Sonotube: Lighter and Stiffer Than I Remembered

I went to American Construction Supply in south Boise today to get a 20" ID Sonotube.

I was a bit concerned if the weight estimates here for Sonotube were correct -- they seemed awfully light for something that you fill with concrete and from which I was going to make a telescope.  I picked up a pretty enormous section of it, and had no doubts.

Getting it home was quite an operation.  The piece above is 74" tall and 20" ID -- and yes, it just fit in the Corvette's passenger seat, once I removed the top.  (But I did get some strange looks on the way back to the office, and even more when I carried into the office because I was concerned about rain.)

I have absolutely no question about the weight or the stiffness.  I just weighed this beast, and it was less than 26 pounds -- or a bit more than four pounds per foot.  It is also quite stiff (although not quite like fiberglass).  I still intend to add a layer of polyester  resin (and even a layer of fiberglass cloth) to improve stiffness and water resistance, but I do not expect that to add more than a couple of pounds of weight -- and then I will have a rock-solid tube.

My father and I built an 8" reflector back in the 1970s using a piece of Sonotube.  I remember that it cost what seemed like a pretty heavy amount of money -- like $30 or so -- but that was what we could afford at the time, and in retrospect, it worked pretty darn well, and we did not even know to coat it with fiberglass.  We just painted it inside and out.

UPDATE: Part of why it is lighter and stiffer than I remembered (aside from the flaws of memory of events and objects now decades away) is that Sonotube, like carbon fiber and fiberglass, is a composite material: paper combined with glue, producing a combination that is stiffer than you expect of paper, and stronger than you expect of glue.

I sanded it inside and out, and now it is hanging, ready to be fiberglassed tomorrow.  I was measuring the amount of cloth that I have, so that I know how much more to buy before I get started tomorrow:

The gray thing in the bottom of the picture is the cat skulking away.  No, it's not a mouse hunting tool, Tater.

What Was I Thinking?

Even the reinforcements didn't do the job...and the quotes for stiffer material are pretty horrendous.  So I find myself asking, "What if I used a solid tube instead?"

It turns out that Sonotube (the traditional home telescope maker material) weighs 4 pounds per foot in 20" ID size.  A 6 foot section would weigh 24 pounds; the lower cage, including the mirror and cell, currently weighs 34 pounds.  The tube for the lower cage weighs 6.26 pounds, so the mirror and cell weighs less than 28 pounds.  Adding the Sonotube, the mirror and cell, comes to 52 pounds.  The finder, the diagonal mirror, the spider, and the focuser, weigh about five pounds.  That's 57 pounds.  I would bolt the Sonotube directly to the dovetail (perhaps with a couple supports on either side to prevent rocking), which weighs 2 pounds, 6.6 ozs.  That's still less weight than what I currently have, and far stiffer.  (How stiff is Sonotube?  It is used to pour concrete columns that are 20 inches diameter and many feet high.)

If I fiberglass the Sonotube (a common technique to both improve water resistance and appearance), I can't imagine it would add more than a pound or two to it.  It won't be terribly portable, but big deal -- I don't need to transport it.

I'll go hunt up a 73" length of this tomorrow.  (It ought to fit into the passenger seat of the Corvette with the top off.  I shudder to think what others will think of it.)  I think I became so enamored of a sophisticated portable design that I completely lost sight of the simplest solution.

UPDATE: $49.48 + tax for 6 feet of it.

New Book Out By A Friend

Robert J. Cottrol, The Long, Lingering Shadow: Slavery, Race, and Law in the American Hemisphere (University of Georgia Press, Studies in the Legal History of the South, 2013)

From the press release:

 Students of American history know of the law’s critical role in systematizing a racial hierarchy in the United States. Showing that this  history is best appreciated in a comparative perspective, The Long, Lingering Shadow (University of Georgia Press, February, 2013) looks at the parallel legal histories of race relations in the United States, Brazil, and Spanish America. Robert J. Cottrol takes the reader on a journey from the origins of New World slavery in colonial Latin America to current debates and litigation over affirmative action in Brazil and the United States, as well as contemporary struggles against racial discrimination and Afro-Latin invisibility in the Spanish-speaking nations of the hemisphere.

Ranging across such topics as slavery, emancipation, scientific racism,  immigration policies, racial classifications, and legal processes, Cottrol  unravels a complex odyssey. By the eve of the Civil War, the U.S. slave system was rooted in a legal and cultural foundation of racial exclusion unmatched in the Western Hemisphere. That system’s legacy was later echoed in Jim Crow, the practice of legally mandated segregation. Jim Crow in turn caused leading Latin Americans to regard their nations as models of racial equality because their laws did not mandate racial discrimination—a belief that masked very real patterns of racism throughout the Americas. And yet, Cottrol says, if the United States has had a history of more-rigid racial exclusion, since the Second World War it has also had a more thorough civil rights revolution, with significant legal victories over racial discrimination. Cottrol explores this remarkable transformation and shows how it is now inspiring civil rights activists throughout the Americas.

Bob teaches at George Washington University School of Law, and sat on my master's thesis committee (which was a stretch, since that was at Sonoma State University).  Bob has also been heavily involved in scholarly work about the gun control issue, unsurprisingly, having authored several very important papers about gun control and its racist origins.

You can order a copy from Amazon by clicking here.

Several of My Works Cited in a Recent Fifth Circuit Decision

Unfortunately, in the dissent from the en banc decision to not rehear NRA et al. v. BATF (5th Cir. 2013) concerning the legal minimum age for sales of handguns to 18-20 year olds.  There are actually at least four citations to my work there.

An Interesting Person Passed During The Night

My stepmother-in-law, Bette Thorne, died of pneumonia in a hospital in British Columbia.  She has been declining for a number of years now from Alzheimer's, and it is hard to see this as a great tragedy.  Still, she had some interesting stories to tell when she was doing better.  Shortly before my father-in-law died of cancer, my wife and I drove Better and him up from Orange County, and I spent a bit of time talking to her about her life, and there was much of interest to learn from her.

She grew up in a rough section of Chicago where the Catholic kids regularly called her a "Christ-killer" because she was Jewish.  (This was the 1930s.)  She served with the U.S. Army Air Force during World War II, and one of her duty assignments was in New Guinea, deep in a jungle airbase.  She met her husband, who was also with the AAF, and after the war, they raised their twin sons in Manila, living what was apparently a very charmed, almost colonial life.

Talk to everyone that you have a chance to meet: they all have some interesting stories to tell.

A Tasty Gluten-Free Dessert

My daughter came up with this recently -- Rainbow-Vanilla Cake.

Another Brilliant Idea From My Readers

Glen's comments below were pretty useful: he pointed out that had I mounted the truss block where the mirror is, it would have reduced the bending moment.  I can't mount that where the mirror cell (too many things in the way) but I can mount the truss blocks below the mirror cell (although of course, with longer tubes).  I have an intuitive sense that this would pretty well eliminate the bending of the tube, since the center of gravity would now be above where the truss blocks are.  Does this make sense to everyone?

I am assuming that I can find 1", .050" wall aluminum tubing from my local vendor.  They won't be black, but I can flat black paint them easily enough, and there won't be any shipping charges.  Better, I can have them cut me the lengths that I want, and I won't be paying for the excess that I don't need.  It increases the weight by a few ounces per tube, but that probably is not much worse than the weight of adding a coat of fiberglass to the lower tube.

So, what's everyone's opinion?  Will putting the truss blocks below the weight of the mirror on the lower cage substantially reduce this problem?

UPDATE: Another possibility is to have another ring like the last one rolled by the elves (I mean, Chicago Metal Rolled Products), but with a 1/4" wall instead of a 1/8" wall like the current one.  That would add almost six pounds to it, unfortunately, but it would be, I think, quite sufficiently stiff.  I have requested a quote from them.  Perhaps I would put more effort into cutting down the walls of the C-channel to take a couple of pounds off there -- what is in place currently has no measurable deflection.

UPDATE 2: It just occurred to me that there is another approach that doesn't involve spending a penny.  The 1" square tube that used to be used for connecting the upper and lower cages is still sitting around.  I could use these on the exterior of the lower cage (where there are still attachment holes) to increase stiffness of the tube in the very area where the bending is taking place.  Perhaps put a thin piece of aluminum plate (which I have lying around) on the inside of the tube to increase stiffness.