Measurement of telescope transmission using a Collimated Beam Projector

With the increasingly large number of type Ia supernova being detected by current-generation survey telescopes, and even more expected with the upcoming Rubin Observatory Legacy Survey of Space and Time, the precision of cosmological measurements will become limited by systematic uncertainties in flux calibration rather than statistical noise. One major source of systematic error in determining SNe Ia color evolution (needed for distance estimation) is uncertainty in telescope transmission, both within and between surveys. We introduce here the Collimated Beam Projector (CBP), which is meant to measure a telescope transmission with collimated light. The collimated beam more closely mimics a stellar wavefront as compared to flat-field based instruments, allowing for more precise handling of systematic errors such as those from ghosting and filter angle-of-incidence dependence. As a proof of concept, we present CBP measurements of the StarDICE prototype telescope, achieving a standard (1 sigma) uncertainty of 3 % on average over the full wavelength range measured with a single beam illumination

Absolute radiant flux measurement of the angular distribution of synchrotron radiation

We have measured the absolute radiant flux of synchrotron radiation as a function of the angle above and below the orbital plane with high accuracy at the Synchrotron Ultraviolet Radiation Facility (SURF III) at the National Institute of Standards and Technology (NIST), and the results were compared with theoretical calculations. The radiant flux of synchrotron radiation was measured at effective wavelengths of 256.5, 397.8, and 799.8 nm using three calibrated narrow-band filter radiometers with electron energies ranging from 180 to 380 MeV at SURF III. The filter radiometers were positioned inside a beamline with an unobstructed view of synchrotron radiation. The measured radiant flux agrees with theoretical Schwinger formulation to better than 0.5% for angles up to several milliradians

Development of Transfer Standard Spectrographs: Implications for Earth Remote Sensing

During NASA’s Earth Observing System-era, a series of source radiance validation campaigns were planned and executed by the EOS Project Office with the goal of validating the radiances assigned to laboratory calibration sources, principally lamp-illuminated integrating spheres, and establishing uncertainty budgets for the disseminated radiance scale. Based on an analysis of 7 years’ worth of data, Butler et al.1 assigned an uncertainty in disseminated radiance scales of 2% to 3% in the Vis/NIR (silicon) region, increasing to 5% in the short-wave infrared region. The uncertainty in the radiance scale met EOS requirements. For example, the radiance uncertainty requirement for MODIS was 5% in the Vis/NIR spectral region and the uncertainty in disseminated radiance scales met or exceeded sensor calibration requirements. Looking to the future, uncertainty requirements for radiance are reduced below 1% [2]. Uncertainties in the radiance scale in disseminated sources from NIST cannot be reduced and alternate approaches need to be considered to meet future satellite sensor uncertainty requirements. In this presentation, the radiometric characterization and calibration of a spectrograph along with its uncertainty budget are discussed. Its long-term stability is presented and its potential use as a transfer standard radiometer is discussed. The combined standard uncertainty in the spectrograph responsivity is estimated to be 0.25% (k=1), representing a potential order of magnitude reduction in the uncertainty in the radiance of laboratory calibration sources. [1] Butler, J. J., et al., Validation of radiometric standards for the laboratory calibration of reflected-solar Earth observing satellite instruments, Proc. SPIE 6677, 667707 (2007). [2] Ohring, G., et al. (eds.), Satellite Instrument Calibration for Measuring Global Climate Change, NISTIR 7047 (2004)

Advances in SI-traceable Detector Standards for the Reflected Solar Region

Current uncertainty requirements for the measurement of global variables relevant to climate change modeling in the reflected solar range are 0.5 % to 1 % for solar and lunar spectral irradiance and 0.5 % to 2 % for Earth reflected radiance. Future uncertainty requirements, as defined by CLARREO mission requirements, include laboratory calibration uncertainties are 0.2 % over the 500 nm to 900 nm spectral range and 1 % or less over the rest of the spectral region from 320 nm to 2300 nm. The uncertainties in lamp-illuminated integrating spheres, a principal calibration artifact, ranging from 2% to 3 % in the visible to near-infrared and 3 % to 5 % in the short-wave infrared, do not meet either current or future uncertainty requirements. NIST uncertainties in broad-band lamp-based calibration artifacts are traceable to primary standard blackbodies. The irradiance and radiance responsivity scales disseminated by the NIST laser-based SIRCUS calibration facility are held by primary standard irradiance meters traceable to the Primary Optical Watt Radiometer and dimensional metrology from the NIST Aperture Area Facility. The SIRCUS facility has demonstrated that moving from source-based scales traceable to primary standard blackbodies to detector-based scales offer opportunities to reduce the uncertainties in disseminated standards. This presentation discusses existing primary radiometric standards and the uncertainties in detector-based radiance and irradiance scales, validation of their uncertainty budgets through measurements of primary standard metal melting point blackbodies, development of low temperature active cavity radiometers operating in irradiance mode, and the development of transfer standard spectrographs. Finally, this paper discusses possible further developments at NIST aimed at supporting future climate benchmark instrument uncertainty requirements

A Study of Out-of-band Uncertainties for On-orbit Ocean Color Measurements Based on Laser Calibration of Flight Radiometers

Laser-based laboratory calibrations in the facility for Spectral Irradiance and Radiance responsivity Calibrations using Uniform Sources (SIRCUS) of the National Institute of Standards and Technology (NIST) have achieved detector calibration with uncertainties less than 0.1 % in the silicon spectral region. Because of the high power of lasers, there is also a drastic increase in dynamic range covering more than 5 orders of magnitude. The low measurement uncertainty and high dynamic range allow accurate assessment of out-of-band performance for flight instruments such as the Suomi National Polar-orbiting Partnership (NPP) Visible and Infrared Imager Radiometer Suite (VIIRS) sensor which was calibrated in 2010 for Absolute Spectral Responsivity (ASR) with full aperture illumination using a tunable laser and a large integrating sphere with an approximately 0.5% uncertainty in radiance responsivity. With the calibration data, it is now possible to fully evaluate the effect of out-of-band (OOB) contribution on at-sensor water-leaving radiance and derive strategy to alleviate variations from OOB scattering and reduce data product uncertainties. In this work, we present a sensitivity analysis of the Top-of-Atmosphere (TOA) measurement for varying chlorophyll concentrations and column water vapor based on measured detector Relative Spectral Responsivity (RSR). A conventional band-averaged radiance approach is taken and histograms are presented for different bands of radiometers to illustrate the data uncertainty from these variations as well as the global seasonal differences. We show how conventional lamp calibration at ground and on board solar calibration resulted in large deviation from OOB scattering because of mismatch with ocean color spectra. We will also present similar analysis for the detectors of the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Sea-viewing Wide Field-of-view Sensor (SeaWiFS)

Pre-flight Testing of an Ocean Radiometer for Carbon Assessment (ORCA) Prototype with a Realistic Scene from a Hyperspectral Image Projector (HIP)

We projected a multispectral MODerate resolution Imaging Spectroradiometer (MODIS) scene into a prototype National Aeronautics and Space Administration (NASA) Ocean Radiometer for Carbon Assessment (ORCA) [1] instrument using a National Institute of Standards and Technology (NIST) Hyperspectral Image Projector (HIP) [2]. The MODIS scene included some clouds over ocean, similar to what ORCA would measure during its flight. During the tests, the ORCA scan mirror was parked, and the projected scene was scrolled in the track direction in order to simulate along-track platform motion. The ORCA instrument measured the projected scene, and we compared the ORCA measured scene to the projected scene, in the spatial and spectral domains, in the near infrared spectral region. The results show reasonable agreement between the measured and projected scenes, and serve to provide confidence that the system was working as expected. We present the results and describe some of the details, which serve to form an example of how the HIP can be used to perform pre-flight testing of sensors with realistic scenes. [1] M. E. Wilson, et al. “Optical Design of the Ocean Radiometer for Carbon Assessment,” Proc. SPIE 8153, Earth Observing Systems XVI, 81530S (2011). [2] J. P. Rice, et al., “Hyperspectral image projector applications,” Proc. SPIE 8254, 82540R (2012)

Measurement of telescope transmission using a Collimated Beam Projector

International audienceWith the increasingly large number of type Ia supernova being detected by current-generation survey telescopes, and even more expected with the upcoming Rubin Observatory Legacy Survey of Space and Time, the precision of cosmological measurements will become limited by systematic uncertainties in flux calibration rather than statistical noise. One major source of systematic error in determining SNe Ia color evolution (needed for distance estimation) is uncertainty in telescope transmission, both within and between surveys. We introduce here the Collimated Beam Projector (CBP), which is meant to measure a telescope transmission with collimated light. The collimated beam more closely mimics a stellar wavefront as compared to flat-field based instruments, allowing for more precise handling of systematic errors such as those from ghosting and filter angle-of-incidence dependence. As a proof of concept, we present CBP measurements of the StarDICE prototype telescope, achieving a standard (1 sigma) uncertainty of 3 % on average over the full wavelength range measured with a single beam illumination