Wooden Telescope

[George.]

How do you build a telescope out of wood? You can start with some transparent wood and sand it into a parabolic mirror. I chose the easy path and started with a used objective lens, bought for a fraction of the price of a telescope. The idea was to build a second, smaller telescope to use in outreach sessions, and do it on a tight budget. We have very dark and clear skies in the dry season, and we bring local school kids out to the wilderness to experience the night sky without any lights around.

First step: make a hexagonal fitting around the 5" triplet lens - that's an objective made of three separate lenses, which together are meant to focus the image while avoiding the splitting of the colors like a prism, which makes for blurry rainbows instead of sharp images.

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[Peerie Maa]

You might enjoy this, built by an Air Raid Warden in WWII from anything he could collect from bomb sites and scrap heaps. The mirror mount is a modified brass porthole casting.

[Beowolf]

Nice.

I used to have a video of John Dobson, grinding out an objective mirror by hand.

“Caveman work.” As he described it.

[Jimmy W]

Nice.

I used to have a video of John Dobson, grinding out an objective mirror by hand.

“Caveman work.” As he described it.

This one?

[L.W. Baxter]

i don't even believe you have a wooden telescope

[skuthorp]

My grandfather built his own telescope, grinding a 5" mirror by hand. It was a wondrous thing in my chlidhood.
The observatory was a disused outhouse moved by tractor to the top of the hill and mounted on an artiliery wheeled (3) revolving platform. But rough but it worked well enough..

[TomF]

My dad ground the mirror for a telescope he built too - they were living in northern Manitoba at the time, way the hell away from any light pollution. Dunno what happened to it in the ensuing 70 years ...

[George.]

Next, I made some baffles of the same shape as the lens holder. Baffles will stiffen the hexagonal tube and keep stray light from bouncing around inside the tube and hitting the eyepiece.

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[George.]

I also cut 8 thin boards to the right length and planed and sanded them, checking with a bevel, until they had the right edge to form an octagonal tube.


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[Beowolf]

Next, I made some baffles of the same shape as the lens holder. Baffles will stiffen the hexagonal tube and keep stray light from bouncing around inside the tube and hitting the eyepiece.

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Given the stated purpose of baffles, would rounding over the inner edge of the baffles helped to prevent “stray light rays” from diffracting into the optics?

[ahp]

These are far to complicated for a newby. There are several books on home made telescopes. Try "Sky Publishing Co." Cambridge, MA.

[George.]

Given the stated purpose of baffles, would rounding over the inner edge of the baffles helped to prevent “stray light rays” from diffracting into the optics?

Actually, I glued a cardboard baffle, with a slightly smaller central hole, to each plywood baffle, precisely to get that "sharp edge" effect.

[Beowolf]

Actually, I glued a cardboard baffle, with a slightly smaller central hole, to each plywood baffle, precisely to get that "sharp edge" effect.

Essentially “peeling away” the light at the outer edges?

[George.]

Found a picture of it:

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Ideally the edge would be about one carbon atom thick, so I sanded the cardboard to make it as sharp as I dare. The plywood kept the whole thing stiff and fixed in place.

[George.]

I then temporarily taped the 8 strips of wood together into a tube. I followed a method learned from a thread on another forum, where someone smarter than me posted step-by-step pictures of how he built an even bigger scope.

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[George.]

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[George.]

With a telescope you need the objective lens and eyepiece lens to align perfectly, centered on the same axis and square to one another, and not ever shifting, so... epoxy! . A couple of drops on each baffle and endpiece, after lots of measuring, squaring, and checking.


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[George.]

With a telescope you need the objective lens and eyepiece lens to align perfectly, centered on the same axis and square to one another, and not ever shifting, so... epoxy! . A couple of drops on each baffle and endpiece, after lots of measuring, squaring, and checking.


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[WX]

I have an 8 inch pyrex mirror my late brother Kevin was building. Still needs polishing. I should sell it or give it away.

[Beowolf]

With a telescope you need the objective lens and eyepiece lens to align perfectly, centered on the same axis and square to one another, and not ever shifting, so... epoxy! . A couple of drops on each baffle and endpiece, after lots of measuring, squaring, and checking.


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Now we’re getting to the part I was curious about.

Details, man… how’d you do it?

[George.]

Details, man… how’d you do it?

I resorted to boatbuilding methods. I drew out profile plans of the optical tube on a piece of plywood at 1:1 scale, with wood pieces represented at actual thickness. I quadruple-checked every angle and measurement. I traced out sightline and optical axis. Then I marked, cut, and assembled all the parts against the drawings.

You can just see part of my "plans" on the plywood under the newly cut baffles.


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[George.]

I fitted the two end pieces very deliberately, checking constantly to make sure they were square. At first I glued each piece with white glue, so I could pull it apart if necessary. Once everything checked out I applied drops of epoxy at strategic places to make it never shift.


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[Beowolf]

I had assumed that some degree of adjustability would be incorporated into the lens installation.

How do you columate it? Can you adjust the lens in its factory bracket?

[George.]

No, the lens doesn't adjust from side to side on its bracket - only its tilt is adjustable, but it has to be aligned to the optical axis before you do that.

I temporarily assembled the tube with tape and screws, put the objective lens on one end and the focuser and backplate on the other, and then taped a piece of paper with a pinhole at the center over the lens. I put a barlow on the focuser and adjusted the position of the lens with little wooden wedges until the crosshairs on the barlow aligned with the pinhole. Then I screwed the objective firmly in place.

[George.]

[Beowolf]

No, the lens doesn't adjust from side to side on its bracket - only its tilt is adjustable, but it has to be aligned to the optical axis before you do that.

I temporarily assembled the tube with tape and screws, put the objective lens on one end and the focuser and backplate on the other, and then taped a piece of paper with a pinhole at the center over the lens. I put a barlow on the focuser and adjusted the position of the lens with little wooden wedges until the crosshairs on the barlow aligned with the pinhole. Then I screwed the objective firmly in place.

Well…. That answers that.

Keep going. What was the next part?

[George.]

Then I took the whole contraption out to check it by focusing on some distant trees - no mount yet, so no good way to point at stars.

[George.]

I still hadn't collimated the lens for tilt, but the image looked sharp enough, even through a handheld phone camera.

[George.]

So at this point I had confirmation that the lens and backplate were square and aligned with the optical axis. I marked the position of everything in case it shifted, removed the lens and backplate, and started carefully taking the tube apart bit by bit and epoxying everything firmly back in place, until only the top of the eight strips remained unepoxied, screwed on so that I could access the inside for painting.

[ahp]

Alignment is discussed in "Norton's 2000.0 Star Atlas and Reference Handbook" Edited by Ian Ridpath.

If you want to go way back in time, the 1920's and 1930's when amateur astronomers had less money, see if you can find "Amateur Telescope Making". It is in three volumes by Scientific American. Maybe Sky Publishing Co. sells it. I have not looked.

[gypsie]

Makes you wonder how Galileo did it, or marvel that he did it at all.

[AndyG]

Galileo? I don't believe for a moment that he did.

The first telescope is attributed to Hans Lippershey. Or maybe Zacharias Jansen. Or Jacob Metius. Who knows? All Dutch, all in the early 1600s. Galileo's claim is from around 1610.

And yet...

Roger Bacon, in the 1260s, talks of "lenses that are contrived so that the most distant objects appear near at hand." These lenses would be able to "read the smallest letters at an incredible distance, we may see objects however small they may be, and we may cause the stars to appear wherever we wish."

He notes the Milky Way is not a mere splot, but made up of dim stars. Three and a half centuries before the official invention of the telescope.

I think the Arabs got it, myself, late 12th, early 13th century. They knew their optics.

Andy

[George.]

Galileo didn't invent the telescope. He pioneered its use to study the heavens. Before him people used it to look at wooden boats and such.

[George.]

So with the tube epoxy curing, I turned to the mount. At this point it was clear that I would need a stable mount to do the testing required to collimate the lens. I decided to make a tabletop mount, easier to transport - a table for it can always be found. I cut, glued, and screwed some plywood:

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[George.]

The contraption above would rest and spin on the turntable below. On a whim, I decided to make the bearings on which the telescope tube would rest in the shape of crescent moons.

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