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The NGST ACTDP cooler features a 5–6 K Joule-Thomson (JT) cooler pre-cooled by a three-stage Pulse Tube (PT) cryocooler. Our design provides remote cooling of ∼60 mW at 5 to 6 K and, optionally, remote cooling at 18 K. The NGST ACTDP cooler is being developed under contract with the Jet Propulsion Laboratory to enable active cooling of instruments including large space telescopes, in particular the Mid InfraRed Instrument (MIRI) of the James Webb Space Telescope (JWST). This paper describes the design of the NGST ACTDP cooler and presents test results, including the integrated tests of the two major subsystems making up the thermal-mechanical cryocooler, the closed loop JT cooler and the PT pre-cooler. Both the JT and PT subsystems have been demonstrated to provide the cooling required for MIRI in laboratory tests using flight like hardware with excess capacity. These tests enable a reliable projection of the total electrical power of the NGST MIRI cooler of 200–296 W for steady state operation under end of life conditions, depending on the margins added to the thermal loads and parasitics. NGST’s design offers a low risk approach to providing cooling of MIRI making maximum use of flight proven hardware. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]