Saturday, May 18, 2013

Telescope Project Marches On

There was some pooling of paint at the bottom of the tube when I painted the first coat with the tube on end, so for the second coat, I went back to the suspension approach that I used when applying the fiberglass resin.

The method of holding the tube in the air at that stage wasn't very good, because sometimes the tube started rocking, and then the aluminum tube holding it would go flying off the chairs.  So I came up with a way to prevent the aluminum tube from getting too far afield:

Unlike the first coat, which did not stick spectacularly well to the fiberglassed Sonotube, the second coat stuck quite well.  It still wasn't very even, so I sanded it after the second coat as well, which again evened out the surfaces a bit (although taking off a bit of paint in the process).  There were still some pretty big depressions caused by air pockets in the area where I had used fiberglass cloth, so I mixed a bit more resin, and tried to fill in the holes.

I still won't call the result, after the third coat, "beautiful," but I think it will do, at least until I can talk myself into believing that the big money for a carbon fiber composite tube that weighs 13 pounds less makes sense:

I mentioned a few days back that I was going to epoxy some steel rod on either side of the center line of the tube where it attaches to the dovetail plate to prevent rocking, which would eventually damage the tube.  Then, yesterday, I mentioned that it would be better to have tube rings that lock onto the outside of the tube and screw directly to the dovetail plate.

I am still waiting on a price quote on those rings, and I started thinking.  I noticed that at the edges of the dovetail plate there are a bunch of 1/4" holes.  Hmmm.  Could I mount supports in those holes to prevent rocking?  My first thought was to machine some 1/4" (or perhaps slightly longer) plugs that would provide the support?  I could even just use 1/4" hex head bolts; the heads would provide just the right elevation to prevent rocking (although at the risk of marring the surface of the tube).

But the more I thought about it (and was glad that enamel dries so slowly), it occurred to me there was an even better solution that gives me the flexibility to rotate the tube, not drill permanent mounting holes, solve the rocking problem, and involves minimal use of materials.

1. I take the C-channel which was used to mount the old scope to the dovetail plate, and cut off two 2" sections (preferably the ones that have 1/4" through holes already).

2. Shorten the legs of the C-channel down to 1/16" inch (since I no longer need long legs for stiffness of the section).  Now I have a round tube to flat base adapter.

3. Drill and tap 1/4"-20 holes in each side of the two sections.

4. Buy four leather belts at the thrift store.  (I need about 60 inches total length, and preferably 1 3/4" wide belts.)

5. Use a 1/4"-20 bolt and a washer to hold one end of each belt to each side of the C-channel.

6. Use the belt buckles to secure the tube to the C-channel sections.  (This means that I will need to get roughly similar belt buckles when picking out belts.)

7. Perhaps drill some holes in these belts to get them in a position where I can tighten down the tube without crushing it marring the surface much.

Now I have the 60 pound or so telescope load held by two belts, which is, I think, sufficient to prevent anything from moving or working loose.

An alternative would be to look for some flexible 2" wide stainless steel straps, and apply felt to the inside to protect the tube.  But then I have to figure out a way to secure the steel straps so that they can be loosened without having falling completely off.  The Cave Optical mount that I had long ago used this approach, with a screw brazed into one end of the strap, so that you could loosen the straps at one end, rotate the tube, then retighten.  If I could find something like this (with roughly 60" long straps) that had some way to tighten and loosen tension, this would be preferable.  But the belts might be a quick way to get the telescope operational, and then worry about rings at a later time.

UPDATE: Or Velcro?  This claims that the closure shear strength is 11.0 pounds per square inch.  I can buy a 15 foot by 2" wide piece of Velcro at Home Depot for $28.97, what they call industrial strength Velcro.  If I had ten inches of overlap of hook and loop (and if I understand what they are claiming for the closure shear strength), that would be 220 pounds per strap.  That seems more than enough.  It would not be as elegant as aluminum rings, but it would be light, I could pick up the Velcro tomorrow, and put the telescope together tomorrow afternoon.

UPDATE 2: Or perhaps use nylon webbing with buckles.  The webbing has a tensile strength of 5500 pounds (probably more than the buckles that come with it or where I would attach it).

UPDATE 3: The more I think about it, nylon webbing and Velcro are likely to stretch under load, and that sounds a bit dangerous.  If I can find some steel straps at Home Depot, I could put a bolt through the adjustment end, and use a wing nut to tension it.

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