A telescope old and new

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No, it’s not a home-made pumpkin cannon.

Instead it’s a trap for photons.

Or more conventionally, it’s a fine astronomical telescope** made by a teenager named Stewart S about 50 years ago.

That’s the part inside the white metal tube.

The plywood box that looks a bit like a Civil War cannon is actually a fairly conventional Dobsonian-style plywood alt-az mount that I made over the past few gloriously clear days in the driveway of my house in Brookland (Northeast DC). #

Yes, that tube is LONG – over seven feet long. It’s finely welded aluminum plate, bent into a cylinder and formed by one of my predecessors in leading telescope-making classes in the DC area, Hoy Walls (whom I never met).

The plywood mount used almost all of a full four-feet-by-eight-feet sheet of 3/4″ hardwood plywood.

I was pleased to see that my calculations were all correct, so that the scope just barely fit inside the plywood pieces and that the mount as a whole behaves well. No filing or last-minute sanding was needed. And the balance is pretty good – all I needed to do was add an old three-pound Barbell-type weight to the front end – it’s the dangly thing near the flange in the front. Having that weight imbalance is actually a good thing, because it gives us leeway to add a small finder scope to help aim the scope at objects of interest.

The flange is actually a plywood ring that I cut with a router and a decent commercial template for circles. The plywood ring fit perfectly, which was gratifying. No sanding or filing was needed — a first! I added it because the front end of the tube had been banged up or pressed hard at some point and was no longer circular.

I made an adjustable clamp inside the scope that seems to be working quite well. One can loosen the clamp and then rotate the tube to make it easier for shorter people can reach the eyepiece. Or one can move the tube forward or backwards to fix any future balance changes. And then clamp the tube back into its new position.

The optics were in very good shape – very clean, not a bit of dust or insect debris, on a beautifully smooth and fully-polished out mirror. I checked the optics briefly with a Ronchi test, and found that it had no turned down edge (which is a good thing) on its surface. I did not have time to use the Couder-Foucault Zonal Knife Edge test to calculate how well-corrected the optics are. Correction is a technical term involving changing a near-sphere to a near-paraboloid by carefully removing less than 2 cubic millimeters of glass — all together — in just the right spots, over a period of weeks or months to try to reach perfection.&&

Since the scope is done, the normal final test is to try it out on Polaris or some other fairly bright star with a short-focal-length eyepiece, looking at the diffraction rings when you roll the eyepiece into and out of focus.

Which brings me to why I had to build or modify the mount that it came with.

The mount that Stewart and Hoy made and that Stewart donated to NCA was very, very heavy, being a classical example of a heavy-duty – modified – plumbing – and – automotive – parts style of telescope making that many amateurs used around 30 to 60 years ago, before John Dobson revolutionized amateur telescope making with his eponymous telescope mounts and his unusual mirror-making methods using cast-off naval portholes as substrates for mirrors.

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That’s me above on the right, wearing an Escher tessellation Y-shirt, at Almost Heaven Star Party near Spruce Knob, WVa, next to a green telescope I made with Nagesh K. It’s called a Lurie-Houghton telescope design because of the geometry of the lenses and the mirror. Bob B did most of the lathe work for the aluminum finder scope that Jan appears to be touching with his left hand. Unfortunately, this telescope is so far a complete and utter failure although all of the individual pieces seem OK or great. Until we figure out what went wrong, we are stuck in limbo. The mount, but not the scope design, is a Dobsonian.

 

 

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The late John Dobson visited our telescope-workshop once, perhaps 10 years ago. He is the oldest person in this picture, facing us, with a bright blue jaket and white hear. It looks like I am doing some sort of incantation over the mysterious batch of molten pitch, but it’s all scientific — no magic spells. *

Unfortunately, the mount on the long aluminum telescope as it came to us was impossible even to roll through doors and was incredibly heavy. We (Mike L, Bob B and I) tried to fix the wheels on the platform that held the mount (sorry, I can’t find any of the photos I took of it), but we then found that the scope literally would not hold still. As a result there was so much backlash that you could not aim it anything and actually look at the object for any length of time at all. I considered putting three jack-screws so that the platform could be jacked up (much the way cranes and RVs will jack themselves up off their wheels) to be stable, but I couldn’t think of any way that wouldn’t require an enormous amount of time or money.

I also wanted to see if I could make a Dob mount in about three days.

I did.

And it worked.

Right now, you can aim the 10-inch f/long scope at an object and the object will stay there in the finder except for the Earth’s rotation. The tube is reasonably well-balanced. With some muscle and the clamp I made, you can change the balance point. Unfortunately, the mount really could use a little bit of beefing up as far as the base is concerned, and I’m contemplating how to do that without adding too much weight.

Alan T helped me take out the guts of the telescope, wrestle the tube into the plywood mount, and then re-assemble the optics. We looked through it at Hopewell Observatory last night. The Moon, Mars, and Saturn were well placed and looked pretty good, and it seemed like the air was pretty stable. We did not crank up the magnification very far, and the Moon was so bright we all cast shadows on the grass, so very few stars and no Milky Way were visible. A formal star test will need to come later.

On behalf of National Capital Astronomers, I would again like to thank Stewart S for donating this fine telescope.

 

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# I hope my neighbors will forgive me. I didn’t work on it at night.

* Some of us amateur telescope mirrors will admit to uttering curses from time to time, but unlike what happens at Hogwarts, they never work. Guess we are just Muggles, condemned to obeying the laws of physics and the other sciences.***

** This scope is not my personal property, nor does it belong to the Hopewell Observatory in northern VA where this photo was taken and of which I’m a member and current president. The telescope tube and a different mount were donated by Stewart S to National Capital Astronomers and is currently housed at Hopewell as a service to NCA. Stewart is obviously no longer a teenager!

*** Actually, I’m rather glad that the laws of physics and other sciences don’t seem to be under the personal command of any individual, and that they seem to be exactly the same for all people and – as far as we can tell so far — everywhere else in the universe. (Yeah, I know that some astrophysicists and cosmologists make claims that certain basic constants of nature change in certain ways over time, but I’m reserving judgment on that. OTOH I hope I live long enough for someone to figure out what ‘dark matter’ is. It remains spooky and awesomely mysterious that with all of our current state of ever-expanding scientific knowledge, most astrophysicists still believe that the vast majority of the matter and energy in the universe is still completely unknown. We don’t know what it is. I find that the idea that the universe has immutable rules, many of which have not been discovered, much more comforting than having a universe where somebody else could get mad at you or me and change the laws of physics in your immediate vicinity with the purpose of doing us harm — and that all those powers are accessible by certain individuals who claim that they have been in personal communication with a personal deity, who uttered cryptic messages and incantations and rules that must never be questioned. How could powers like that NOT corrupt somebody?

 

&& like a small sand grain.

Published in: on June 8, 2014 at 9:17 pm  Comments (3)  
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3 CommentsLeave a comment

  1. Guy, never having built a scope, I’m wondering what keeps the tube from sliding when pointed near the zenith.
    Thanks,
    Paul – NOVAC

    Like

    • I incorporated a clamp into the rectangular ‘cradle’ that holds the tube. It’s not visible in the photo. It has two wooden knobs that allow you to press a semi-circular wooden piece against the metal tube, holding it gently but firmly. There are four springs that will pull the clamp back out of the way when you loosen the clamp.

      Like

  2. Very nice!

    Like


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