Astrophotography

Astrophotography, also known as astronomical imaging, is the practice of photographing celestial objects and events.

This field includes imaging the Moon, the Sun, planets, and deep-sky objects like nebulae and galaxies. While professional astronomers use these images to collect scientific data, the term often refers to amateur astronomers seeking beautiful images. Modern techniques allow us to see things the human eye cannot detect. We can capture dim stars and objects outside the visible spectrum of light.

The primary mechanism of astrophotography is the long exposure. Because celestial objects are often very faint, photographers must let the camera collect light for long periods. During these exposures, both film and digital sensors accumulate and sum photons. Photons are the tiny particles that make up light. To increase the amount of light reaching the sensor, photographers use larger primary optics, or objectives.

Specialized optical filters are also used to limit photons to specific wavelengths. This allows for the imaging of specific parts of the light spectrum.

Capturing clear images requires managing the Earth's rotation, known as diurnal motion. As the Earth rotates, stars appear to move across the sky. To keep a target centered, telescopes use specialized mounts. These include equatorial mounts or computer-controlled altazimuth mounts. Even with these, tracking errors can occur. These errors come from imperfect motor drives, mechanical sag, or atmospheric refraction. To correct this, photographers use a process called guiding. They use a second telescope called a guide scope or an off-axis guider. This device uses a prism or beam splitter to help keep a guide star centered during the exposure.

Astrophotography has many specialized subdisciplines. These include star cartography, which is the mapping of the sky, and astrometry, the measurement of positions. Other fields include stellar classification, photometry, and spectroscopy. Spectroscopy involves studying the light spectra of objects. Some photographers use CCD cameras, which are electronic sensors. These cameras can be cooled to reduce thermal noise. Cooling the sensor also allows the detector to record images in other spectra, such as infrared astronomy.

The history of this science began in the mid-19th century. Early experimenters faced many technological hurdles. The daguerreotype process was very slow and could only capture bright objects. The wet plate collodion process was also limited by how long a plate stayed wet. In 1839, Louis Jacques Mandé Daguerre attempted the first astronomical photograph of the Moon. It was an indistinct fuzzy spot due to tracking errors. However, John William Draper succeeded in 1840. He took a 20-minute-long daguerreotype of the Moon using a reflecting telescope.

Significant milestones followed in the 19th century. In 1851, Johann Julius Friedrich Berkowski captured the first image of the Sun's corona during an eclipse.

In 1850, William Cranch Bond and John Adams Whipple took the first photograph of a star other than the Sun, which was Vega. By 1880, Henry Draper used the dry plate process to take the first photograph of a nebula, the Orion Nebula.

Shortly after, in 1883, Andrew Ainslie Common used long exposures to show stars too faint for the human eye to see.

The transition from film to digital changed the field entirely. Starting in the 1970s, Charge-Coupled Devices, or CCDs, began replacing photographic plates. CCDs are much more sensitive to light. Unlike film, they do not suffer from reciprocity failure, which is a loss of sensitivity during long exposures. They also allow for easier storage of data and a wider spectral range. Today, almost all observational astronomy relies on these advanced imaging techniques to explore the universe.