Thursday, April 12, 2012

More On The Big Bertha Rebuild

More on the rebuild effort.  It turns out that Moonlite Telescope Accessories truss tube blocks can only be had sized to handle an ID of .625" and larger, so I have three choices:

1. Make my own.  Probably more work than it is worth.

2. Use .625" ID carbon fiber tubes, which will give me greater stiffness, and a pound or so more weight (not exactly a tragic amount), and another $120 or in cost.

3. Build an adapter of aluminum or carbon fiber composite tube that is .625" ID, and epoxies onto the smaller tubes that I was planning to use.

Concerning the C-channel part:

1. I do not need to thread anything, so this simplifies the process immensely.  I can use through holes for this.

2. It turns out that I may not need the carbon fiber composite C-channel.  I can cut the existing 72" long aluminum C-channel to 24" long.  Because it is somewhat thicker than the composite, the calculated deflection goes from .0006" (because all but five inches of the channel are actually supported by the very thick dovetail plate), to .0004".  On the downside, instead of shedding eleven pounds, I would only shed a bit more than seven pounds.  Because the stiffness is already so good on this, I might add some lightening holes, especially in the portion that is supported by the dovetail plate.  It also saves $179.50 for the carbon fiber C-channel.

UPDATE: The more I look at this, the more sense it makes to machine some adapters to fit the .52" OD tubes into a .625" ID insert for the blocks.  The .625" ID carbon fiber tubes are $45 each, while the .52" OD tubes are $25 each.  More importantly, the larger tubes come in a 70 inch length, when I really only need 60 inches.

I also remeasured and discovered that the base of the trusses is 19", not 17", which reduces the sag for one truss to .0097", and for all three trusses together, something like .0064" at the horizon--small enough to not even worry about.  Other parts of the optical path will move more than that.

Machining adapters out of aluminum to fit the ends of the carbon fiber tubes has another advantage: the ends of carbon fiber composite tubes tend to have little slivers that really hurt!  These will cover the ends.  I'll start with a 3/4" piece of aluminum rod (or perhaps I can find some 3/4" OD, 1/2" ID aluminum tube) and machine it to fit  the inserts on end end, and the carbon fiber tube on the other, and make these .010" fits on the carbon fiber tubes, and perhaps .020" for the inserts.  This will make them a very tight fit on the carbon fiber, and a bit more relaxed onto the inserts.

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