Astrophotography
Photography using extended exposure-times revolutionized the field of professional astronomical research, recording hundreds of thousands of new stars, and nebulae invisible to the human eye.
Specialized and ever-larger optical telescopes were constructed as essentially big cameras to record images on photographic plates.
Astrophotography had an early role in sky surveys and star classification but over time it has used ever more sophisticated image sensors and other equipment and techniques designed for specific fields.
This is accomplished by using either equatorial or computer-controlled altazimuth telescope mounts to keep celestial objects centered while Earth rotates.
Tracking errors are corrected by keeping a selected aiming point, usually a guide star, centered during the entire exposure.
Astronomical photography was one of the earliest types of scientific photography[2] 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.
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.
He later gave an account of his attempt and the Daguerreotype photographs he obtained, in which he wrote: 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.
Dr. August Ludwig Busch, the Director of the Königsberg Observatory gave instructions for a local daguerreotypist named Johann Julius Friedrich Berkowski to image the eclipse.
Commencing immediately after the beginning of totality, Berkowski exposed a daguerreotype plate for 84 seconds in the focus of the telescope, and on developing an image of the corona was obtained.
[7] More detailed photographic studies of the Sun were made by the British astronomer Warren De la Rue starting in 1861.
[citation needed] 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.
Techniques ranges from basic film and digital cameras on tripods up to methods and equipment geared toward advanced imaging.
Commercially available color film stock is subject to reciprocity failure over long exposures, in which sensitivity to light of different wavelengths appears to drop off at different rates as the exposure time increases, leading to a color shift in the image and reduced sensitivity over all as a function of time.
This can also be compensated for by using the same technique used in professional astronomy of taking photographs at different wavelengths that are then combined to create a correct color image.
Since the film is much slower than digital sensors, tiny errors in tracking can be corrected without much noticeable effect on the final image.
Film astrophotography is becoming less popular due to the lower ongoing costs, greater sensitivity, and the convenience of digital photography.
Simple digital devices such as webcams can be modified to allow access to the focal plane and even (after the cutting of a few wires), for long exposure photography.
Many commercially available DSLR cameras have the ability to take long time exposures combined with sequential (time-lapse) images allowing the photographer to create a motion picture of the night sky.
[18] "Lucky imaging" is a secondary technique that involves taking a video of an object rather than standard long exposure photos.
Images attempting to reproduce the true color and appearance of an astronomical object or phenomenon need to consider many factors, including how the human eye works.
Another method used by amateurs to avoid light pollution is to set up, or rent time, on a remotely operated telescope at a dark sky location.
Exposure times must be short (under a minute) to avoid having the stars point image become an elongated line due to the Earth's rotation.
Camera lens focal lengths are usually short, as longer lenses will show image trailing in a matter of seconds.
[25] For example, with a 35 mm lens on an APS-C sensor, the maximum time is 500/35 × 1.5 ≈ 9.5 s. A more accurate calculation takes into account pixel pitch and declination.
Star trackers rely on the user ensuring the mount is polar aligned with high accuracy, as it is unable correct in the secondary declination axis, limiting exposure times.
However using an auto-guiding system, the secondary declination axis can also be driven, compensating for errors in polar alignment, allowing for significantly longer exposure times.
