The warm-hot phase coronal gas, known as the circumgalactic matter (CGM), around galaxy halos plays a critical role in the evolution of galaxies. However, the morphology of the CGM is poorly understood as it is difficult to detect the extreme-UV (EUV) emission from this diffuse gas. Aspera, a small satellite telescope, is designed to map the EUV emission from the CGM of nearby galaxies. In order to achieve high observation efficiency, the payload has a unique dual-channel optical layout sharing a Microchannel plate (MCP) detector. The spectroscopy layout benchmarked the Rowland Circle ( OAP + slit + curved diffractive optics) and adopted a special coating to enhance reflectivity at EUV (~103.2 nm). The unique dual-channel layout requires co-pointing alignment of both channels and tight tolerance of each channel alignment. We reviewed the tolerance and sensitivity analyses of the optical system. The comprehensive Monte Carlo simulation provides the required alignment accuracy, and we proposed appropriate alignment plans using the interferometer and computer-generated hologram (CGH). The proposed alignment strategy is available for quantitative and decomposed evaluation of misalignin
Aspera is the UV small-satellite mission to detect and map the warm-hot phase gas in nearby galaxy halo. Aspera was chosen as one of NASA's Astrophysics Pioneers missions in 2021 and employs a FUV long-slit spectrograph payload, optimized for low-surface brightness O VI emission line detection at 103-104 nm. The mission incorporates state-of-the-art UV technologies such as high-efficiency micro-channel plates and enhanced LiF coating to achieve a high level of diffuse-source sensitivity of the payload, down to 5.0E-19 erg/s/cm^2/arcsec^2. The combination of the high sensitivity and a 1-degree by 30-arcsecond long-slit field of view enables efficient 2D mapping of diffuse halo gas through step and stare concept observation. Aspera is presently in the critical design phase, with an expected launch date in mid-2025. This work provides a current overview of the Aspera payload design.
The integration of a new calibration system into FIREBall-2 (Faint Intergalactic Redshifted Emission Balloon-2) allows in-flight calibration capability for the upcoming Fall 2023 flight. This system is made up of a calibration box that contains zinc and deuterium lamp sources, focusing optics, electronics, and sensors, and a fiber-fed calibration cap with an optical shutter mounted on the spectrograph tank. We discuss how the calibration cap is optimized to be evenly illuminated through nonsequential modeling for the near-UV (200-208nm). Then, we present the pre-flight performance testing results of the calibration system and their implications for in-flight measurements.
We present Hyperion, a mission concept recently proposed to the December 2021 NASA Medium Explorer announcement of opportunity. Hyperion explores the formation and destruction of molecular clouds and planet-forming disks in nearby star-forming regions of the Milky Way. It does this using long-slit high-resolution spectroscopy of emission from fluorescing molecular hydrogen, which is a powerful far-ultraviolet (FUV) diagnostic. Molecular hydrogen (H2) is the most abundant molecule in the universe and a key ingredient for star and planet formation but is typically not observed directly because its symmetric atomic structure and lack of a dipole moment mean there are no spectral lines at visible wavelengths and few in the infrared. Hyperion uses molecular hydrogen’s wealth of FUV emission lines to achieve three science objectives: (1) determining how star formation is related to molecular hydrogen formation and destruction at the boundaries of molecular clouds, (2) determining how quickly and by what process massive star feedback disperses molecular clouds, and (3) determining the mechanism driving the evolution of planet-forming disks around young solar-analog stars. Hyperion conducts this science using a straightforward, highly efficient, single-channel instrument design. Hyperion’s instrument consists of a 48-cm primary mirror with an f/5 focal ratio. The spectrometer has two modes, both covering 138.5- to 161.5-nm bandpasses. A low resolution mode has a spectral resolution of R ≥ 10,000 with a slit length of 65 arcmin, whereas the high-resolution mode has a spectral resolution of R ≥ 50,000 over a slit length of 5 armin. Hyperion occupies a 2-week-long high-earth lunar resonance TESS-like orbit and conducts 2 weeks of planned observations per orbit, with time for downlinks and calibrations. Hyperion was reviewed as category I, which is the highest rating possible but was not selected.
We present a comprehensive stray light analysis and mitigation strategy for the FIREBall-2 ultraviolet balloon telescope. Using nonsequential optical modeling, we identified the most problematic stray light paths, which impacted telescope performance during the 2018 flight campaign. After confirming the correspondence between the simulation results and postflight calibration measurements of stray light contributions, a system of baffles was designed to minimize stray light contamination. The baffles were fabricated and coated to maximize stray light collection ability. Once completed, the baffles will be integrated into FIREBall-2 and tested for performance preceding the upcoming flight campaign. Given our analysis results, we anticipate a substantial reduction in the effects of stray light.
This conference presentation was prepared for the Space Telescopes and Instrumentation 2022: Ultraviolet to Gamma Ray conference at SPIE Astronomical Telescopes and Instrumentation, 2022.
Aspera is an extreme-UV (EUV) Astrophysics small satellite telescope designed to map the warm-hot phase coronal gas around nearby galaxy halos. Theory suggests that this gas is a significant fraction of a galaxy’s halo mass and plays a critical role in its evolution, but its exact role is poorly understood. Aspera observes this warm-hot phase gas via Ovi emission at 1032 °A using four parallel Rowland-Circle-like spectrograph channels in a single payload. Aspera’s robust-and-simple design is inspired by the FUSE spectrograph, but with smaller, four 6.2 cm × 3.7 cm, off-axis parabolic primary mirrors. Aspera is expected to achieve a sensitivity of 4.3×10−19 erg/s/cm2/arcsec2 for diffuse Ovi line emission. This superb sensitivity is enabled by technological advancements over the last decade in UV coatings, gratings, and detectors. Here we present the overall payload design of the Aspera telescope and its expected performance. Aspera is funded by the inaugural 2020 NASA Astrophysics Pioneers program, with a projected launch in late 2024.
Molecular clouds are a crucial stage in the lifecycle of a star, and the far ultraviolet (FUV) spectral range is a prime observation band. Hyperion is an FUV space telescope that investigates the birth clouds of stars using a high-resolution spectrometer. To meet the scientific requirements, we developed and evaluated a spectrometer that covers the 140.5 to 164.5 nm wavelength range with a spectral resolution higher than 30,000. We employed on-axis and on-plane dispersive optic layouts to control the aberration from a large aspect ratio slit (10 arcmin × 2.5 arcsec, aspect ratio R = 240). The cross-dispersion isolates three orders from the échelle grating (n = − 19, −18, and −17), and the subsequent two-mirror freeform imaging optics form a two-dimensional spectral distribution on a 50 mm × 50 mm detector array. The geometrical and spectral performances of this innovative design are evaluated.
The IUCAA digital sampling array controller (IDSAC) is a flexible and generic yet powerful CCD controller that can handle a wide range of scientific detectors. Based on an easily scalable modular backplane architecture consisting of single board controllers (SBC), IDSAC can control large detector arrays and mosaics. Each of the SBCs offers the full functionality required to control a CCD independently. The SBCs can be cold swapped without the need to reconfigure them. IDSAC is also available in a backplane-less architecture. Each SBC can handle data from up to four video channels with or without dummy outputs at speeds up to 500-kilo pixels per second (kPPS) per channel with a resolution of 16 bits. Communication with a Linux-based host computer is through a USB3.0 interface, with the option of using copper or optical fibers. A field programmable gate array (FPGA) is used as the master controller in each SBC, which allows great flexibility in optimizing performance by adjusting gain, timing signals, bias levels, etc., using user-editable configuration files without altering the circuit topology. Elimination of thermal kTC noise is achieved via digital correlated double sampling (DCDS). The number of digital samples per pixel (for both reset and signal levels) is user configurable. We present the results of noise performance characterization of IDSAC through simulation, theoretical modeling, and actual measurements. The contribution of different types of noise sources is modeled using a tool to predict noise of a generic DCDS signal chain analytically. The analytical model predicts the net input referenced noise of the signal chain to be 5 electrons for 200-k pixels/s per channel readout rate with three samples per pixel. Using a cryogenic test setup in the lab, the noise is measured to be 5.4 e (24.3 μV), for the same readout configuration. With a better-optimized configuration of 500-kPPS readout rate, the measured noise is down to 3.8 electrons RMS (17 μV), with three samples per interval.
A new field re-configuration technique, Multiple Rooks of Chess, for multiple deployable Integral Field Spectrographs has been developed. The method involves a mechanical geometry as well as an optimized deployment algorithm. The geometry is found to be simple for mechanical implementation. The algorithm initially assigns the IFUs to the target objects and then devises the movement sequence based on the current and the desired IFU positions. The reconfiguration time using the suitable actuators which runs at 20 cm/s is found to be a maximum of 25 seconds for the circular DOTIFS focal plane (180 mm diameter). It is similar to some of the fastest schemes currently available. The Geometry Algorithm Combination (GAC) has been tested on several million mock target configurations with object-to-IFU ( τ ) ratio varying from 0.25 to 16. The configuration had both contiguous and sparse distribution of targets. The MRC method is found to be extremely efficient in target acquisition in terms of field revisit and deployment time without any collision or entanglement of the fiber bundles. The efficiency of the technique does not get affected by the increase of number density of target objects. For field with τ >1 prioritization of target objects is an optional feature and not necessary. The GAC can be modified for an instrument with higher or lower number of IFUs and different field size without any significant change in the flow. The technique is compared with other available methods based on sky coverage, flexibility and overhead time. The proposed geometry and algorithm combination is found to have advantage in all of the aspects.
Long-slit astronomical spectroscopy has various limitations when dealing with optimum slit width, atmospheric dispersion, extended source spectroscopy, etc. to name a few. Most of these issues can be solved by the use of optical fibers as the light carrier from the telescope focal plane to the spectrograph. The approach is technically and scientifically flexible in terms of instrument modularity and target acquisition. Implementation of Integral Field Unit (IFU) provides a continuous sampling of extended objects and has a distinct advantage over the single fiber. Using a microlens array in front of the fibers improves the sky coverage by increasing the fill factor. Devasthal Optical Telescope Integral Field Spectrograph (DOTIFS) is a novel instrument being built by the Inter-University Centre for Astronomy and Astrophysics, Pune for the 3.6m Devasthal Optical Telescope (DOT) constructed by Aryabhatta Research Institute of Observational Sciences, Nainital. Each of the 16 DOTIFS IFUs consist of 12x12 spatial elements (spaxels) distributed in a hexagonal honeycomb structure covering 8.7"x7.8" in the sky. Each IFU is made by a photolithography technique to transfer the corresponding microlens array pattern to create a mask which holds the fibers at the focal plane end of an integral field unit. These masks are aligned with the microlens array and fibers are inserted before gluing and polishing. The fiber array can be positioned with a peak positioning error less than 5 μm from the desired position within a fiber array, compared to a requirement of 10 μm. The slit end is made by wire EDM cutting technology and fibers are placed with an accuracy of ~0.3 pixels compared to a 6.75 pixel center-to-center gap between two spectra on the detector. In this paper we provide details of deriving requirements and error budgets. The process of photolithography and the use of generated masks to create an IFU are also discussed. The technique allows very cost effective mass production of IFUs which are very accurately matched with the corresponding microlens array.
We present fore-optics and calibration unit design of Devasthal Optical Telescope Integral Field Spectrograph (DOTIFS). DOTIFS fore-optics is designed to modify the focal ratio of the light and to match its plate scale to the physical size of Integral Field Units (IFUs). The fore-optics also delivers a telecentric beam to the IFUs on the telescope focal plane. There is a calibration unit part of which is combined with the fore-optics to have a light and compact system. We use Xenon-arc lamp as a continuum source and Krypton/Mercury-Neon lamps as wavelength calibration sources. Fore-optics and calibration unit shares two optical lenses to maintain compactness of the overall subsystem. Here we present optical and opto-mechanical design of the calibration unit and fore-optics as well as calibration scheme of DOTIFS.
Devasthal Optical Telescope Integral Field Spectrograph (DOTIFS) is a new multi-Integral Field Unit (IFU) instrument, planned to be mounted on the 3.6m Devasthal optical telescope in Nainital, India. It has eight identical, fiber-fed spectrographs to disperse light coming from 16 IFUs. The spectrographs produce 2,304 spectra over a 370-740nm wavelength range simultaneously with a spectral resolution of R=1200-2400. It is composed of all-refractive, allspherical optics designed to achieve on average 26.0% throughput from the telescope to the CCD with the help of high transmission spectrograph optics, volume phase holographic grating, and graded coated e2v 2K by 4K CCD. We present the optical and opto-mechanical design of the spectrograph as well as current development status. Optics and optomechanical components for the spectrographs are being fabricated.
Devasthal Optical Telescope Integral Field Spectrograph (DOTIFS) is a new multi-object Integral Field Spectrograph
(IFS) being designed and fabricated by the Inter-University Center for Astronomy and Astrophysics (IUCAA), Pune,
India, for the Cassegrain side port of the 3.6m Devasthal Optical Telescope, (DOT) being constructed by the Aryabhatta
Research Institute of Observational Sciences (ARIES), Nainital. It is mainly designed to study the physics and
kinematics of the ionized gas, star formation and H II regions in the nearby galaxies. It is a novel instrument in terms of
multi-IFU, built in deployment system, and high throughput. It consists of one magnifier, 16 integral field units (IFUs),
and 8 spectrographs. Each IFU is comprised of a microlens array and optical fibers and has 7.4” x 8.7” field of view with
144 spaxel elements, each sampling 0.8” hexagonal aperture. The IFUs can be distributed on the telescope side port over
an 8’ diameter focal plane by the deployment system. Optical fibers deliver light from the IFUs to the spectrographs.
Eight identical, all refractive, dedicated spectrographs will produce 2,304 R~1800 spectra over 370-740nm wavelength
range with a single exposure. Volume Phase Holographic gratings are chosen to make smaller optics and get high
throughput. The total throughput of the instrument including the telescope is predicted as 27.5% on average. Observing
techniques, data simulator and reduction software are also under development. Currently, conceptual and baseline design
review has been done. Some of the components have already been procured. The instrument is expected to see its first
light in 2016.
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