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.
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.
KEYWORDS: Ultraviolet radiation, Astronomy, Earth's atmosphere, Atmospheric monitoring, Absorption, Near ultraviolet, Ozone, Solar processes, Signal to noise ratio, Wind energy
The ozone layer has a complex spectral absorption profile at NUV wavelengths. It is dependent on seasonal effects due to solar intensity, as well as atmospheric circulation of the ozone layer. Getting above this then becomes imperative for getting a usable SNR for scientific observations. GLUV is an affordable, long duration, high altitude balloon experiment which will fly a network of NUV telescopes at altitudes of 20-30 km. GLUV Pathfinder is a spectrometer based system to identify the sky background in the NUV, measuring this as a function of altitude, latitude, and seasonal phase in the regimes that the final GLUV project will experience. The development of dedicated NUV instrumentation is highly important for supernovae astronomy, as these higher energy wavelengths reveal their initial detonation conditions. GLUV is expected to capture the initial shocks of these events at a rate of 10+ per year of operation, well in excess of the few instances that have been seen to date
SING is a near ultraviolet (NUV) spectrograph with a size close to 6U CubeSat form-factor. The spectrograph operates in the wavelength range from 1800 Å to 3000 Å, with a spectral resolution of 2 Å at the central wavelength. The spectrograph is intended to operate in low Earth orbit (LEO), with the primary goal to generate spectral maps of the regions of the sky covered within the field of view (FOV) of the instrument, constrained by the orbital inclination of the spacecraft. The payload consists of the telescope in Cassegrain configuration focusing the light on the slit, the corrector lens, the concave grating, the detector, and other required electronics components. As the event rate in the UV is low, the spectrograph employs a photon-counting detector because of its low noise performance. The payload uses FPGA as the main system controller to handle the detector readout, data compression and storage, health monitoring, and telemetry. In this work, we present the optical design and its analysis, along with a brief description of the architecture of electronic subsystems of the payload.
GLUV is a balloon-based near UV survey telescope under development. The primary objective of GLUV is high- cadence observations of transient events such as early UV observations of supernova. The ozone layer absorbs a considerable amount of UV radiation, so it is important for a UV balloon telescope to achieve ight altitude above the ozone layer. The ozone layer density varies with respect to latitude, altitude and season. The structure and behaviour of ozone distribution is an important factor to be studied to account for astronomical ultraviolet observations. GLUV pathfinder is a spectrometer based system to identify the sky background at intended ight altitudes and atmosphere transmission GLUV will experience. In this presentation, we will present the design and development of the GLUV pathfinder system.
The Pesit/IIA Observatory for the Night Sky(PIONS) is a near UV imaging telescope to be flown on a small satellite. The instrument is a 150mm RC telescope that covers a wavelength range of 180-280 nm. We are using an intensified CMOS detector with a solar blind photocathode, to be operated in photon counting mode. The telescope has a wide field of view of 3 degrees and an angular resolution of 13”. We plan to point the telescope to scan the sky continuously along the sun pointing axis to look for variable UV sources such as flare stars, AGNs, and other transient events. We can detect objects as faint as 21 magnitude and perform their photometric analysis. Since the aperture and the effective area of the telescope are comparatively small, it can be pointed to relatively brighter parts of the UV sky which were not accessible to larger mission due to detector limitations.
The Lunar Ultraviolet Cosmic Imager (LUCI) is an innovative all-spherical mirrors telescope, proposed to fly as a scientific UV imaging payload on a lunar mission in collaboration with Indian Aerospace Company-TeamIndus, Axiom Research Labs Pvt. Ltd. Observations from the Moon provide a unique opportunity to observe the sky from a stable platform far above the Earths atmosphere. LUCI will observe at a fixed elevation angle and will detect stars in the near ultraviolet (200-320 nm) to a limiting magnitude of 12 AB, with a field of view of around 0.5 degrees. The primary science goal is to search for transient sources and flag them for further study. The instrument has been assembled in the class 1000 clean room at the M.G.K Menon Laboratory for Space Sciences. Here we will describe the optomechanical assembly procedures we have carried out during the optical alignment and integration of the payload. Opto-mechanical alignment of the instrument was carried out by using alignment telescope cum autocollimator (for coarse alignment) and ZYGO interferometer (fine alignment). We will also discuss the ground calibration tests performed on the assembled telescope. The results from the ground calibration activities will help in establishing the full calibration matrix of the instrument once operational.
Spatial Heterodyne Spectroscopy (SHS) is a relatively novel interferometric technique similar to Fourier transform spectroscopy and shares design similarities with a Michelson Interferometer. An Imaging detector is used at the output of a SHS to record the spatially heterodyned interference pattern. The spectrum of the source is obtained by Fourier transforming the recorded interferogram. The merits of the SHS -its design, including the lack of moving parts, compactness, high throughput, high SNR and instantaneous spectral measurements - makes it suitable for space as well as ground observatories. The small bandwidth limitation of the SHS can be overcome by building it in tunable configuration (Tunable Spatial Heterodyne Spectrometer(TSHS)). In this paper, we describe the wavelength calibration of the tunable SHS using a Halogen lamp and Andor monochromator setup. We found a relation between the fringe frequency and the wavelength.
Spatial heterodyne spectroscopy (SHS) is an interferometric technique similar to the Fourier transform spectroscopy with heritage from the Michelson interferometer. An imaging detector is used at the output of an SHS to record the spatially heterodyned interference pattern. The spectrum of the source is obtained by Fourier transforming the recorded interferogram. The merits of the SHS—its design, including the absence of moving parts, compactness, high throughput, high SNR, and instantaneous spectral measurements—make it suitable for space as well as for ground observatories. The small bandwidth limitation of the SHS can be overcome by building it in tunable configuration [tunable spatial heterodyne spectrometer (TSHS)]. We describe the design, development, and simulation of a TSHS in refractive configuration suitable for optical wavelength regime. Here we use a beam splitter to split the incoming light compared with all-reflective SHS where a reflective grating does the beam splitting. Hence, the alignment of this instrument is simple compared with all-reflective SHS where a fold mirror and a roof mirror are used to combine the beam. This instrument is intended to study faint diffuse extended celestial objects with a resolving power above 20,000 and can cover a wavelength range from 350 to 700 nm by tuning. It is compact and rugged compared with other instruments having similar configurations.
We describe the development and implementation of a light-weight, fully autonomous 2-axis pointing and stabilization system designed for balloon-borne astronomical payloads. The system is developed using off-the-shelf components such as Arduino Uno controller, HMC 5883L magnetometer, MPU-9150 inertial measurement unit, and iWave GPS receiver unit. It is a compact and rugged system which can also be used to take images/video in a moving vehicle or in real photography. The system performance is evaluated from the ground, as well as in conditions simulated to imitate the actual flight by using a tethered launch.
The ultraviolet (UV) window has been largely unexplored through balloons for astronomy. We discuss here the development of a compact near-UV spectrograph with fiber optics input for balloon flights. It is a modified Czerny-Turner system built using off-the-shelf components. The system is portable and scalable to different telescopes. The use of reflecting optics reduces the transmission loss in the UV. It employs an image-intensified CMOS sensor, operating in photon counting mode, as the detector of choice. A lightweight pointing system developed for stable pointing to observe astronomical sources is also discussed, together with the methods to improve its accuracy, e.g. using the in-house build star sensor and others. Our primary scientific objectives include the observation of bright Solar System objects such as visible to eye comets, Moon and planets. Studies of planets can give us valuable information about the planetary aurorae, helping to model and compare atmospheres of other planets and the Earth. The other major objective is to look at the diffuse UV atmospheric emission features (airglow lines), and at column densities of trace gases. This UV window includes several lines important to atmospheric chemistry, e.g. SO2, O3, HCHO, BrO. The spectrograph enables simultaneous measurement of various trace gases, as well as provides better accuracy at higher altitudes compared to electromechanical trace gas measurement sondes. These lines contaminate most astronomical observations but are poorly characterized. Other objectives may include sprites in the atmosphere and meteor ashes from high altitude burn-outs. Our recent experiments and observations with high-altitude balloons are discussed.
We describe the characterization and removal of noises present in the Inertial Measurement Unit (IMU) MPU- 6050, which was initially used in an attitude sensor, and later used in the development of a pointing system for small balloon-borne astronomical payloads. We found that the performance of the IMU degraded with time because of the accumulation of different errors. Using Allan variance analysis method, we identified the different components of noise present in the IMU, and verified the results by the power spectral density analysis (PSD). We tried to remove the high-frequency noise using smooth filters such as moving average filter and then Savitzky Golay (SG) filter. Even though we managed to filter some high-frequency noise, these filters performance wasn't satisfactory for our application. We found the distribution of the random noise present in IMU using probability density analysis and identified that the noise in our IMU was white Gaussian in nature. Hence, we used a Kalman filter to remove the noise and which gave us good performance real time.
We are developing a compact UV Imager using light weight components, that can be own on a small CubeSat or a balloon platform. The system has a lens-based optics that can provide an aberration-free image over a wide field of view. The backend instrument is a photon counting detector with off-the-shelf MCP, CMOS sensor and electronics. We are using a Z-stack MCP with a compact high voltage power supply and a phosphor screen anode, which is read out by a CMOS sensor and the associated electronics. The instrument can be used to observe solar system objects and detect bright transients from the upper atmosphere with the help of CubeSats or high altitude balloons. We have designed the imager to be capable of working in direct frame transfer mode as well in the photon-counting mode for single photon event detection. The identification and centroiding of each photon event are done using an FPGA-based data acquisition and real-time processing system.
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