A dewar is, simply, a thermos bottle used to keep an instrument as cold as possible. In infrared astronomy, we have to keep our sensors at temperatures of a few degrees above absolute zero. To do this we build a cylindrical container that contains at one end, two small holes connected to thin pipes that lead into the interior of the dewar and connect to two cryogenic reservoirs. One will contain liquid nitrogen at about 70 K, and the second reservoir will contain liquid helium at 1.5 K. Attached to the helium 'can' is a copper cold plate about 1 - 2 centimeters thick, onto which is attached the infrared detectors. You have to make certain that the cans do not come into physical contact with each other, or with the outer shell of the dewar, otherwise a pathway develops along which heat can be conducted very rapidly. The entire dewar has to be pumped-down to very low pressures about 1 million times lower that sea level pressure to get rid of as much gas as possible which could also conduct heat to the very cold surfaces inside the dewar. Thermos bottles used to be vacuum pressurized 30 years ago, but today people use various kinds of foam insulation for hot and cold drinks. For research- grade dewars, you have to use high tech vacuum pumps.