  Observations of the Cosmic Microwave Background rely on cryogenic
instrumentation with cold detectors, readout, and optics providing the low
noise performance and instrumental stability required to make more sensitive
measurements. It is therefore critical to optimize all aspects of the cryogenic
design to achieve the necessary performance, with low temperature components
and acceptable system cooling requirements. In particular, we will focus on our
use of thermal filters and cold optics, which reduce the thermal load passed
along to the cryogenic stages. To test their performance, we have made a series
of in situ measurements while integrating the third receiver for the BICEP
Array telescope. In addition to characterizing the behavior of this receiver,
these measurements continue to refine the models that are being used to inform
design choices being made for future instruments.
