For most of the Two Weeks on Mars project, the 36" was used "stopped down" (i.e. with its aperture reduced) to 20 inches, a practice which lessens the light gathering power of the telescope but reduces aberrations in the image. Since Mars is very bright, the light loss was negligible while the gain in image quality was significant.

Here Rem Stone is seen putting the 20" stop in place. He has inverted the telescope with the eyepiece end high in the air, to gain access to the 36" lens.