there without re-focusing it for her, and

that cuts down the number of people across the eyepiece from one or two thousand to about a hundred. At Ghirardelli Square we often get a

thousand people a night, sometime-, two thousand. In the national parks, we get something like three to four hundred people a night. At the

Grand Canyon one time, the rangers asked us to keep tabs on it, and it turned out that in sixteen days and nights we entertained twenty thousand

visitors. We had sun telescopes in the daytime, slide shows in the evening, and six or eight telescopes at night.


I'll tell you another story. One time at the Grand Canyon I had a sun telescope, like the one out there, and I asked a well dressed man

in a business suit if he wanted to look through it. After he had looked, I said, "Come this evening, we'll entertain the Hell out of you." He

stood up to his full height and said, "There is no Hell in us; we are Christians, (Laughter) but we would like to see the works of the Creator."

(More laughter)


Here we're cutting the tailgate. For a ten inch tube, we want it eight inches square. Then you draw a ten inch circle on it and cut off

the corners, where the circle crosses, and it just fits into the tube.  The corners will be held in tight with sheet metal screws.


Here we're finding the center for our circle. It just hits the corners; so you just have a tiny bit of sawing to do.


Oh, we're back to the first slide. Go back, please, and take it out. I don't usually have all this help. (Ashley McDermott, who had

introduced John, was running the projector.) You know, you people down here are very nice to us. (Ashley said, "We like you too." (Laughter))

Thank you! (Applause)


Here we're cutting off the little corners. You see, I wanted you to see everything; so that even a retard can get the job done. (Laughter)

Just because you have problems doesn't mean that you don't need to see. Everybody has to see. It is stupid to live in this universe and never

see it.


To put in the bolts, we draw a six inch circle, divide it into six parts, and put a bolt hole on alternate spots, one at the top and

one on either side. We have a pair of twelve inch binoculars called The Kangaroo. It weighs about 300 lbs. and operates with both eyes straight

at the moon at 90x. But those bolts are not put in this way. The outside bolts are put on the outside edge; so that you can make it look

cross-eyed to see the trees a mile away. It makes the bushes stand out from the mountains at fifteen miles. (Gasps) I looked through that at

the moon the first day the astronauts were on it. That very first night I was like a little kid, running in to watch the TV and running out to

look through the Kangaroo and the 10 1/2 inch Psychedelic Giraffe. I looked through both of them at the moon on the first day the astronauts

footprinted it.


Here we're drilling the bolt holes, right through the wood. Then we use a "church key" to wind them in. We want them tight. If the bolts

go in loosely, the kids at the star parties will wind them up tight. (Laughter)


Then we'll glue a thin cardboard triangle to the inside of the tailgate, with three little pads of masonite on the corners; so that

the mirror doesn't run into the bolts. If, by accident, you drop your telescope on the concrete, tailgate first, you don't want your mirror

to run out of the holes like bath salts; (Laughter) so it's cushioned by little pads of masonite glued to the cardboard. You can't glue the

masonite to the bolts. It won't stick. This cardboard should be floppy.  If it doesn't flop rightly, put a pencil under there and bend it down

till -.11e pads stay flat over the bolts. I don't know much about metal technology, but cardboard and wood I understand. (Laughter)


Now we're putting furring nails at the front ends of the blocks that center the mirror in the tube and keep it from running out the

front end. In my telescope classes I always say, "Booboo number one in this class is dropping and breaking your objective, and if you're going

to do it, do it at once. (Laughter) Don't wait till it's finished. And booboo number one and a half is tipping your front end down, without

having furring nails in the blocks, and rolling it out past the diagonal and loosing them both." (Much laughter)


Our mirrors are thin.; sc we use three inch blocks and put an ice pick hole in the middle, 1 1/2" from the end. Then we put a screw hole

in the tube 2 1/2" from the end. That puts the ends of the blocks one inch inside the end of the tube, and the tailgate butts against all four

blocks. If the tailgate touches only three, vou can glue a penny or a piece of cardboard to one of the blocks, or you can use a hammer. I use

a hammer. I'm like the man who had five cat holes in his kitchen door. When someone asked, "Can't they all go out through the same hole?", he

said, "When I say scram, I mean scram." (Much laughter)


The mirror is going to rest on two of these blocks, and there will be two at the top with furring nails in them; so that even if you tip

your mirror forward it can't roll down the tube. The tailgate will be vertical, with hoKizontal bottom and top, and we put the blocks and

tailgate in with /4", #8 sheet metal screws. The bolts are only one inch lone on the threads. We want it to be impossible for anyone to

get to the back and wind them up till the mirror touches the furring nails. The nails are not part of the telescope in operation. they are

just to prevent booboo number one and a half. (Much laughter)


Some of these slides are not very clear. Sorry about that, but the original pictures are very, very clear.


There's the objective mirror. We can reach our fingers in past cur tailgates and turn our mirrors around to see if they have astigmatism.

Also, those holes are used for ventilation. You mustn't keep your telescope in a warm place and then get it out under the cold, night

sky, or the air currents in the tube will make it impossible for you to read the defects in your mirror. 


For lining up our optics, we put a half inch colored decal at the center of the aluminized objective. Then we line up the diagonal

so that, looking down the eyepiece tube with an extender in it, we see that decal straight down the axis of the eyepiece tube. After that you

fiddle with the tailgate bolts till the decal covers the iris of your eye, centered in the eyepiece tube, as seen in the reflection from the

objective mirror. There are only two things to line up, and if you do them in the right order, it takes only five minutes.


That's looking into the front end of the eyepiece tube with an aluminized mirror in there. That's what you'd see if your optics are

properly lined up.


This is an out-of-focus diffraction pattern drawn by yours truly. It is just to explain to my classes that when we figure these things at

the eyepiece we choose a bright spot of light, either a star or the glint of sunlight from a power-pole insulator, and then we throw the

eyepiece out of focus, too far in and too far out. What we do in figuring is to match the distribution of light in the out-of-focus discs.

If the distribution is the same, inside and outside, as seen by you, it may be okay. If it's the same as seen by me, it's okay. (Much laughter,

followed by applause) That's the end of the slides.


(Ashley asks, "Shall I use the third tray?") No, no, I don't think we need to see it. (The audience expresses disappointment.) If

you want to see some more, we could show it. The next will be photographs of people making telescopes. Do you want to see them? (Applause)

I was told I wouldn't have much time; so I talked loud, fast and out of tune. (Cliff Holmes says, "John, you've got about twenty

minutes.) Alright, that's fine.


This is yours truly, about fifteen years ago, before his hair got grey. I'm doing some fine grinding here because I had a mirror to grind

on the day that Bruce Sams was grinding his. Bruce is now at Caltech, but in these pictures he's nine years old. He was one of the first

three Sidewalk Astronomers, he and Jeff Roloff and I, and he is the person on whose account the Sidewalk Astronomers were founded. His mother

called me up and said that "Brucie" needs someone to talk to about telescopes and about astronomy. He's taken the astronomy course at the Morrison

Planetarium twice and needs something more. 


This is a posed picture. She wanted me out of the way. Here we are on a pitch lap, and, in the real world, there is Bruce

Sams. He's now at Caltech and much taller than I. (Laughter) He could pick me up and throw me against the wall. (Much laughter) In the real

world he did his own work, but his mother wanted pictures of me. When he was thirteen he made an eighteen incher and won the California State

first prize in the Science Fair with his telescope, but he was three years too young to go to the national finals. (Laughter)


Here we're putting in the tailgate. And here we're building the box. This is plywood out of a debris box on the street. The rocker

also is discarded plywood from the building of an apartment house. We're recycling junk. In our mirrors, we're recycling the Navy, and a

lot of this other stuff is recycled junk.


Here he's testing his focal length by shining the reflected beam on his brother's tummy. That's not a smart idea. Don't do that to your

brother! (Long laughter) This is only a four percent reflection, and it's not very dangerous, but if it got in his eye it might be damaging.


When he had finished this ten and a half incher, he was my assistant at the Jewish Community center, but he was so small they wouldn't

listen to what he said; so I gave them a tongue lashing. (Laughter 


That's one I built in Sacramento long ago. It's made of those old hose reels. This is a "four-barrel-twelve". There are four 22" pieces

in there. It's still one of our workhorses, called Stellatrope. Its companion was called Heliotrope and now lives in Colorado. I had to

sell it for diagonal mirror glass.


I do sloppy work, but some of my friends save my reputation. See, he even upholsters the rear end. (Loud laughter)


Here's another kid making a ten and a half incher and grinning from ear to ear.


Now you see, we do not have grinding and polishing pictures for the Sidewalk Astronomers telescope book. Also, in How to Build a Dobsonian

Telescope, there is no information on grinding, polishing and figuring. It all ought to be in there in the same book. It's unfair

to have it the way it is. I think it should be edited with these telescope building pictures in there, and with some pictures showing how

the grinding and polishing are done, and how the figuring is done if you want to read your mirror at the eyepiece on stars or power-pole insulators.


Here he's drilling the holes through his tailgate, and here he's catching the reflection of the sun to measure his focal length.


And here he's being regurgitated by the tube. (Loud laughter) 


Here he's making the eyepiece hole, and the next slides are of someone else.


Now I want you to watch that six incher and see what happens.  (Gasps and laughter) That's a twelve incher. Fred Dilts made a six

incher and a twelve incher, and he painted them just alike. They're painted with seven coats of marine enamel. Don't ever do it. I have

argued for years that there is no earthly reason why telescopes should be white outside. The problem is how do they handle infrared, not how

do they handle sunlight. They're not used in the daytime; so that has never been a problem. What you need to find out is what paint to use

for the infrared, and it's not marine enamel. This telescope will dew down one hour after sundown on a night when no other telescope will dew

down. (Laughter) I've been at the Lawrence Hall of Science in Berkeley on a sunny day when the sun was coming straight down on the back of it,

and the belly of the tube was warmer than the top. The top can handle the sunlight, but the belly can't handle the infrared radiated up from

the sidewalk.


This is some side bearing detail. We use wooden discs, wrap sheet metal around them, overlap the ends and drill holes through them. Then

we drive a screw through the holes and into the notch in the disc till it pulls the sheet metal tight over the shoulders. The metal runs on

teflon. All our telescopes run on teflon. 


That's the Giraffe. It was built by a sixth grader in the '60's.  It shows six stars in the Trapezium, and is still one of our Workhorses.


This is a nine incher, and the father of the girl that made it traded catfish for the aluminum side bearings. (Laughter) It belonged

to a fifteen year old girl named Mo, and when we were figuring her mirror, it looked real good; so I said to her, "It's already to an eighth

of a wave. If you want, we could put a little extra time in it and get it to a sixteenth of a wave." She said, "What's the use; it's only a

nine incher?" (Laughter) My hero! This is just to show how easily portable they are.


This is a little sun telescope, and this is the front end of our big sun telescope called Heliotrope. It used to operate at 93 power so

you see the sun as if you were one million miles away behind a big welder's glass. In five minutes the welder's glass would evaporate,

 and the rest of you would go in the next three minutes. (Laughter) The sun telescope-out there shows you the sun the way you'd see it from two

million miles.


Here's the twentyfour incher at Fremont Peak with a ten and a little sun telescope. Here's the twentyfour up at Glacier Point in Yosemite.

That's where the Sidewalk Astronomers would like to see a permanent, public observatory with something like fifteen or twenty eighteen

inchers. (Murmurs) You cannot handle 400 people with one telescope in one evening. Don't flatter yourselves. It takes fifteen or eighteen

eighteen inchers to handle a crowd of several hundred people and get them all to see a whole lot of things on the same evening. With perhaps

a dozen people in each line, and all telescopes pointed at Jupiter, one man with a bull horn can talk to them all about Jupiter. Then you turn

them all to Saturn and talk about Saturn. You can have a lot of people on one evening and give a lot of information. The problem is information.

The reason - the only reason I have ever been interested in telescope making is because the people in this world have to see where the

Heck they live. (Applause begins.) They have to see it and they have to understand it. (Applause continues.)


Here we are at Glacier Point with the sun telescope re-painted.  

Here we are at Buttonwillow. My old van broke down near Buttonwillow, and when Brian Rhodes pushed me in there to work on my engine,

I set the sun telescope up in the street. By evening I had made arrangements to go to the school the next day with the sun telescope, and to

give them all a slide show. The entire school got to look through the sun telescope, and we got to see the murals in the halls. They belong

in the Metropolitan Museum of Art. If you have the good fortune to break down in Buttonwillow while school is on, go and speak to the superintendent

of schools - he's the principal, there's only one school - (Laughter) and get in there and see the murals in those halls before

they go.


A lot of you know Brian Rhodes. He and Jeff Roloff rebuilt my engine and we were out of there in two days. (Murmurs)


This is Bruce Lusignan. He's in communications, and teaches at Stanford. Here he's building a sixteen incher. See, he has a TV show

to watch. (Laughter)


Here we are in the Sierra at seven thousand feet, at Soda Springs. This is the twentyfour incher, and this is Ruthie over here. That's

an eighteen inch unobstructed reflector. It has the diagonal mirror just inside the edge, and the light comes from the objective over to

the side, hits the diagonal and goes immediately into the eyepiece.


Here is the black widow spider mount of the 24 incher. (Laughter) These pieces are garbage from the burned down hotel at Glacier Point.

The first time we took the 24 there, I told Brian that that must be the best place in the world for a public observatory, but there's that

awful hotel there and there'll be mercury vapor lamps. We went there anyway and found that the hotel had just burned down. (Laughter) So

this telescope has been there for more than 100 nights already. We've sized up the seeing conditions there for a lot of years, over a lot of

nights, and it's a fabulous place for a public observatory. It's right at the top of a cliff. Whatever cold air comes down over it, and it's

not very much, comes down through a forest very, very slowly and falls immediately off the cliff. It's cozy and warm there.


This is Thanksgiving Day. I'm down at Palo Alto to figure that 16 incher you saw a little while ago. I'm running my thumb around near

the edge, with cerium oxide and water on there, to take down a hill near the edge.


This is Jeannie Lusignan pressing the pitch lap. These pictures are long ago.


Here we are at Death Valley. That is a very long focal length 16 incher. It's ten feet long, and it's called the Magnificat because it

magnifies. This is Furnace Creek Visitor Center just about New Year's Day. He has hardware to put the two halves of his tube together. That

whole telescope, and six of us, rode in that little camper. 


Those are the Funeral Mountains behind us. This is below sea level. People sometimes ask us, "Why do you go below sea level?" We

go below sea level because you go below sea level. We take the telescopes to where you go.


Here we're looking at Saturn. That's all.  (Long applause)

(At the end he invited the entire audience to join the Sidewalk Astronomers.)


If we had 500 Sidewalk Astronomers, world wide, we could operate all summer long. We're not in conflict with the other groups; so join

some other group, but if you want to help get the telescopes join the Sidewalk Astronomers.