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 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.
NASA's Space Infrared Telescope Facility (SIRTF) is a 1- meter class cryogenically-cooled space observatory currently in the design and development phase. It is planned for launch in December 2001 by a Delta rocket into a heliocentric orbit. The SIRTF Observatory is comprised of the Cryogenic Telescope Assembly (CTA), the Spacecraft, and the three Science Instruments. The CTA has an 85 cm diameter aperture telescope which is cooled to its lowest operating temperature of 5.5 K by effluent vapor from the 360-liter superfluid helium cryostat. The three Science Instruments, which span an operating wavelength range from 3 to 180 micrometers , will be maintained at a temperature of 1.4 K inside the cryostat. The required SIRTF mission lifetime is >= 2.5 years. The CTA system and subsystem design as well as their technical challenges are described.
Self-focusing is a phenomena that is induced in certain materials when high irradiance laser light interacts with the material. High irradiances are most readily achieved with focused ultrashort laser pulses. Past theoretical calculations using the nonlinear wave equation have calculated the critical power for self-focusing by tightly focused beams in water at 580 nm to be 1 MW. The recent pulse propagation model by Feng et al. has been used to find the pulse duration where the self-focusing threshold can be most easily found. In addition, a first-order model of laser-induced breakdown developed by Kennedy has predicted that the threshold for breakdown at each pulse duration is independent of spot size. Thus self-focusing can be seen from a precise measurement of spot size and breakdown threshold. With several optical setups with different predicted spot sizes, we measured the spot size by knife- edge technique at energies far below the breakdown or self- focusing thresholds for a pulse duration of 2.4 ps, 800 fs, and 126 fs. We also measured the laser-induced breakdown threshold for each of these optical setups. The laser- induced breakdown irradiance threshold was constant for those spot sizes that were below the self-focusing threshold, as predicted by Kennedy's model. The measurements of self-focusing for ultrashort laser pulses in water and its implications on retinal damage will be discussed in this paper.
NASA's Cosmic Background Explorer (COBE) was launched into a polar orbit from the Vandenberg Air
Force Base, California on November 18, 1989. The COBE conthin three scientific instruments. Two of
these are infrared instruments housed within a 660 liter toroidal superfluid helium cryogen tank. The
tank is designed to maintain the base of the instruments below 1.6 K for the duration of the planned one
year mission. Boil-off helium is vented from the cryogen tank through a porous plug liquid vapor phase
separator, and then overboard from the spacecraft.
We discuss here the initial thermal set-up and operation of the dewar in general, and the helium vent
system in particular. During the initial cooldown of the dewar from 1.72 K to 1.41 K, short term
(1 mm ≤ t ≤ 3 mm) temperature and pressure oscillations were observed in the porous plug and in the vent
line. These oscillations have continued throughout the mission life. A detailed flow model was developed
to describe this phenomenon and is described below. We further detail the slow establishment of a steady
state, 'mission mode' operation of the dewar. The various factors leading to a two week time to mission
mode equilibrium for the dewar and cryogenic instruments are discussed.
Finally we summarize the performance of the dewar and instruments through the first six months, and
we project the expectations for the remainder of the mission through the final depletion of the liquid
The evolution of design approaches for high-performance superfluid helium dewars containing large-aperture telescopes are discussed. Particular attention is given to thermal-math modeling for the IRAS and the Cosmic Background Explorer (COBE) dewars. Correlation of the recent COBE flight data with the dewar thermal-math model is presented, and apparent predictive deficiencies of the model are discussed.