Which has more strength, 28mm cast aluminum, 28mm steel tubing, or 28mm aluminum tubing?
Due to economies of scale, it is usually considerably cheaper to purchase a manufactured item rather than building it yourselfOf course if your goal is to be creative, purchasing is not the way to goBut otherwise there is no substituteBuilding a car out of parts would cost many times what a manufactured automobile would costSimple economics.
Well, I have to agree with those above - get a pre-manufactured mirror; it'll be far more accurate than one you can grind by handbut, if you'd want to learn more - go to the library, and check out a book by Robert (?) Dobson - he's the guy who invented the Dobsonian mount telescope, and it'll guide you through the steps you need.
Its far easier to buy a polished mirror with 1/6 wave than it is to make one yourself, these are extremely cheap to buyMaking telescopes is a dying artIf you really want to do it there are some excellent books on telescope mirror making which tell you everything you want to knowLearn the procedures in those books before you startAnyway I doubt there is anybody on Answers who can help you here.
If you are comparing 3 tubes of the same dimensions, then the steel tubing may be sourced with hihger strength than the aluminumIt depends on the grade of steel and aluminumSome high strength aluminum (7075) may be stronger in yield strength than some lower grades of steelThe best steels will be stronger than the strongest aluminumsThe steel will always have a harder surface than the aluminum The cast aluminum may be stronger if it is solidIt depends on the application Engineers make these decisions based on many factors in design.
