The Big Bertha Rebuild Project

I mentioned this yesterday, and I know that some of you are very interested in it (or are really desperate for something to read). For those who are wondering why I care about a deflection measured in hundredths of inch, when I almost certainly can't make all the parts that accurately--it's very simple. With the telescope sticking straight up in the air, there will be no deflection. With the telescope pointing at the horizon, a few hundredths of an inch of deflection will screw up the collimation of the optical train. If I collimate for one position, that much deflection will screw up collimation in the other position. As far as I am concerned, deflection needs to be down in the thousandths of an inch area before I am happy.

I went to Metals Supermarket today to look at what they had in stock, and see if comparing the stiffness of the square aluminum tubing with what the formulas tell me passed the giggle test. Yup! I tried to bend a 3", .125" wall piece of 6063T6 aluminum, and a 3", .25" wall piece of 6061T6 aluminum. Yes, extremely stiff!

There's no difference in stiffness between 6061 and 6063--although 6061 is a bit harder. Both have a 68.9 gigapascal modulus of elasticity.

The good news is that when I went out to measure the dimensions of Big Bertha, I discovered that some of my assumptions about the dimensions were wrong. The mirror weighs 26 pounds, and it is only 23 inches from the balance point for the telescope. This lower weight and shorter length substantially reduces the point load length and somewhat reduces the beam load length. This lets me use either a somewhat smaller tube, or get less deflection. Using real data, a 3" square tube with .125" wall would give me .001" total deflection from beam load and point weight load--and at least at this point, it appears that my total telescope weight will be somewhere around 46 pounds.

I can get the total deflection below .001" by going to a 4.25" tube, which brings the telescope weight up to 48 pounds--still acceptable. I am still trying to find out if using two smaller tubes is additive--if it distributes the load across both tubes, and thus cuts the deflection in half. I have an email into a friend with a PhD in Mechanical Engineering--I'm hoping that he is educated enough to answer the question!

As I mentioned, I have to build my own mirror cell to fit the rather odd geometry of not having a tube, but I think have come up with a design involving a hexagon that will work. I can't turn a piece of aluminum 17.5" inside diameter (as tempting as it is), but I think the solution is to make a hexagon from pieces of aluminum bar stock, cutting 60 degree corners, then drill, tap, and screw them together at the corners. I can use a similar, although slightly larger hexagon to suspend the diagonal mirror from, and on which to mount the eyepiece focuser and finder.

I may build a small version of this first to house the 3" f/4.5 reflector I built some years ago--a chance to verify the design in Delrin. If it works in Delrin, aluminum should be no problem. Yes, the weight of something like this goes up with the cube of the increase in linear dimension, but aluminum has a somewhat higher modulus of elasticity than Delrin, so I suspect that if it works for the 3", I won't have to do much to make it work for Big Bertha.