Reddit reviews The Perfect Machine: Building the Palomar Telescope

There's a great book I read years ago about the Mt Palomar scope - I think it's this one. They overcame a huge number of problems to build it - it's well worth a read. When you consider the size of the thing coupled with the tiny tolerances they had to build it within - it's a really incredible that they pulled it off so successfully.

Here is a picture I like. Even cooler though are the sketches Russell Porter did of the telescope from the blueprints. If you are interested in learning about the construction of the telescope I would recommend The Perfect Machine by Ronald Florence

I recently read The Perfect Machine which was a really excellent history of early modern telescope design and construction, and gets into the fundamentals, abilities, and limitations of large telescopes. It was very enjoyable!

To amplify Wagamos's comment: it's heat. IIRC, when doing the final figuring on the 200" mirror in the Mt. Palomar telescope, they could only polish for a few minutes at a time due to glass changing shape as it warmed. Read The Perfect Machine for details. There's not much you can do to get around it: polishing creates friction which creates heat which changes the shape and properties of the glass. If you want a good mirror, you can only go so fast.