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Your search keyword '"Hoffmann, William A."' showing total 321 results
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321 results on '"Hoffmann, William A."'

1. Evaluating the GeoSnap 13-$\mu$m Cut-Off HgCdTe Detector for mid-IR ground-based astronomy

Author

Leisenring, Jarron M., Atkinson, Dani, Bowens, Rory, Douence, Vincent, Hoffmann, William F., Meyer, Michael R., Auyeung, John, Beletic, James, Cabrera, Mario S., Greenbaum, Alexandra Z., Hinz, Phil, Ives, Derek, Forrest, William J., McMurtry, Craig W., Pipher, Judith L., and Viges, Eric

Subjects
Astrophysics - Instrumentation and Methods for Astrophysics, Physics - Instrumentation and Detectors
Abstract

New mid-infrared HgCdTe (MCT) detector arrays developed in collaboration with Teledyne Imaging Sensors (TIS) have paved the way for improved 10-$\mu$m sensors for space- and ground-based observatories. Building on the successful development of longwave HAWAII-2RGs for space missions such as NEO Surveyor, we characterize the first 13-$\mu$m GeoSnap detector manufactured to overcome the challenges of high background rates inherent in ground-based mid-IR astronomy. This test device merges the longwave HgCdTe photosensitive material with Teledyne's 2048x2048 GeoSnap-18 (18-$\mu$m pixel) focal plane module, which is equipped with a capacitive transimpedance amplifier (CTIA) readout circuit paired with an onboard 14-bit analog-to-digital converter (ADC). The final assembly yields a mid-IR detector with high QE, fast readout (>85 Hz), large well depth (>1.2 million electrons), and linear readout. Longwave GeoSnap arrays would ideally be deployed on existing ground-based telescopes as well as the next generation of extremely large telescopes. While employing advanced adaptive optics (AO) along with state-of-the-art diffraction suppression techniques, instruments utilizing these detectors could attain background- and diffraction-limited imaging at inner working angles <10 $\lambda/D$, providing improved contrast-limited performance compared to JWST MIRI while operating at comparable wavelengths. We describe the performance characteristics of the 13-$\mu$m GeoSnap array operating between 38 and 45K, including quantum efficiency, well depth, linearity, gain, dark current, and frequency-dependent (1/f) noise profile., Comment: 17 pages, 17 figures. Published in special edition of Astronomische Nachrichten / Astronomical Notes as a contribution to SDW2022

Published
2023
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9. Decadal changes in fire frequencies shift tree communities and functional traits

Author

Pellegrini, Adam F. A., Refsland, Tyler, Averill, Colin, Terrer, César, Staver, A. Carla, Brockway, Dale G., Caprio, Anthony, Clatterbuck, Wayne, Coetsee, Corli, Haywood, James D., Hobbie, Sarah E., Hoffmann, William A., Kush, John, Lewis, Tom, Moser, W. Keith, Overby, Steven T., Patterson, III, William A., Peay, Kabir G., Reich, Peter B., Ryan, Casey, Sayer, Mary Anne S., Scharenbroch, Bryant C., Schoennagel, Tania, Smith, Gabriel Reuben, Stephan, Kirsten, Swanston, Chris, Turner, Monica G., Varner, J. Morgan, and Jackson, Robert B.

Published
2021
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12. Stem heating results in hydraulic dysfunction in Symplocos tinctoria: implications for post-fire tree death.

Author

Hoffmann, William A, Sherry, Catherine D K, and Donnelly, Tallis M

Subjects
*HYDRAULIC conductivity, *PLANT-water relationships, *FIELD research, *STOMATA, *CAVITATION, *AIRBORNE lasers
Abstract

Fire-induced heating of stems can impair plant water transport by deforming xylem and increasing vulnerability to cavitation, but it is not clear whether these effects can result in tree death, or how quickly this may occur. In field experiments, we heated stems of Symplocos tinctoria (L.) L'Hér saplings to 90 °C using a thin-film resistive heater, and we monitored stomatal conductance, leaf water potential, sap flow and hydraulic conductivity until stem death. Sap flow and stomatal conductance declined quickly after heating, while whole-plant hydraulic conductance and leaf water potential remained high for the first week. In fact, leaf water potential increased during the first days after heating, indicating that stomatal closure was not initially caused by leaf water deficit induced by impaired water transport. After 1 week, leaf water potential and whole-plant conductance declined below unheated controls, while stomatal conductance and sap flow continued declining, approaching zero after 2 weeks. To better understand the cause of these declines, we directly measured hydraulic conductivity of heated stems. Stems underwent a progressive decline in conductivity after heating, and by the time that samples were severely wilted or desiccated, the heated portion of stems had little or no conductivity. Importantly, conductivity of heated stems was not recovered by flushing stems to remove embolisms, suggesting the existence of physical occlusions. Scanning electron micrographs did not reveal deformed cell walls, nor did it identify alternative causes of blockages. These results reveal that stem heating can result in xylem dysfunction and mortality, but neither response is immediate. Dysfunction was likely caused by wound responses rather than embolism, but improved understanding of the mechanisms of heat-induced hydraulic failure is needed. [ABSTRACT FROM AUTHOR]

Published
2024
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25. Evaluating the GeoSnap 13‐μ$$ \mu $$m cutoff HgCdTe detector for mid‐IR ground‐based astronomy.

Author

Leisenring, Jarron M., Atkinson, Dani, Bowens, Rory, Douence, Vincent, Hoffmann, William F., Meyer, Michael R., Auyeung, John, Beletic, James, Cabrera, Mario S., Greenbaum, Alexandra Z., Hinz, Philip, Ives, Derek, Forrest, William J., McMurtry, Craig W., Pipher, Judith L., and Viges, Eric

Subjects
INFRARED detectors, ASTRONOMY, DETECTORS, ADAPTIVE optics, FOCAL planes, ANALOG-to-digital converters
Abstract

New mid‐infrared HgCdTe (MCT) detector arrays developed in collaboration with Teledyne Imaging Sensors (TIS) have paved the way for improved 10‐μ$$ \mu $$m sensors for space‐ and ground‐based observatories. Building on the successful development of longwave HAWAII‐2RGs for space missions such as NEO Surveyor, we characterize the first 13‐μ$$ \mu $$m GeoSnap detector manufactured to overcome the challenges of high‐background rates inherent in ground‐based mid‐IR astronomy. This test device merges the longwave HgCdTe photosensitive material with Teledyne's 2048×2048$$ 2048\times 2048 $$ GeoSnap‐18 (18‐μ$$ \mu $$m pixel) focal plane module, which is equipped with a capacitive transimpedance amplifier (CTIA) readout circuit paired with an onboard 14‐bit analog‐to‐digital converter (ADC). The final assembly yields a mid‐IR detector with high QE, fast readout (>85 Hz), large well depth (>1.2 million electrons), and linear readout. Longwave GeoSnap arrays would ideally be deployed on existing ground‐based telescopes as well as the next generation of extremely large telescopes. While employing advanced adaptive optics (AO) along with state‐of‐the‐art diffraction suppression techniques, instruments utilizing these detectors could attain background‐ and diffraction‐limited imaging at inner working angles <10 λ$$ \lambda $$/D, providing improved contrast‐limited performance compared with JWST MIRI while operating at comparable wavelengths. We describe the performance characteristics of the 13‐μ$$ \mu $$m GeoSnap array operating between 38 and$$ and $$ 45 K, including quantum efficiency, well depth, linearity, gain, dark current, and frequency‐dependent (1/f$$ 1/f $$) noise profile. [ABSTRACT FROM AUTHOR]

Published
2023
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31. Comment on “The extent of forest in dryland biomes”

Author

Griffith, Daniel M., Lehmann, Caroline E. R., Strömberg, Caroline A. E., Parr, Catherine L., Pennington, R. Toby, Sankaran, Mahesh, Ratnam, Jayashree, Still, Christopher J., Powell, Rebecca L., Hanan, Niall P., Nippert, Jesse B., Osborne, Colin P., Good, Stephen P., Anderson, T. Michael, Holdo, Ricardo M., Veldman, Joseph W., Durigan, Giselda, Tomlinson, Kyle W., Hoffmann, William A., Archibald, Sally, and Bond, William J.

Published
2017
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33. Savanna Vegetation-Fire-Climate Relationships Differ Among Continents

Author

Lehmann, Caroline E. R., Anderson, T. Michael, Sankaran, Mahesh, Higgins, Steven I., Archibald, Sally, Hoffmann, William A., Hanan, Niall P., Williams, Richard J., Fensham, Roderick J., Felfili, Jeanine, Hutley, Lindsay B., Ratnam, Jayashree, San Jose, Jose, Montes, Ruben, Franklin, Don, Russell-Smith, Jeremy, Ryan, Casey M., Durigan, Giselda, Hiernaux, Pierre, Haidar, Ricardo, Bowman, David M. J. S., and Bond, William J.

Published
2014

35. Herbaceous vegetation responses to experimental fire in savannas and forests depend on biome and climate.

Author

Gold, Zachary J., Pellegrini, Adam F. A., Refsland, Tyler K., Andrioli, Romina J., Bowles, Marlin L., Brockway, Dale G., Burrows, Neil, Franco, Augusto C., Hallgren, Steve W., Hobbie, Sarah E., Hoffmann, William A., Kirkman, Kevin P., Reich, Peter B., Savadogo, Patrice, Silvério, Divino, Stephan, Kirsten, Strydom, Tercia, Varner, J. Morgan, Wade, Dale D., and Wills, Allan

Subjects
FOREST fires, BIOMES, HERBACEOUS plants, FOREST plants, SAVANNAS
Abstract

Fire–vegetation feedbacks potentially maintain global savanna and forest distributions. Accordingly, vegetation in savanna and forest ecosystems should have differential responses to fire, but fire response data for herbaceous vegetation have yet to be synthesized across biomes. Here, we examined herbaceous vegetation responses to experimental fire at 30 sites spanning four continents. Across a variety of metrics, herbaceous vegetation increased in abundance where fire was applied, with larger responses to fire in wetter and in cooler and/or less seasonal systems. Compared to forests, savannas were associated with a 4.8 (±0.4) times larger difference in herbaceous vegetation abundance for burned versus unburned plots. In particular, grass cover decreased with fire exclusion in savannas, largely via decreases in C4 grass cover, whereas changes in fire frequency had a relatively weak effect on grass cover in forests. These differential responses underscore the importance of fire for maintaining the vegetation structure of savannas and forests. [ABSTRACT FROM AUTHOR]

Published
2023
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