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.