Digital micromirror devices (DMDs) can be used as rapidly reconfigurable "slit mask" object-selectors in space- based UV multi-object spectrometers (MOS). There are several missions currently in the planning process, which are developing concepts for multi-object spectrometers. For example, both LUVOIR and HabEx plan to include such an instrument, working into the deep UV. Currently, DMDs are the only alternative technology to microshutter arrays, which were developed for the infrared MOS on the James Webb Space Telescope. However, the deep UV (100 - 300 nm) reflectivity of DMDs needs to be substantially higher for efficient mission operation. We have re-coated commercially available DMDs (which use aluminum alloy mirrors) with high reflectivity aluminum, which is protected from oxidation by a AlF3 overcoat. We found that DMDs remain functional after being re-coated and show a dramatic reflectance improvement in the region of 100 - 300 nm. The scattering properties of re-coated DMDs can be further improved by masking the gaps between individual micromirrors during the coating process.
KEYWORDS: Astronomical imaging, Spectroscopy, Digital micromirror devices, Molybdenum, Micromirrors, Astronomy, Light scattering, Spectral resolution, Signal to noise ratio
Multi-object spectrometers (MOSs) are astronomical instruments capable of accurately acquiring spectra of up to several hundreds of objects of interest in a single exposure. Digital micromirror devices (DMDs) have proven to be an excellent candidate for use as slit masks in both terrestrial and space-based MOSs because they are highly reliable and rapidly re-configurable. The Rochester Institute of Technology Multi-Object Spectrometer (RITMOS) is a terrestrial DMD-based MOS, which uses a newer generation DMD, with improved scattered light characteristics. RITMOS utilizes a 0.700 XGA DMD with a micromirror pitch of 13.68 microns and a micromirror flip angle of 12 degrees. By design, RITMOS covers the spectral range 3900 - 4900 angstroms, with a dispersion of 0.7 angstroms per pixel; the resolving power is R∼5300. Performance evaluation has been conducted both in the laboratory and on-sky. The results presented here show that DMD-based MOSs are highly capable instruments, offering great observational flexibility, while achieving excellent signal-to-noise ratios by optimally rejecting the sky background.
The astronomical community continues to be interested in suitable programmable slit masks for use in multiobject spectrometers (MOSs) on space missions. There have been ground-based MOS utilizing digital micromirror devices (DMDs), and they have proven to be highly accurate and reliable instruments. This paper summarizes the results of a continuing study to investigate the performance of DMDs under conditions associated with space deployment. This includes the response of DMDs to accelerated heavy-ion radiation, to the vibration and mechanical shock loads associated with launch, and the operability of DMD under cryogenic temperatures. The optical contrast ratio and a study of the long-term reflectance of a bare device have also been investigated. The results of the radiation testing demonstrate that DMDs in orbit would experience negligible heavy-ion-induced single event upset (SEU) rate burden; we predict an SEU rate of 5.6 micromirrors/24 h. Vibration and mechanical shock testing was performed according to the NASA General Environmental Verification Standard; there were no failed mirrors in the devices tested. The results of low temperature testing suggest that DMDs are not affected by the thermal load and operate smoothly at temperatures at least as low as 78 K. The reflectivity of a bare DMD did not measurably change even after being exposed to ambient conditions over a period of 13 months even. The measured contrast ratio (“on state” versus “off state” of the DMD micromirrors) was greater than 6000∶1 when illuminated with an f/4 optical beam. Overall DMDs are extremely robust and promise to provide a reliable alternative to microshutter arrays to be used in space as remotely programmable slit masks for MOS design.
There is a pressing need in the astronomical community for space-suitable multiobject spectrometers (MOSs). Several digital micromirror device (DMD)-based prototype MOSs have been developed for ground-based observatories; however, their main use will come with deployment on a space-based mission. Therefore, the performance of DMDs under exoatmospheric radiation needs to be evaluated. DMDs were rewindowed with 2-μm thick pellicle and tested under accelerated heavy-ion radiation (control electronics shielded from radiation), with a focus on the detection of single-event effects (SEEs) including latch-up events. Testing showed that while DMDs are sensitive to nondestructive ion-induced state changes, all SEEs are cleared with a soft reset (i.e., sending a pattern to the device). The DMDs did not experience single-event induced permanent damage or functional changes that required a hard reset (power cycle), even at high ion fluences. This suggests that the SSE rate burden will be manageable for a DMD-based instrument when exposed to solar particle fluxes and cosmic rays in orbit.
Digital micromirror devices (DMDs) are a mature commercial technology, with several potential applications in space-based instruments. In particular, DMDs are currently the only practical alternative to microshutter arrays as slit mask generators for space-based multi-object spectrometers (MOS). A DMD is an array of micromirrors which can be addressed individually and tilted into one of two states (+/- 12 w.r.t. the device plane), which makes it a very versatile binary light modulator. These devices are widely utilized in a variety of optical systems, especially projectors. Recently, the use of DMDs for ground-based multi-object spectrometers has been demonstrated. The compact size and small weight of DMDs makes them especially attractive for a space- based MOS, where the only current alternative is an array of microshutters. DMDs were originally designed for visible range applications; therefore the protective glass window they are supplied with does not have sufficient throughput in the UV or IR and has to be replaced. In this work, we describe the procedure by which we replaced the standard window with UV-grade fused silica, sapphire and magnesium fluoride. We performed initial shock and vibrational tests to evaluate the mechanical robustness of the re-windowed devices, to investigate the ability of these devices to survive launch conditions. We performed residual gas analysis to study the outgassing properties of the new DMDs and evaluate the ability of the new seals to protect the device. The tested devices show near-hermetic seals before and after the mechanical testing.
Digital micromirror devices (DMDs) are commercial micro-electromechanical systems, consisting of millions of mirrors which can be individually addressed and tilted into one of two states (±12°). These devices were developed to create binary patterns in video projectors, in the visible range. Commercially available DMDs are hermetically sealed and extremely reliable. Recently, DMDs have been identified as an alternative to microshutter arrays for space-based multi-object spectrometers (MOS). Specifically, the MOS at the heart of the proposed Galactic Evolution Spectroscopic Explorer (GESE) uses the DMD as a reprogrammable slit mask. Unfortunately, the protective borosilicate windows limit the use of DMDs in the UV and IR regimes, where the glass has insufficient throughput. In this work, we present our efforts to replace standard DMD windows with custom windows made from UV-grade fused silica, low-absorption optical sapphire (LAOS) and magnesium fluoride (MgF2). We present transmission measurements of the antireflection coated windows and the reflectance of bare (window removed) DMDs. Furthermore, we investigated the long-term stability of the DMD reflectance and experiments for coating DMD active area with a layer of pure aluminum (Al) to boost reflectance performance in the UV spectral range (200−400 nm).
There is a pressing need in the astronomical community for space-suitable multi-object spectrometers (MOSs). Several digital micromirror device (DMD)-based prototype MOSs have been developed for ground-based observatories; however, their main use will come with deployment on a space based mission. Therefore, performance of DMDs under exoatmospheric radiation needs to be evaluated. In our previous work we demonstrated that DMDs are tolerant to heavy ion irradiation in general and calculated upset rate of 4.3 micromirrors in 24 hours in orbit for 1-megapixel device. The goal of this additional experiment was to acquire more data and therefore increase the accuracy of the predicted in-orbit micromirror upset rate. Similar to the previous experiment, for this testing 0.7 XGA DMDs were re-windowed with 2 μm thick pellicle and tested under accelerated heavy-ion radiation (with control electronics shielded from radiation) with a focus on detection of single-event upsets (SEUs). We concentrated on ions with low levels of linear energy transfer (LET) 1.8 – 13 MeV•cm2•mg-1 to cover the most critical range of the Weibull curve for those devices. As during the previous experiment, we observed and documented non-destructive heavy ion-induced micromirror state changes. All SEUs were always cleared with a soft reset (that is, sending a new pattern to the device). The DMDs we tested did not experience single-event induced permanent damage or functional changes that required a hard reset (power cycle), even at high ion fluences. Based on the data obtained in the experiments we predict micromirror in-orbit upset rate of 5.6 micromirrors in 24 hours in-orbit for the tested devices. This suggests that the heavy-ion induced SEU rate burden for a DMD-based instrument will be manageable when exposed to solar particle fluxes and cosmic rays in orbit.
KEYWORDS: Digital micromirror devices, Astronomical imaging, Scattering, Spectroscopy, Mirrors, Light scattering, Space telescopes, Signal to noise ratio, Molybdenum, Spectrographs
Digital micromirror devices (DMDs) are micro-electro- mechanical systems, originally developed to display images in projector systems. A DMD in the focal plane of an imaging system can be used as a reprogrammable slit mask of a multi-object spectrometer (MOS) by tilting some of the mirrors towards the spectrometer and tilting the rest of the mirrors away, thereby rejecting the unwanted light (due to the background and foreground objects). A DMD-based MOS can generate new, arbitrary slit patterns in seconds, which significantly reduces the overhead time during astronomical observations. Critically, DMD-based slit masks are extremely lightweight, compact and mechanically robust, which makes them attractive for use in space-based telescopes. As part of a larger effort to investigate the use of DMDs in space telescopes (sponsored by a NASA Strategic Astrophysics Technologies grant), we characterized the optical performance of Texas Instruments DMDs to determine their suitability for use in multi-object spectrometers. The performance of a DMD-based MOS is significantly affected by its optical throughput (reflectance), contrast ratio (the ability of the DMD to reject unwanted light) and scattering properties (which could lead to crosstalk and reduced signal-to-noise ratio in the spectrometer). We measured and quantified the throughput and contrast ratio of a Texas Instruments DMD in several configurations (which emulate the operation of a typical DMD-based MOS) and investigated the scattering properties of the individual DMD mirrors. In this work we present the results of our analysis, describe the performance of a typical DMD- based MOS and discuss the practical limitations of these instruments (such as maximum density of sources and expected signal-to- noise ratio).
There is a need for a space-suitable solution to the selection of targets to be observed in astronomical multiobject spectrometers (MOS). A few digital micromirror device (DMD) - based prototype MOS have been developed for use at ground observatories, However their main use will come in deploying a space based mission. The question of DMD performance under in-orbit radiation remains unanswered. DMDs were tested under accelerated heavy-ion radiation (with the control electronics shielded from radiation), with a focus on detection of single-event effects (SEEs) including latch-up events. Testing showed that DMDs are sensitive to non-destructive ion-induced state changes; however, all SEEs were cleared with a soft reset (that is, sending a new pattern to the device). The DMDs did not experience single-event induced permanent damage or functional changes that required a hard reset (power cycle), even at high ion fluences. This suggests that the SSE rate burden will be manageable for a DMD-based instrument when exposed to solar particle fluxes and cosmic rays on orbit.
KEYWORDS: Wavefronts, Wavefront sensors, Binary data, Sensors, Fourier transforms, Modulation, Photomasks, Near field optics, Reconstruction algorithms, Signal to noise ratio
High-resolution wavefront sensors are used in a wide range of applications. The Shack-Hartmann sensor is the industry standard and mostly used for this kind of analysis. However, with this sensor the analysis can only be performed for narrowband radiation, the recoverable curvature of the wavefront slopes is also restricted by the size of a single lens in the microlens array. The high-resolution Shack Hartmann wavefront sensor (>128×128) is also significantly expensive. The optical differentiation wavefront sensor, on the other hand, consists of only simple and therefore inexpensive components, offers greater signal to noise ratio, allows for high-resolution analysis of wavefront curvature, and is potentially capable of performing broadband measurements. When a transmission mask with linear attenuation along a spatial direction modulates the far field of an optical wave, the spatial wavefront slope along that direction can be recovered from the fluence in the near field after modulation. With two orthogonal measurements one can recover the complete wavefront of the optical wave. In this study the characteristics of such a wavefront sensor are investigated when the linear transmission modulation is implemented with a pixelated binary filter. Such a filter can be produced as a gray-scale quasi-continuous transmission pattern constructed using arrays of small (e.g., 10-micron) transparent or opaque pixels and therefore it can simply be fabricated by conventional lithography techniques. Simulations demonstrate the potential ability of such a pixelated filter to match the performance of a filter with continuously varying transmission, while offering the advantage of better transmission control and reduction of fabrication costs.
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