The CANDLE Engineering Demonstration Unit (EDU) was selected by the 2022 APRA program to develop and demonstrate the ability to reach the flux accuracy and range required for an artificial flux calibration star. A critical issue in producing accurate and reliable flux calibration is systematic effects; this EDU is providing a path to deploying an artificial star calibration payload outside Earth’s atmosphere with SI-traceable calibration that enables accurate throughput characterization of astronomical and earth science observatories in space and on the ground. Such a payload could be carried independently on a dedicated platform such as an orbiting satellite, e.g. the Orbiting Configurable Artificial Star (ORCAS), by a star shade at L2, or some other independent platform to enable accurate end-to-end throughput vs. wavelength calibration that can be measured repeatedly throughout the operational lifetime of an observatory. Once calibrated, the observatory is enabled to carry out astrophysical programs whose science objectives demand high accuracy and/or high precision observations. One specific and immediate application is establishing SI-traceable standard stars beyond the current limited set. We show in this paper the progress made in developing this EDU.
The Ultraviolet Transient Astronomy Satellite (ULTRASAT) is a space-borne near UV telescope with an unprecedented large field of view (200 deg2 ). The mission, led by the Weizmann Institute of Science and the Israel Space Agency in collaboration with DESY (Helmholtz association, Germany) and NASA (USA), is fully funded and expected to be launched to a geostationary transfer orbit in Q2/Q3 of 2025. With a grasp 300 times larger than GALEX, the most sensitive UV satellite to date, ULTRASAT will revolutionize our understanding of the hot transient universe, as well as of flaring galactic sources. We describe the mission payload, the optical design and the choice of materials allowing us to achieve a point spread function of ∼ 10 arcsec across the FoV, and the detector assembly. We detail the mitigation techniques implemented to suppress out-of-band flux and reduce stray light, detector properties including measured quantum efficiency of scout (prototype) detectors, and expected performance (limiting magnitude) for various objects.
Here we present the methodology and results of transferring UV–NIR flux calibration from NIST photodiodes to a set of 20 picoammeters. These are to be deployed as flux reference sensors on the SCALA calibration system at the University of Hawaii 2.2m telescope on Maunakea as part of a systematic upgrade aimed at improving the existing flux calibration for dark energy and exoplanet host star measurements beyond the ∼ 4 mmag / 100 nm we have already achieved at optical wavelengths with SCALA. Our robotic light source for performing the photodiode calibration transfer provides monochromatic light spanning 230 to 1200 nm with a dynamic range of 106 , while our new picoammeters have a noise floor of 10 fA in 4 s at 25 ◦C, with saturation around 400 pA. Our robotic gantry enabled the measurement of the spatial and angular response of our picoammeters. In preparation for the calibration transfer, a number of tests were performed to establish the measurement uncertainties, and these tests revealed subtle systematic effects that required correction. These includes polarization effects, leading to the redesign of part of the optics in the gantry head, implementation of a Holmium-Didymium filter as a precision wavelength transfer between arc and continuum light sources, and further suppression of stray light. We find that our calibration transfers are consistent with the NIST calibration to within ∼0.1%
The Ultraviolet Transient Astronomical Satellite (ULTRASAT) is a scientific UV space telescope that will operate in geostationary orbit. The mission, targeted to launch in 2024, is led by the Weizmann Institute of Science (WIS) in Israel and the Israel Space Agency (ISA). Deutsches Elektronen Synchrotron (DESY) in Germany is tasked with the development of the UV-sensitive camera at the heart of the telescope. The camera's total sensitive area of ≈90mm x 90mm is built up by four back-side illuminated CMOS sensors, which image a field of view of ≈200 deg2. Each sensor has 22:4 megapixels. The Schmidt design of the telescope locates the detector inside the optical path, limiting the overall size of the assembly. As a result, the readout electronics is located in a remote unit outside the telescope. The short focal length of the telescope requires an accurate positioning of the sensors within ±50 μm along the optical axis, with a flatness of ±10 μm. While the telescope will be at around 295K during operations, the sensors are required to be cooled to 200K for dark current reduction. At the same time, the ability to heat the sensors to 343K is required for decontamination. In this paper, we present the preliminary design of the UV sensitive ULTRASAT camera.
The Ultraviolet Transient Astronomical Satellite (ULTRASAT) is a scientific space mission carrying an astronomical telescope. The mission is led by the Weizmann Institute of Science (WIS) in Israel and the Israel Space Agency (ISA), while the camera in the focal plane is designed and built by Deutsches Elektronen Synchrotron (DESY) in Germany. Two key science goals of the mission are the detection of counterparts to gravitational wave sources and supernovae.1 The launch to geostationary orbit is planned for 2024. The telescope with a field-of-view of ≈ 200 deg2, is optimized to work in the near-ultraviolet (NUV) band between 220 and 280 nm. The focal plane array is composed of four 22:4-megapixel, backside-illuminated (BSI) CMOS sensors with a total active area of 90 x 90mm2.2 Prior to sensor production, smaller test sensors have been tested to support critical design decisions for the final flight sensor. These test sensors share the design of epitaxial layer and antireflective coatings with the flight sensors. Here, we present a characterization of these test sensors. Dark current and read noise are characterized as a function of the device temperature. A temperature-independent noise level is attributed to on-die infrared emission and the read-out electronics' self-heating. We utilize a high-precision photometric calibration setup3 to obtain the test sensors' quantum efficiency relative to PTB/NIST-calibrated transfer standards (220-1100 nm), the quantum yield for λ >300 nm, the non-linearity of the system, and the conversion gain. The uncertainties are discussed in the context of the newest results on the setup's performance parameters. From the three ARC options Tstd, T1 and T2, the last assists the out-of-band rejection and peaks in the mid of the ULTRASAT operational waveband. We recommend ARC option T2 for the final ULTRASAT UV sensor.
The SCALA system provides a physical flux calibration for the SuperNova Integral Field Spectrograph (SNIFS) mounted to the University of Hawaii 2.2m telescope on Mauna Kea by transferring the flux scale from a NIST- traceable photodiode to SNIFS. This calibration is then applied to CALSPEC standard stars. We thereby remove stellar atmospheric models from the calibration chain. Measurement results for supernova cosmology are directly improved, as systematic uncertainties in the flux calibration limit them. Using the existing SCALA setup we achieved a calibration that agrees with the CALSPEC and Hayes6 systems to within ~4 mmag / 1000 A over a wavelength range from 4500 A to 9000 A. We are now upgrading the SCALA system to reach measurement uncertainties below 0.5 %. To provide the flux references for the new system and to perform tests of the improved components, we have built a laboratory light source enabling measurements with sub-percent uncertainties. The light source provides monochromatic light (FWHM 1.8/3.6 nm) spanning UV to IR, with wavelength accuracy and reproducibility of ≤ 1A. Neutral density filters enable fluxes that induce photodiode currents between fA and µA. A subsystem allows linearity testing for detectors with their readout system. Using a gantry robot, we can measure our detectors’ spatial response and angular acceptance with active areas up to 0.5 m2.
The SNIFS CALibration Apparatus (SCALA), a device to calibrate the Supernova Integral Field Spectrograph on the University Hawaii 2.2m telescope, was developed and installed in Spring 2014. SCALA produces an artificial planet with a diameter of 1° and a constant surface brightness. The wavelength of the beam can be tuned between 3200 Å and 10000 Å and has a bandwidth of 35 Å. The amount of light injected into the telescope is monitored with NIST calibrated photodiodes. SCALA was upgraded in 2015 with a mask installed at the entrance pupil of the UH88 telescope, ensuring that the illumination of the telescope by stars is similar to that of SCALA. With this setup, a first calibration run was performed in conjunction with the spectrophotometric observations of standard stars. We present first estimates for the expected systematic uncertainties of the in-situ calibration and discuss the results of tests that examine the influence of stray light produced in the optics.
Observational cosmology employing optical surveys often require precise flux calibration. In this context we present SNIFS Calibration Apparatus (SCALA), a flux calibration system developed for the SuperNova Integral Field Spectrograph (SNIFS), operating at the University of Hawaii 2.2 m telescope. SCALA consists of a hexagonal array of 18 small parabolic mirrors distributed over the face of, and feeding parallel light to, the telescope entrance pupil. The mirrors are illuminated by integrating spheres and a wavelength-tunable (from UV to IR) light source, generating light beams with opening angles of 1°. These nearly parallel beams are flat and flux-calibrated at a subpercent level, enabling us to calibrate our “telescope + SNIFS system” at the required precision.
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