25 research outputs found
Specific Heat of Liquid Helium in Zero Gravity very near the Lambda Point
We report the details and revised analysis of an experiment to measure the
specific heat of helium with subnanokelvin temperature resolution near the
lambda point. The measurements were made at the vapor pressure spanning the
region from 22 mK below the superfluid transition to 4 uK above. The experiment
was performed in earth orbit to reduce the rounding of the transition caused by
gravitationally induced pressure gradients on earth. Specific heat measurements
were made deep in the asymptotic region to within 2 nK of the transition. No
evidence of rounding was found to this resolution. The optimum value of the
critical exponent describing the specific heat singularity was found to be a =
-0.0127+ - 0.0003. This is bracketed by two recent estimates based on
renormalization group techniques, but is slightly outside the range of the
error of the most recent result. The ratio of the coefficients of the leading
order singularity on the two sides of the transition is A+/A- =1.053+ - 0.002,
which agrees well with a recent estimate. By combining the specific heat and
superfluid density exponents a test of the Josephson scaling relation can be
made. Excellent agreement is found based on high precision measurements of the
superfluid density made elsewhere. These results represent the most precise
tests of theoretical predictions for critical phenomena to date.Comment: 27 Pages, 20 Figure
The Nancy Grace Roman Space Telescope Coronagraph Instrument (CGI) technology demonstration
The Coronagraph Instrument (CGI) on the Nancy Grace Roman Space Telescope will demonstrate the highcontrast technology necessary for visible-light exoplanet imaging and spectroscopy from space via direct imaging of Jupiter-size planets and debris disks. This in-space experience is a critical step toward future, larger missions targeted at direct imaging of Earth-like planets in the habitable zones of nearby stars. This paper presents an overview of the current instrument design and requirements, highlighting the critical hardware, algorithms, and operations being demonstrated. We also describe several exoplanet and circumstellar disk science cases enabled by these capabilities. A competitively selected Community Participation Program team will be an integral part of the technology demonstration and could perform additional CGI observations beyond the initial tech demo if the instrument performance warrants it
Paving the Way to Future Missions: the Roman Space Telescope Coronagraph Technology Demonstration
This document summarizes how far the Nancy Grace Roman Space Telescope
Coronagraph Instrument (Roman CGI) will go toward demonstrating high-contrast
imaging and spectroscopic requirements for potential future exoplanet direct
imaging missions, illustrated by the HabEx and LUVOIR concepts. The assessment
is made for two levels of assumed CGI performance: (i) current best estimate
(CBE) as of August 2020, based on laboratory results and realistic end-to-end
simulations with JPL-standard Model Uncertainty Factors (MUFs); (ii) CGI design
specifications inherited from Phase B requirements. We find that the predicted
performance (CBE) of many CGI subsystems compares favorably with the needs of
future missions, despite providing more modest point source detection limits
than future missions. This is essentially due to the challenging pupil of the
Roman Space Telescope; this pupil pushes the coronagraph masks sensitivities to
misalignments to be commensurate with future missions. In particular, CGI will
demonstrate active low-order wavefront control and photon counting capabilities
at levels of performance either higher than, or comparable to, the needs of
future missions.Comment: 10 pages, 3 tables. Revised version v2: added some co-author
Paving the Way to Future Missions: the Roman Space Telescope Coronagraph Technology Demonstration
This document summarizes how far the Nancy Grace Roman Space Telescope Coronagraph Instrument (Roman CGI) will go toward demonstrating high-contrast imaging and spectroscopic requirements for potential future exoplanet direct imaging missions, illustrated by the HabEx and LUVOIR concepts. The assessment is made for two levels of assumed CGI performance: (i) current best estimate (CBE) as of August 2020, based on laboratory results and realistic end-to-end simulations with JPL-standard Model Uncertainty Factors (MUFs); (ii) CGI design specifications inherited from Phase B requirements. We find that the predicted performance (CBE) of many CGI subsystems compares favorably with the needs of future missions, despite providing more modest point source detection limits than future missions. This is essentially due to the challenging pupil of the Roman Space Telescope; this pupil pushes the coronagraph masks sensitivities to misalignments to be commensurate with future missions. In particular, CGI will demonstrate active low-order wavefront control and photon counting capabilities at levels of performance either higher than, or comparable to, the needs of future missions