What are two problems with building a refracting telescope with a 10-meter diameter lens?

1. Weight.

2. Large lens will tend to deflect under its own weight

Explanation:

Hello,

Weight is a big part of it. There’s a reason the largest working refractor on Earth (the Clark refractor at Yerkes Observatory in Wisconsin) has a 102-cm objective; a 125-cm lens was created for the Paris Exposition of 1900, but the accompanying telescope was a pain to use (very hard to aim) and was scrapped when no one wanted to buy it.

Lens can only be supported at its edges unlike mirrors, and a large lens will tend to deflect under its own weight unless it’s so thick that it won’t transmit much light. It is also extremely difficult to cast and polish a glass blank of such huge size, which is why (see List of largest optical refracting telescopes - Wikipedia) almost no one has tried building one in over a century.

Best regards.

lol for the pointsss