Continual Revision of the Big Bertha Redesign

Continual Revision of the Big Bertha Redesign

It turns out that Big Bertha's current diagonal mirror is larger than it should be (reducing image quality and light)--4.25". It turns out that if I reduce the mirror size to 3.1"--and increase tube diameter to 20.4" from 20"--I can get a nearly optimum combination of full illumination of the eyepiece and minimal obstruction. Of course, that means slightly enlarging the base plate the mirror cell (which I have not yet ordered), which increases the weight by a few ounces, and the weight of the two tube assemblies by an ounce or two. On the other hand, a 3.1" diagonal mirror is much lighter than the current diagonal.

However: my choices on the square tubes that hold the tube ends apart are becoming frustrating. I had plugged in the wrong value for Young's modulus--using the stiffness of carbon fiber composite instead of aluminum. Now my choices are to go to a larger, considerably heavier set of aluminum square tubes, or spend the money on carbon fiber composite. The net effect is that using 1", .125" wall aluminum square tube gives me a total telescope weight of 67 pounds to get the total deflection down into the thousandths of an inch range--just too heavy. Going to .995", .060" wall carbon fiber composite gets me a 48 pound telescope with a comparable worst case deflection down below .004". The price of this stuff, however, is breathtaking--like $400 for the four tubes. Unless, of course, you know of a surplus carbon fiber composite dealer...

Of course, I may be going too far on this. The deflection calculations are for the worst case--the telescope is pointing at the horizon. In practice, there is probably enough flex in a solid telescope tube that perhaps I can accept a few hundredths of an inch of deflection. I suppose that the worst that happens is I use somewhat smaller aluminum square tube (or two instead of four). For two square tubes, this gives me a total telescope weight of 57 pounds, and a maximum deflection of .014". If I need to, I suppose that I can add two more tubes.