The telescope has access to stars of magnitude 18. Can you see a double star in it, each component of which is magnitude 19?

By the definition of stellar magnitude, an n-th magnitude star is 100 ^ 1/5 ≈ 2.5 times brighter than an (n + 1) th magnitude star. Two stars of 19 magnitude are only 2 times brighter than one star of 19 magnitude. Consequently, such a binary is fainter than an 18th magnitude star, and is inaccessible to the telescope.