  The Simons Observatory (SO) will provide precision polarimetry of the cosmic
microwave background (CMB) using a series of telescopes which will cover
angular scales from arc-minutes to tens of degrees, contain over 60,000
detectors, and observe in frequency bands between 27 GHz and 270 GHz. SO will
consist of a six-meter-aperture telescope initially coupled to ~35,000
detectors along with an array of 0.5m aperture refractive cameras, coupled to
an additional 30,000+ detectors. The large aperture telescope receiver (LATR)
is coupled to a six-meter crossed Dragone telescope and will be 2.4m in
diameter, weigh over 3 tons, and have five cryogenic stages (80 K, 40 K, 4 K, 1
K and 100 mK). The LATR is coupled to the telescope via 13 independent optics
tubes containing cryogenic optical elements and detectors. The cryostat will be
cooled by by two Cryomech PT90 (80 K) and three Cryomech PT420 (40 K and 4 K)
pulse tube cryocoolers, with cooling of the 1 K and 100 mK stages by a
commercial dilution refrigerator system. The second component, the small
aperture telescope (SAT), is a single optics tube refractive cameras of 42cm
diameter. Cooling of the SAT stages will be provided by two Cryomech PT420, one
of which is dedicated to the dilution refrigeration system which will cool the
focal plane to 100 mK. SO will deploy a total of three SATs. In order to
estimate the cool down time of the camera systems given their size and
complexity, a finite difference code based on an implicit solver has been
written to simulate the transient thermal behavior of both cryostats. The
result from the simulations presented here predict a 35 day cool down for the
LATR. The simulations suggest additional heat switches between stages would be
effective in distribution cool down power and reducing the time it takes for
the LATR to cool. The SAT is predicted to cool down in one week, which meets
the SO design goals.
