Abstract
Current problems in astrophysics relating to the determination of stellar properties, properties of exoplanet host stars, and observation of the expanding universe due to dark energy depend on accurate measurements of flux (a.k.a. spectral irradiance) from stellar sources. Flux calibrations of observatories are typically based on natural standard stars that currently have tenuous SI traceability where additional uncertainty occurs from differences in the reference source and target observations during the scale transfer measurements. CANDLE (Calibration using an Artificial star with NIST-traceable Distribution of Luminous Energy), is a prototype instrument payload to demonstrate an artificial light source that could be flown in space and enable flux calibrations of ground or space observatories with uncertainty less than 0.5% (k=1) across the visible to near-infrared spectral range from 350 nm to 2000 nm with rigorous SI traceability.
CANDLE will reduce the uncertainty of stellar flux measurements by providing an on-orbit source of calibrated spectral irradiance. The instrument consists of three illumination modes that will be SI-traceable to the NIST primary optical watt radiometer (POWR) through physical standards that are available on-orbit. One mode will consist of a set of mirrors that direct sunlight to a target observatory and which is traceable through solar irradiance measurements by other space-based instruments. A second mode will consist of a broadband monochromator and the third mode will consist of a set of single-mode fiber-coupled laser diodes at discrete wavelengths spanning the spectral range and falling within target observatory band-pass filters. In these latter two cases, SI traceability will be provided by on-board reference standard detectors (an electrical substitution radiometer and set of photodiodes) that carry the flux responsivity scale to orbit. These on-orbit reference sources will better mimic target star observations and help reduce systematic effects by cross-checks available from the three illumination modes.
This talk concerns the single-mode fiber-coupled laser illumination mode of CANDLE. It will detail current progress towards the fiber-optical design, understanding of the estimated flux level, and target observatory calibration and uncertainty budget. It will also detail progress made towards radiometric testing of a single-mode fiber-coupled laser diode calibrated irradiance source and efforts towards improving the technology readiness level of the laser illumination mode. This presentation will show that the chosen lasers can survive vibration, vacuum, and radiation testing and that measurements of the irradiance from a single-mode fiber laser can be made at the 0.1% stability level, which is a target for achieving observatory response calibrations with total uncertainty less than the 0.5% level (k=1).
Characterization and Calibration of Single-Mode Fiber-Coupled Laser Diodes as Irradiance Sources for Astronomical Observatories
Current problems in astrophysics relating to the determination of stellar properties, properties of exoplanet host stars, and observation of the expanding universe due to dark energy depend on accurate measurements of flux (a.k.a. spectral irradiance) from stellar sources. Flux calibrations of observatories are typically based on natural standard stars that currently have tenuous SI traceability where additional uncertainty occurs from differences in the reference source and target observations during the scale transfer measurements. CANDLE (Calibration using an Artificial star with NIST-traceable Distribution of Luminous Energy), is a prototype instrument payload to demonstrate an artificial light source that could be flown in space and enable flux calibrations of ground or space observatories with uncertainty less than 0.5% (k=1) across the visible to near-infrared spectral range from 350 nm to 2000 nm with rigorous SI traceability.
CANDLE will reduce the uncertainty of stellar flux measurements by providing an on-orbit source of calibrated spectral irradiance. The instrument consists of three illumination modes that will be SI-traceable to the NIST primary optical watt radiometer (POWR) through physical standards that are available on-orbit. One mode will consist of a set of mirrors that direct sunlight to a target observatory and which is traceable through solar irradiance measurements by other space-based instruments. A second mode will consist of a broadband monochromator and the third mode will consist of a set of single-mode fiber-coupled laser diodes at discrete wavelengths spanning the spectral range and falling within target observatory band-pass filters. In these latter two cases, SI traceability will be provided by on-board reference standard detectors (an electrical substitution radiometer and set of photodiodes) that carry the flux responsivity scale to orbit. These on-orbit reference sources will better mimic target star observations and help reduce systematic effects by cross-checks available from the three illumination modes.
This talk concerns the single-mode fiber-coupled laser illumination mode of CANDLE. It will detail current progress towards the fiber-optical design, understanding of the estimated flux level, and target observatory calibration and uncertainty budget. It will also detail progress made towards radiometric testing of a single-mode fiber-coupled laser diode calibrated irradiance source and efforts towards improving the technology readiness level of the laser illumination mode. This presentation will show that the chosen lasers can survive vibration, vacuum, and radiation testing and that measurements of the irradiance from a single-mode fiber laser can be made at the 0.1% stability level, which is a target for achieving observatory response calibrations with total uncertainty less than the 0.5% level (k=1).