Friday, May 3, 2013

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.

3 comments:

AlanKH said...

Here's something you can build with any leftover Sonotube.

http://www.hometheaterhifi.com/volume_6_3/diy-11-sonotube-subwoofer-september-99.html

JohnG said...

Clayton, I'm curious. Since mobility seems secondary, why not expend your effort and money on a permanent mount in an observing station in your backyard? Something with a sliding roof would offer a solid mount, protect you from most of the wind, block some light in your eyes, and provide a permanent year-round home for Bertha. Thoughts? John

Clayton said...

I had thought of that before we had the extra garage built. In retrospect, it might have been a better idea.

However:

1. The mount is solid enough; the tube isn't. The Sonotube (with fiberglass on it) should solve that.

2. Block some light? What light? It's DARK here.

3. I can roll it out pretty quickly from garage to observing location in about a minute.