The weight of the glass will increase with the square of the radius. But it will probably increase more than that because as the diameter gets bigger the glass has to be thicker to support itself and also hold its optical figure. That puts additional constraints on the tube design. Even if you keep the focal ratio the same as you increase the aperture, the focal length will continue to elongate the tube.
This means that the moment arm or leverage exerted upon the entire apparatus is increasing as well. If you measure it in inch-lbs you've got both more pounds and more inches. This is going out in the rear direction as well as forward because the tube will be balanced approximately in its center. At the same time you want to keep the light cone from the objective precisely targeting the center of the focuser for optimal photography or eyepieces viewing.
In the old days they basically used industrial pipe to solve these problems.
At the same time a lot of oems want to make the refractor available at a price where it will sell more units. This invites cutting corners in the models which are not in the to-hell-with-price top tier of the market.
In a sense the inexpensive models are underpriced relative to desired performance. That's why so many people who buy these look around for aftermarket focusers. The aftermarket focusers will improve performance but they will be put onto a tube that was not necessarily designed for the additional stress of the fancier focuser.
Over a century ago in professional astronomy the limits of the refractor design in terms of cost and aperture were reached with the 40 inch refractor. Beyond that you got to face the facts, buckle down, and get some kind of alternative design telescope. In amateur astronomy today you face steeply rising curves for refractors after about 5 in.
Edited by gnowellsct, 08 May 2021 - 09:54 AM.