Null test lens for a 30" F/4 mirror?

What size, quality, and specific type of lens would be required to use a
Foucault tester as a Ross Null Tester for a 30" ~f/4 mirror? Does anyone
make such a beast?

Lawrence Sayre
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My philosophy, in essence, is the concept of man as
a moral being, with his own happiness as the moral
purpose of his life, with productive achievement as
his noblest activity, and reason as his only absolute.

Ayn Rand (in the appendix to 'Atlas Shrugged')
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