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Disertaciones Astronómicas Boletín Número 66 De Efemérides Astronómicas 24 De Febrero De 2021

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Disertaciones Astronómicas Boletín Número 66 De Efemérides Astronómicas 24 De Febrero De 2021 Disertaciones astronómicas Boletín Número 66 de efemérides astronómicas 24 de febrero de 2021 Realiza Luis Fernando Ocampo O. ([email protected]). Noticias de la semana. 50 años del CCD. Imagen 1: Un CCD en el instrumento ESPRESSO del Very Large Telescope (VLT). Con 81 millones de píxeles, este es uno de los CCD monolíticos más grandes del mundo. Crédito: ESO / Olaf Iwert. En 1969, dos investigadores diseñaron la estructura básica de un CCD, o dispositivo de carga acoplada, y definieron sus principios operativos. Desde entonces, estos dispositivos han jugado un papel muy importante en la astronomía, así como en nuestra vida diaria. Imagen 2: Willard Boyle, a la izquierda, y George Smith prueban un sensor CCD en 1970 en los laboratorios Bell.AP/. El uso de CCD en la fotografía cotidiana se produjo unos 20 años después de la aplicación de los CCD científicos a la astronomía; los CCD astronómicos fueron los pioneros de la fotografía digital. Pero hay una gran diferencia entre los sensores de imagen CCD científicos que utilizamos en ESO para astronomía y los CCD comerciales, como los que se utilizan en las cámaras de vídeo. Los CCD científicos se adelgazan, se iluminan en la parte trasera y se tratan en la superficie para recopilar la mayor cantidad de información posible sobre el objeto observado. Los CCD científicos también suelen ser monocromáticos, lo que significa que no filtran los colores, por lo que recolectan toda la luz disponible con la mayor eficiencia posible. Los CCD comerciales, por otro lado, producen principalmente imágenes en color. Imagen 3: Imagen que muestra la disposición del sistema de capas y pixeles en un chip CCD. Imagen: Wikipedia. El primer CCD fue desarrollado por dos físicos de Bell Labs, Willard Boyle y George Smith, quienes originalmente estaban interesados en la tecnología de transistores, que se inventó unos 20 años antes que el CCD. Como el primer CCD se produjo utilizando una instalación de fabricación de transistores, yo diría que la invención del CCD al menos requirió la invención del transistor. Como suele ser el caso, los detectores en sí mismos y las tecnologías de fabricación utilizadas se desarrollan en paralelo: los científicos e ingenieros (¡y el empresario ocasional!) Están constantemente ampliando los límites de lo que es posible. Los CCD modernos dependen en gran medida de una serie de tecnologías de fabricación muy bien desarrolladas, y probablemente ocurrirá que los usos novedosos de estas tecnologías den lugar a la próxima generación de detectores para astronomía. ¿cómo funciona exactamente un CCD? Los CCD funcionan convirtiendo la luz en un patrón de carga electrónica en un chip de silicio. Este patrón de carga se convierte en una forma de onda de video, se digitaliza y se almacena como un archivo de imagen en una computadora. El efecto fotoeléctrico es fundamental para el funcionamiento de un CCD: Los átomos de un cristal de silicio tienen electrones dispuestos en bandas de energía discretas. La banda de energía inferior se llama Banda de Valencia, la banda superior es Banda de Conducción. La mayoría de los electrones ocupan la banda de valencia, pero pueden excitarse hacia la banda de conducción por calentamiento o por absorción de un fotón; La energía requerida para esta transición es de 1,26 electronvoltios. Una vez en esta banda de conducción, el electrón puede moverse libremente en la red del cristal de silicio. Deja un "agujero" en la banda de valencia que actúa como un portador cargado positivamente. En ausencia de un campo eléctrico externo, el agujero y el electrón se volverán a combinar rápidamente y se perderán. En un CCD se introduce un campo eléctrico para barrer estos portadores de carga y evitar la recombinación. Los electrones generados térmicamente son indistinguibles de los electrones fotogenerados. Constituyen una fuente de ruido conocida como "corriente oscura" / RUIDO ELECTRÓNICO, y es importante que los CCD se mantengan fríos para reducir su número. 1.26eV corresponde a la energía de la luz con una longitud de onda de 1 micrón. Más allá de esta longitud de onda, el silicio se vuelve transparente y los CCD construidos a partir de silicio se vuelven insensibles. Imagen 4: Analogía del funcionamiento del CCD Imagen: Wikipedia. Un CCD es una matriz bidimensional de millones de píxeles, cada uno de los cuales recoge fotones de luz y los convierte en una carga eléctrica cuando el CCD se expone a la luz. En lugar de usar un cable para detectar la carga de cada píxel, la carga se transfiere primero verticalmente y luego horizontalmente para alcanzar un solo amplificador de salida que mide la cantidad de carga de cada píxel. La analogía clásica es pensar en un CCD como un conjunto de cubos recolectores de lluvia a lo largo de una serie de cintas transportadoras; Primero, las cintas transportadoras mueven los envases en una dirección, sobre una sola cinta transportadora que mueve todos los envases en una dirección perpendicular para verter el agua en un cilindro medidor que contabiliza la cantidad de agua en cada cubo, uno por uno. Una analogía común para el funcionamiento de un CCD es la siguiente: un número de cubos (píxeles) se distribuye en un campo (plano focal de un telescopio) en una matriz cuadrada. Los cubos se colocan encima de una serie de cintas transportadoras paralelas y recogen la lluvia (fotones) en todo el campo. Las cintas transportadoras están inicialmente estacionarias, mientras que la lluvia llena lentamente los cubos (durante el curso de la exposición). Una vez que la lluvia se detiene (el obturador de la cámara se cierra), las cintas transportadoras comienzan a girar y transfieren los cubos de lluvia, uno por uno, a un cilindro de medición (amplificador electrónico) en la esquina del campo (en la esquina del CCD). Primero, abrimos el obturador y dejamos que la lluvia (luz) caiga sobre la matriz, llenando los cubos (píxeles). Al final de la exposición, cerramos el obturador. Ahora, las vasijas se mueven a lo largo de las cintas transportadoras. Vierta el primer juego de cubos en la cinta transportadora especial (el registro de serie) al final de la matriz. Registra también el contenido de este balde y luego se repite hasta que se hayan leído todos los baldes de la cinta transportadora especial. Imagen 5: CCD del instrumento ESPRESSO del The Very Large Telescope (VLT), que tiene 81 millones de pixels. Es uno de los más grandes monolitos del mundo. Crédito: ESO/Olaf Iwert En los últimos 50 años, los CCD han mejorado mucho de muchas maneras. Para empezar, ahora contienen más píxeles. Los píxeles también pueden ser más grandes o más pequeños según el diseño óptico del instrumento. Ahora también tenemos transistores de salida optimizados con menos "ruido de lectura" mientras operamos a una velocidad más alta, transferencia de carga más eficiente, empaquetado mecánico mejorado para un mejor enfriamiento, mayor eficiencia cuántica, menos ruido de corriente oscura, menos defectos dentro del área de imagen, mejor óptica revestimientos, mayor fiabilidad ... ¡la lista continúa! Otro avance es que los CCD ahora pueden diseñarse especialmente para ser óptimamente sensibles a longitudes de onda de luz específicas. Un ejemplo de esto es el uso de detectores optimizados para los extremos azul y rojo del espectro visible, como por ejemplo el que se usa en el instrumento ESPRESSO del Very Large Telescope. Constelación de la semana: Sagitta, La Flecha. Imagen 6: Constelación Sagitta. Sagitta se puede ver sobre Aquila en esta placa del Espejo de Urania (1825). La constelación de Sagitta se encuentra en el cielo del norte. Su nombre significa "la flecha" en latín. Sagitta es una de las constelaciones griegas. Fue catalogado por primera vez por el astrónomo griego Ptolomeo en el siglo II. Representa la flecha de Heracles en la mitología griega. Sagitta es la tercera constelación más pequeña del cielo. No tiene estrellas más brillantes que la cuarta magnitud y contiene pocos objetos notables de cielo profundo. Estos incluyen el cúmulo globular Messier 71 (NGC 6838) y la Nebulosa Collar, una nebulosa planetaria descubierta en 2005. Sagitta es la constelación número 86 en tamaño, ocupando un área de solo 80 grados cuadrados. Las únicas constelaciones más pequeñas que Sagitta son Equuleus y Crux. Sagitta se encuentra en el cuarto cuadrante del hemisferio norte (NQ4) y se puede ver en latitudes entre + 90 ° y -70 °, desde cualquier lugar de la Tierra excepto el Círculo Antártico. Las constelaciones vecinas son Aquila, Delphinus, Hercules y Vulpecula. Sagitta pertenece a la familia de constelaciones de Hércules, junto con Aquila, Ara, Centaurus, Corona Australis, Corvus, Crater, Crux, Cygnus, Hercules, Hydra, Lupus, Lyra, Ophiuchus, Scutum, Sextans, Serpens, Triangulum Australe y Vulpecula. Sagitta contiene un objeto Messier, el cúmulo globular Messier 71 (M71, NGC 6838). No hay lluvias de meteoritos asociadas con la constelación. Sagitta contiene tres estrellas con nombre formal. Los nombres de estrellas aprobados por la Unión Astronómica Internacional (IAU) son Sham, Uruk y Sansuna. Sagitta no tiene estrellas más brillantes que la magnitud 3.00 y contiene solo una estrella ubicada a 10 parsecs (32.6 años luz) de la Tierra. La estrella más brillante de la constelación es Gamma Sagittae, con una magnitud aparente de 3,51. La estrella más cercana es Gliese 745 (clase espectral M1.0VI), ubicada a una distancia de 28,14 años luz de la Tierra. Sagitta tiene tres estrellas con exoplanetas conocidos. HD 231701 (clase espectral F8V) tiene un planeta parecido a Júpiter, descubierto en 2007, y 15 Sagittae (G0V) tiene una compañera enana marrón de largo período, detectada en 2002. La compañera es una enana subestelar de gran masa, solo unas Júpiter tiene masas por debajo del límite de una estrella. HAT-P-34 (F8) también tiene un planeta en tránsito, descubierto en 2012.
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