amazing pictures: Amazing Astronomy Pictures

Amazing Astronomy Pictures Definition

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Astrophotography is a specialized type of photography that entails recording images of astronomical objects and large areas of the night sky. The first photograph of an astronomical object (the Moon) was taken in 1840, but it was not until the late 19th century that advances in technology allowed for detailed stellar photography. Besides being able to record the details of extended objects such as the Moon, Sun, and planets, astrophotography has the ability to image objects invisible to the human eye such as dim stars, nebulae, and galaxies. This is done by long time exposure since both film and digital cameras can accumulate and sum light photons over these long periods of time. In professional astronomical research, photography revolutionized the field, with long time exposures recording hundreds of thousands of new stars and nebulae that were invisible to the human eye, leading to specialized and ever larger optical telescopes that were essentially big "cameras" designed to collect light to be recorded on film. Direct astrophotography had an early role in sky surveys and star classification but over time it has given way to more sophisticated equipment and techniques designed for specific fields of scientific research, with film (and later astronomical CCD cameras) becoming just one of many forms of sensor.[1]

Astrophotography is a large sub-discipline in amateur astronomy where it is usually used to record aesthetically pleasing images, rather than for scientific research,[2] with a whole range of equipment and techniques dedicated to the activity.

With only a few exceptions, astronomical photography employs long exposures since both film and digital imaging devices can accumulate and sum light photons over long periods of time. The amount of light hitting the film or detector is also increased by increasing the diameter of the primary optics (the objective) being used. Urban areas produce light pollution so equipment and observatories doing astronomical imaging are located in remote locations to allow long exposures without the film or detectors being swamped with stray light.

Since the Earth is constantly rotating, telescopes and equipment have to be rotated in the opposite direction to follow the apparent motion of the stars overhead (called diurnal motion). This is accomplished by using either equatorial or computer controlled altazimuth telescope mounts to keep celestial objects centered while the earth rotates. All telescope mount systems suffer from induced tracking error due to imperfect motor drives and mechanical sag of the telescope. Tracking errors are corrected by keeping a selected aiming point, usually a bright guide star, centered during the entire exposure. Sometimes (as in the case of comets) the object to be imaged is moving, so the telescope has to be kept constantly centered on that object. This guiding is done through a second co-mounted telescope called a "guide scope" or via some type of "off-axis guider", a device with a prism or optical beam splitter that allows the observer to view the same image in the telescope that is taking the picture. Guiding was formerly done manually throughout the exposure with an observer standing at (or riding inside) the telescope making corrections to keep a cross hair on the guide star. Since the advent of computer controlled systems this is accomplished by an automated systems in professional and even amateur equipment.

Astronomical photography is one of the earliest types of scientific photography[3] and almost from its inception it diversified into subdisciplines that each have a specific goal including star cartography, astrometry, stellar classification, photometry, spectroscopy, polarimetry, and the discovery of astronomical objects such as asteroids, meteors, comets, variable stars, novae, and even unknown planets. These all require specialized equipment such as telescopes designed for precise imaging, for wide field of view (such as Schmidt cameras), or for work at specific wavelengths of light. Astronomical CCD cameras may use cryogenic cooling to reduce thermal noise and to allow the detector to record images in other spectra such as in infrared astronomy. Specialized filters are also used to record images in specific wavelengths.

Henry Draper with a refractor telescope set up for photography (photo probably taken in the 1860s or early 1870).[4]

The development of astrophotography as a scientific tool was pioneered in the mid-19th century for the most part by experimenters and amateur astronomers, or so-called "gentleman scientists" (although, as in other scientific fields, these were not always men).[5] Because of the very long exposures needed to capture relatively faint astronomical objects, many technological problems had to be overcome. These included making telescopes rigid enough so they wouldn’t sag out of focus during the exposure, building clock drives that could rotate the telescope mount at a constant rate, and developing ways to accurately keep a telescope aimed at a fixed point over a long period of time. Early photographic processes also had limitations. The daguerreotype process was far too slow to record anything but the brightest objects, and the wet plate collodion process limited exposures to the time the plate could stay wet.[6]

The first known attempt at astronomical photography was by Louis Jacques Mandé Daguerre, inventor of the daguerreotype process which bears his name, who attempted in 1839 to photograph the moon. Tracking errors in guiding the telescope during the long exposure meant the photograph came out as an indistinct fuzzy spot. John William Draper, New York University Professor of Chemistry, physician and scientific experimenter managed to make the first successful photograph of the moon a year later on March 23, 1840, taking a 20-minute-long daguerreotype image using a 5-inch (13 cm) reflecting telescope.

The Sun may have been first photographed in an 1845 daguerreotype by the French physicists Léon Foucault and Hippolyte Fizeau. A failed attempt to obtain a photograph of a Total Eclipse of the Sun was made by the Italian physicist, Gian Alessandro Majocchi during an eclipse of the Sun that took place in his home city of Milan, on the 8th of July 1842. He later gave an account of his attempt and the Daguerreotype photographs he obtained, in which he wrote[7]

"…a few minutes before and after totality an iodized plate was exposed in a camera to the light of the thin crescent, and a distinct image was obtained; but another plate exposed to the light of the corona for two minutes during totality did not show the slightest trace of photographic action. No photographic alteration was caused by the light of the corona condensed by a lens for two minutes, during totality, on a sheet of paper prepared with bromide of silver."

The first solar eclipse photograph taken on July 28, 1851 by a daguerrotypist named Berkowski.

The Sun's solar corona was first successfully imaged by a during the Solar eclipse of July 28, 1851. Dr. August Ludwig Busch, the Director of the Konigsberg Observatory gave instructions for a local Kaliningrad daguerreotypist named Berkowski (his first name is not known) to image the eclipse. Busch himself was not present at Konigsberg (now Kaliningrad, Russia), but preferred to observe the eclipse from nearby Rixhoft. The telescope used by Berkowski was attached to the 6 ½-inch Konigsberg Heliometer and had an aperture of only 2.4 inches (6.1 cm), and a focal length of 32 inches (81.2 cm) inches. Commencing immediately after the beginning of totality, Berkowski exposed a daguerreotype plate for 84 seconds in the focus of the telescope, and on development an image of the corona was obtained. He also exposed a second plate for about 40 to 45 seconds but was spoiled when the sun broke out from behind the moon.[8] More detailed photographic studies of the Sun were made by the British astronomer Warren De la Rue starting in 1861.[9]

The first photograph of a star was a daguerreotype of the star Vega by astronomer William Cranch Bond and daguerreotype photographer and experimenter John Adams Whipple, on July 16 and 17, 1850 with Harvard College Observatory's 15 inch Great refractor.[10] In 1863 the English chemist William Allen Miller and English amateur astronomer Sir William Huggins used the wet collodion plate process to obtain the first ever photographic spectrogram of a star, Sirius and Capella.[11] In 1872 American physician Henry Draper, the son of John William Draper, recorded the first spectrogram of a star (Vega) to show absorption lines.[12]

Henry Draper's 1880 photograph of the Orion Nebula, the first ever taken.

One of Andrew Ainslie Common 1883 photograph of the same nebula, the first to show that a long exposure could record new stars and nebulae invisible to the human eye.

Astronomical photography did not become a serious research tool until the late 19th century, with the introduction of dry plate photography.[13] It was first used by Sir William Huggins and his wife Margaret Lindsay Huggins, in 1876, in their work to record the spectra of astronomical objects. In 1880 Henry Draper used the new dry plate process with photographically corrected 11-inch (28 cm) refracting telescope made by Alvan Clark[14] to make a 51-minute exposure of the Orion Nebula, the first photograph of a nebula ever made. A breakthrough in astronomical photography came in 1883, when amateur astronomer Andrew Ainslie Common used the dry plate process to record several images of the same nebula in exposures up to 60 minutes with a 36-inch (91 cm) reflecting telescope that he constructed in the backyard of his home in Ealing, outside London. These images for the first time showed stars too faint to be seen by the human eye.[15] [16]

1887 saw the Astrographic Catalogue and Carte du Ciel, the first all-sky photographic astrometry project. It was conducted by 20 observatories all using special photographic telescopes with a uniform design called normal astrographs, all with an aperture of around 13 inches (330 mm) and a focal length of 11 feet (3.4 m), designed to create images with a uniform scale on the photographic plate of approximately 60 arcsecs/mm while covering a 2° × 2° field of view. The attempt was to accurately map the sky down to the 14th magnitude but it was never completed.

The beginning of the 20th century saw the worldwide construction of refracting telescopes and sophisticated large reflecting telescopes specifically designed for photographic imaging. Towards the middle of the century, giant telescopes such as the 200-inch (5 meter) Hale Telescope and the 48-inch Samuel Oschin telescope at Palomar Observatory were pushing the limits of film photography.

Some progress was made in the field of photographic emulsions and in the techniques of forming gas hypersensitization, cryogenic cooling, and light amplification, but starting in the 1970s after the invention of the CCD, photographic plates have given way to electronic imaging in professional observatories. CCD's are far more light sensitive, do not drop off in sensitivity to light over long exposures the way film does ("reciprocity failure"), have the ability to record in a much wider spectral range, and simplify storage of information. Telescopes now use many configurations of CCD sensors including linear arrays and large mosaics of CCD elements equivalent to 100 million pixels, designed to cover the focal plane of telescopes that formerly used 10-to-14-inch photographic plates.[17]

The late 20th century saw advances in astronomical imaging take place in the form of new hardware, with the construction of giant multi-mirror and segmented mirror telescopes. It would also see the introduction of space based telescopes, such as the Hubble Space Telescope. Operating outside the atmosphere’s turbulence, scattered ambient light and the vagaries of weather allows the Hubble Space Telescope, with a mirror diameter of 2.4 m, to record stars down to the 30th magnitude, some 100 times dimmer than what the 5-meter Mount Palomar Hale telescope could record in 1949