The James Webb Space Telescope (JWST) telescope's secondary mirror and eighteen primary mirror segments are
each actively controlled in rigid body position via six hexapod actuators. The mirrors are stowed to the mirror
support structure to survive the launch environment and then must be deployed 12.5 mm to reach the nominally
deployed position before the Wavefront Sensing and Control (WFSandC) alignment and phasing process begins. The
actuation system is electrically, but not mechanically redundant. Therefore, with the large number of hexapod
actuators, the fault tolerance of the OTE architecture and WFSandC alignment process has been carefully considered.
The details of the fault tolerance will be discussed, including motor life budgeting, failure signatures, and motor life.
The payload of the Spitzer Space Telescope is the Cryogenic Telescope Assembly (CTA), a thermal and optical system that houses the science instruments and provides them a 1.2 K thermal sink. It also provides the 0.85-meter telescope, which is controlled between 5 K and 12 K to achieve the appropriate low photon background for the instruments while conserving helium. This cryogenic system supplies cooling through a combination of passive radiation and controlled vapor flow from a superfluid helium cryostat. Unlike previous cryogenic space infrared telescopes, the CTA allows the users to selectively cool Spitzer for particular science operations. Each science opportunity is both a benefit to the astronomical community and a cost to Spitzer's cryogen lifetime. CTA allows these benefits and costs to be weighed. Launched warm in August 2003 with 49 kg of helium, the CTA has been performing superbly with a current helium loss rate of only 9 kg per year after the initial cool-down period. Remaining helium is measured periodically so that mission lifetime can be accurately determined. Due in large part to the success of Spitzer, various aspects of the warm launch design have become the standard for future cryogenic space telescopes. This report describes the CTA and provides flight performance data for the cryogenic system.
The instruments of the Spitzer Space Telescope are cooled directly by liquid helium, while the optical system is cooled by helium vapor. The greater the power dissipation into the liquid helium, the more vapor is produced, and the colder the telescope. Observations at shorter wavelengths do not require telescope temperatures as low as those required at longer wavelengths. By varying the telescope temperature with observing wavelength, we are extending the mission lifetime by an estimated 9%.
The Cryogenic Telescope Assembly (CTA) on the Spitzer Space Telescope employs a revolutionary warm launch design. Unlike previous space cryogenic telescopes, the Spitzer telescope is mounted outside of the cryostat and was launched at ambient temperature. The telescope was cooled through a combination of passive radiation and controlled vapor cooling from the superfluid helium cryostat. Launched in August 2003 with 49 kg of helium, the 0.85-meter telescope cooled to below 5.5 K within the initial 45 days of flight in accordance with analytical predictions. Despite an aggressive schedule of instrument initialization and checkout testing during the first two months of flight, the CTA met the temperature requirements for all checkout activities. The transient flight performance of this multi-stage thermal/cryogenic system has been found to agree well with pre-launch predictions over the broad temperature range. With an emphasis on early flight cool-down behavior, this report highlights the pre-launch cryostat preparation, the thermal behavior during cryostat blow-down, comparisons to pre- and post-launch model predictions, and in-flight helium mass measurement. The post cool down performance and rate of helium use is also discussed.
The SIRTF Cryogenic Telescope Assembly employs a multi-stage thermal/cryogenic system in which the telescope is cooled bo 5.5K by passive techniques combined with vapor cooling by the effluent from a superfluid helium cryostat. The cryostat and telescope are surrounded by an outer shell, which is passively cooled to an expected temperature of about 35K. Verifying the performance of this system by test cannot be practically accomplished by a single end-to-end test. In the SIRTF-CTA Performance Test, we verified the relationship between helium flow rate and telescope temperature with the outer shell held at its predicted flight temperature. For systems like the SIRTF-CTA with thermal time constraints of several days, schedule is a critical parameter when planning the test procedure. The original plan of two steady-state tests at two different flight-like thermal boundary conditions was supplemented when we discovered that the test-induced (background) heat load to the cryostat and telescope was an order of magnitude larger than the predicted flight levels. With these unforeseen heat loads, we amended the test plan to include multiple changes to the test boundary conditions to quantify the background heat sources. Because of schedule constraints, we did not h ave the luxury of establishing steady-state for the various conditions of interest, and we relied on analysis of the system transient response for verification. Here we present our investigation of test-induced heat loads, our approach to data analysis, a comparison of measured system performance to analytical predictions, and some lessons learned.
The instruments of the Space Infrared Telescope Facility (SIRTF) are cooled directly by liquid helium, while the optical system is cooled by helium vapor. The greater the power dissipation into the liquid helium, the more vapor is produced, and the colder the telescope. Observations at shorter wavelengths do not require telescope temperatures as low as those required at shorter wavelengths. By taking advantage of this, it may be possible to extend the helium and mission lifetime by 10% or even 20%
The Cryogenic Telescope Assembly (CTA) houses the SIRTF Science Instruments and provides them a 1.3 K temperature heat sink. It also provides the telescope, which is maintained at 5.5 K temperature in order to achieve the low photon background required for the 160 micron detector array. This unique cryogenic/thermal system provides the necessary cooling through passive means along with use of vapor cooling from the helium gas vented from the 360 liter superfluid helium cryostat. The cryostat vacuum shell temperature is low enough that the heat load to the helium reservoir is due almost entirely to instrument power dissipation, thus resulting in a predicted lifetime over 5 years. The corresponding helium loss rate is over 7 times lower than achieved by previously flown helium-cooled instrument systems, such as IRAS, COBE, and ISO. This extraordinary performance is made possible by the highly favorable thermal environment achieved in an Earth-trailing solar orbit at a distance of about 0.3 AU from the Earth. Attaining this outer orbit with the slight lift capacity of a Delta-II launch vehicle is made possible by the mass-saving approach of having the telescope outside the cryostat and warm at launch. The general end-to-end system architecture, verification approach, and predicted performance are discussed.