question: What does "camera exposure" mean? What do people mean when they say that a picture of a star/planet is "19 hours exposure"?
comment: That the camera was collecting light for 19 hours. Your average photo for example has exposure of fractions of a second usually.
passages: 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.
   Most digital single-lens reflex (DSLR) cameras are limited to 30 second or shorter exposures. An intervalometer can be used to take long (30 seconds) or very long exposures (minutes or hours) using the "Bulb" setting. Long and very long exposures taken at night can be combined to create time-lapse animations, including star trails. Astrophotographers can use processing techniques with such exposures to create images of deep-sky objects in the night sky, like nebulae and galaxies.
   By taking two exposures of the same section of the sky days or weeks apart, it is possible to find objects such as asteroids, meteors, comets, variable stars, novae, and even unknown planets. By comparing the pair of images, using a device such as a blink comparator, astronomers are able to find objects that moved or changed brightness between the two exposures or simply appear in one image only, as in the case of a nova or meteor. Sometimes objects can even be found in one exposure since a fast moving object will appear as a "line" in a long exposure.
   American astronaut Don Pettit recorded star trails with a digital camera from the International Space Station in Earth orbit between April and June, 2012. Pettit described his technique as follows: "My star trail images are made by taking a time exposure of about 10 to 15 minutes. However, with modern digital cameras, 30 seconds is about the longest exposure possible, due to electronic detector noise effectively snowing out the image. To achieve the longer exposures I do what many amateur astronomers do. I take multiple 30-second exposures, then 'stack' them using imaging software, thus producing the longer exposure."
  
  Section::::Earth's rotation.
   BULLET::::- People and Space - Photographs of the night sky that include people or elements that show the presence or influence of human beings.
  
  BULLET::::- Planets, Comets and Asteroids - Photographs of objects in our solar system, including planets and their satellites, comets, asteroids and other forms of zodiacal debris.
  
  BULLET::::- Skyscapes - Photographs of landscapes, seascapes and cityscapes in which the night sky or twilight sky is a prominent feature.
  
  BULLET::::- Stars and Nebulae - Photographs of deep-space objects in the Milky Way galaxy, including stars, star clusters, supernova remnants, nebulae and other galactic phenomena.
   Frequently, cameras are made from upcycled aluminum or tin cans which can be made light tight and are generally weather proof for the time needed to make the exposure. Apertures usually range in fractions of a millimeter
   BULLET::::- C. L. Stong 1956 interviewing scientist Robert B. Leighton for "Amateur Scientist", "Concerning the Problem of Making Sharper Photographs of the Planets", Scientific American, Vol 194, June 1956, p. 157. Early example of exposure selection with mechanical tip-tilt correction (using cine film and exposure times of 2 seconds or more).
  
  BULLET::::- William A. Baum 1956, "Electronic Photography of Stars", Scientific American, Vol 194, March 1956. Discusses the selection of short exposures at moments when the image through a telescope is sharpest (using image intensifier and short exposures).
  
  Section::::External links.
  
  BULLET::::- Amateur lucky imaging
   The sequence of images below shows how lucky imaging works. From a series of 50,000 images taken at a speed of almost 40 images per second, five different long exposure images have been created. Additionally, a single exposure with very low image quality and another single exposure with very high image quality are shown at the beginning of the demo sequence. The astronomical target shown has the 2MASS ID J03323578+2843554. North is up and East on the left.
   Night photography includes photographing subjects that are lit primarily by starlight. Directly taking images of night sky is also a part of astrophotography. Like other photography, it can be used for the pursuit of science and/or leisure. Subjects include nocturnal animals. In many cases starlight photography may also overlap with a need to understand the impact of moonlight.
  
  Section::::Polarization.
  
  Startlight intensity has been observed to be a function of its polarization.
   Of course there is a downside: taking images at this short an exposure is difficult, and if the object is too dim, not enough light will be captured to make analysis possible. Early uses of the technique in the early 1970s were made on a limited scale using photographic techniques, but since photographic film captures only about 7% of the incoming light, only the brightest of objects could be viewed in this way. The introduction of the CCD into astronomy, which captures more than 70% of the light, lowered the bar on practical applications by an order of magnitude, and today the technique is widely used on bright astronomical objects (e.g. stars and star systems).
   Section::::Trinity.
  
  Brixner was assigned to shoot movies in 16-millimeter black-and-white film, from every angle and distance and at every available speed, of an unknown event beginning with the brightest flash ever produced on Earth. "The theoretical people had calculated a some 10-sun brightness. So that was easy," said Brixner. "All I had to do was go out and point my camera at the sun and take some pictures. Ten times that was easy to calculate."
   After the Pluto times, exposure times of at least 30 seconds were made possible (half a minute), which was also useful for taking reconnaissance images and enabling imaging down to a magnitude of 21. Magnitude in astronomy is way to measure brightness, and higher number is dimmer; two related concepts are Apparent magnitude and Absolute magnitude.
  
  LORRI is pointed by moving the entire spacecraft, which limits the exposure time. The spacecraft does not have reaction wheels and is stabilized by thrusters.
  
  Section::::Jovian system.
  
  While passing by Jupiter in February 2007, the Jovian system was observed using LORRI and other instruments.
   The most basic types of astronomical photographs are made with standard cameras and photographic lenses mounted in a fixed position or on a tripod. Foreground objects or landscapes are sometimes composed in the shot. Objects imaged are constellations, interesting planetary configurations, meteors, and bright comets. 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. Allowing the stars to intentionally become elongated lines in exposures lasting several minutes or even hours, called “Star trails”, is an artistic technique sometimes used.
   Section::::CCD photometry.:Calibrations.
  
  After determining the flux of an object in counts, the flux is normally converted into instrumental magnitude. Then, the measurement is calibrated in some way. Which calibrations are used will depend in part on what type of photometry is being done. Typically, observations are processed for relative or differential photometry.
   Solargraphy is a technique in which a fixed pinhole camera is used to expose photographic paper for a length of time that is typically 6-months but can be as short as a few hours and last well over a year. Depending on the length of the exposure the resulting image shows the path of the Sun across the sky both from dawn until dusk, but also from North to South (or vice versa) between the solstices. Each light streak in a solargraphic image represents one day. Where light streaks are broken or missing indicates obstruction of the sun, which is caused by cloud cover.
   During the beginning of the 21st century, the popularity of digital cameras made it much easier for beginning photographers to understand the complexities of photographing at night. Today, there are hundreds of websites dedicated to night photography.
  
  Section::::Subjects.
  
  Common subjects photographed at night or in low light include the following:
  
  BULLET::::- Celestial bodies – the Moon, stars, planets, etc. (see astrophotography and star trail)
  
  BULLET::::- City skylines
  
  BULLET::::- Factories and industrial areas, particularly those that are brightly lit and are emitting smoke, steam, or another aerosol
  
  BULLET::::- Fireworks
  
  BULLET::::- Nightlife or rock concerts
  
  BULLET::::- Lit caves
   Lucky imaging
  
  Lucky imaging (also called lucky exposures) is one form of speckle imaging used for astrophotography. Speckle imaging techniques use a high-speed camera with exposure times short enough (100 ms or less) so that the changes in the Earth's atmosphere during the exposure are minimal.
  
  With lucky imaging, those optimum exposures least affected by the atmosphere (typically around 10%) are chosen and combined into a single image by shifting and adding the short exposures, yielding much higher angular resolution than would be possible with a single, longer exposure, which includes all the frames.
  
  Section::::Explanation.
   During a transit the amount of the light that reaches us from the star is slightly diminished while the planet is blocking out a small part of it. This little dip in the star's brightness is the signature of the transit and can be captured by a sensitive instrument, a photometer, that accurately measures the light from the star collected by a telescope. The main goal of PicSat is exactly that: To observe the brightness of the star Beta Pictoris continuously, so as to capture the small dip in that light as the planet Beta Pictoris b or its Hill Sphere passes in front of it.
   Because exposure times for star trail photographs can be several hours long, camera batteries can be easily depleted. Mechanical cameras that do not require a battery to open and close the shutter have an advantage over more modern film and digital cameras that rely on battery power. On these cameras, the Bulb, or B, exposure setting keeps the shutter open. Another problem that digital cameras encounter is an increase in detector noise with increasing exposure time.
   Any scene of photographic interest contains elements of different luminance; consequently, the “exposure” actually is many different exposures. The exposure time is the same for all elements, but the image illuminance varies with the luminance of each subject element.
  
  Exposure is often determined using a reflected-light exposure meter. The earliest meters measured overall average luminance; meter calibration was established to give satisfactory exposures for typical outdoor scenes. However, if the part of a scene that is metered includes large areas of unusually high or low reflectance, or unusually large areas of highlight or shadow, the “effective” average reflectance
   BULLET::::- She then turned to infrared astronomy. She was the Principal Investigator of the ISOCAM camera on board the Infrared Space Observatory of the European Space Agency (ESA), which flew between 1995 and 1998. As such, she has led the ISOCAM central programme, which studied, in a coordinated way, the infrared emission from a variety of galactic and extragalactic sources and yielded new and exciting results on star formation and galactic evolution. These were consolidated through further observations with the ESO VLT, the satellites Spitzer and now Herschel.
  
  Section::::Awards and distinctions.
  
  BULLET::::- Recipient of the 1998 COSPAR Space Sciences Award.
   One example of this is a single six-month exposure taken by photographer Justin Quinnell, showing sun-trails over Clifton Suspension Bridge between 19 December 2007 and 21 June 2008. Part of the "Slow light: 6 months over Bristol" exhibition, Quinnell describes the piece as capturing "a period of time beyond what we can perceive with our own vision." This method of solargraphy uses a simple pinhole camera securely fixed in a position which won't be disturbed. Quinnel constructed his camera from an empty drink can with a 0.25mm aperture and a single sheet of photographic paper.
   Section::::Scientific activity.:Extra-galactic astronomy.
   In 2010, the full list of films available on the International Space Station came to the attention of Shooting People. Concerned that the only off-world film library was consumed by mainstream American films, they conducted a poll and wrote to NASA with some more varied suggestions from members. NASA responded that individual films were selected by its crew members and that the list would be forwarded to the crew office.
  
  Section::::Work.
   The survey is broken up into 26 observation sectors, each sector being , with an overlap of sectors at the ecliptic poles to allow additional sensitivity toward smaller and longer-period exoplanets in that region of the celestial sphere. The spacecraft will spend two 13.7-day orbits observing each sector, mapping the southern hemisphere of sky in its first year of operation and the northern hemisphere in its second year. The cameras actually take images every 2 seconds, but all the raw images would represent much more data volume than can be stored or downlinked. To deal with this, cutouts around 15,000 selected stars (per orbit) will be coadded over a 2-minute period and saved on board for downlink, while full-frame images will also be coadded over a 30-minute period and saved for downlink. The actual data downlinks will occur every 13.7 days near perigee. This means that during the 2 years, TESS will continuously survey 85% of the sky for 27 days, with certain parts being surveyed across multiple runs. The survey methodology was designed such that the area that will be surveyed, essentially continuously, over an entire year (351 observation days) and makes up about 5% of the entire sky, will encompass the regions of sky (near the ecliptic poles) which will be observable at any time of year with the JWST.
  