Astrophotography

Astrophotography is photography of astronomical objects, celestial events, and 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.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.20sec exposure photograph taken with a tripod mounted DSLR camera with 18-55mm lensFixed tripod mounted camera capturing 'star trails'Star trails photographed in earth orbit from the International Space StationFixed tripod image of a solar eclipse using a digital-SLR camera with a 500 mm lens1 minute exposure using ISO 800 film, wide angle lens, piggybacked on an equatorial telescopeComet Hale-Bopp, camera with a 300mm lens piggybackedFilm image of the Andromeda Galaxy shot at the prime focus of an 8' f/4 Schmidt–Newton telescopeLagoon and Trifid Nebulae in a montage of two film exposures with an 8' Schmidt–Newton telescope, manually guidedImage of the moon taken with a Nikon Coolpix P5000 digital camera via Afocal projection through an 8-inch Schmidt–Cassegrain telescopeThe Moon photographed using the Afocal technique, using 10 seconds of video stacked to create a final image.A composite of several Digital-SLR photos compiled in Photoshop taken via eyepiece projection from an 8-inch Schmidt Cassegrain telescope.Saturn image using negative projection (Barlow lens) with a webcam attached to a 250mm Newtonian telescope. It is a composite images made from 10% of the best exposures out of 1200 images using freeware image stacking and sharpening software (Giotto)Jupiter photographed using the Afocal technique, using 10 seconds of video stacked to create a final image. Astrophotography is photography of astronomical objects, celestial events, and 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. Photography revolutionized the field of professional astronomical research, with longtime 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 record light using photographic plates. 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 image sensors becoming just one of many forms of sensor. Today, astrophotography is mostly a subdiscipline in amateur astronomy, usually seeking aesthetically pleasing images rather than scientific data. Amateurs use a wide range of special equipment and techniques. With a few exceptions, astronomical photography employs long exposures since both film and digital imaging devices can accumulate and 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 often 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 are 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 errors due to imperfect motor drives, mechanical sag of the telescope and atmospheric refraction. Tracking errors are corrected by keeping a selected aiming point, usually a 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 was one of the earliest types of scientific photography 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 often 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 cool the sensor 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. 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). 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 would not 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. 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 July 8, 1842. He later gave an account of his attempt and the Daguerreotype photographs he obtained, in which he wrote: