Astrophotography and Photometry
Astrophotography is the photography of astronomical objects, events, or areas of the night sky. The first image of an astronomical object (the Moon) was taken in 1840, but it was not until the late 19th century that advances in technology allowed detailed star photography. As well as being able to record details of distant objects such as the Moon, Sun, and planets, modern astrophotography has the ability to image objects invisible to the human eye such as faint stars, nebulae, and galaxies. This is achieved through long exposure, as both film and digital cameras or dedicated Astro-cameras (CMOS/CCD) can accumulate and sum photons over long periods of time.
Long-exposure photography has revolutionized professional astronomy, recording hundreds of thousands of new stars and nebulae invisible to the human eye. Dedicated and increasingly larger optical telescopes were constructed as large cameras to record images on photographic plates. Astrophotography plays a leading role in sky surveys and star classification.
Today, astrophotography is mostly a sub-discipline of amateur astronomy, usually looking for aesthetic images rather than scientific data. Amateurs use a wide range of specialized equipment and techniques suitable for amateur and limited professional purposes. Astrophotography can be one of many expensive hobbies, but it can also be a tool for scientific research and in some cases a little of both. It may come as a surprise to some, but amateur astronomers have contributed greatly to the modern understanding of the universe.
Under a partnership program with Scorpion Shipping Ltd, the Institute for Advanced Physical Studies has a system for astrophotography, variable star photometry, and astronomical observations. The equipment is in active use, regularly generating data for the international astronomical database AAVSO (The American Association of Variable Star Observers), and consists of the following main components:
- Telescope: Sky-Watcher Explorer PDS 250/1200 mm Newtonian
- Camera: ZWO ASI533MM Pro (mono) cooled
- Mount: Sky-Watcher EQ6-R Pro (Equatorial Go-To)
- Scientific Filters: Baader Bessel (Johnson) V and B (photometric)
- Astrophotography Filters: ZWO LRGB Filter Set; Astronomik SHO
- Pixel Scale: 0,64 arc-sec/pixel
- FOV: 32×32 arc-min
- Resolution: 3008×3008
- Location: Meshtitsa, Pernik (42º 42′ 00″; 23º 00′ 13″; 695m)
Photometry of Variable Stars, Novae and Supernovae
Astrophotography is one of the earliest types of scientific photography and almost from its beginning has been divided into sub-disciplines, each with a specific purpose, including stellar cartography, astrometry, stellar classification, photometry, spectroscopy, polarimetry, and the detection of astronomical objects such as asteroids, meteors, comets, and variable stars. They often require specialized equipment such as long-focal length catadioptric telescopes. Astronomical CCD/CMOS cameras have active cooling to reduce thermal noise and allow the sensor to record images in other spectra. Specialized narrowband filters are also used to obtain images in specific wavelengths, and photometric filters are used for photometry.
Photometry is the measurement of the intensity of starlight, or in other words, it is a methodology that measures the brightness of a star based on an image of it. Each pixel of a CCD/CMOS captures a certain number of photons from the observed object during its exposure. The photons are transformed into electrons, which are stored until they are read by specialized software.
Variable stars are interesting for many different reasons, but ultimately we study them because they are like physics labs. If we understand how the light from a variable star changes, we can learn more about how stars work. The same fundamental physical processes that operate here on Earth (e.g. gravity, fluid mechanics, electromagnetism, light and heat, chemistry, and nuclear physics) operate in exactly the same way throughout the universe. By observing how stars change over time, we can learn why they change. Observations provide the raw material that feeds subsequent analysis and scientific study.
Nikola Antonov is a member of the American Association of Variable Star Observers, whose mission is to give a wide range of laypeople access to participate in astronomy-related observational science initiatives. As part of this collaboration, the Institute for Advanced Physical Studies participates in the AAVSO International Database (AID), which brings together data from thousands of observers. Researchers turn to AID because of its reputation as a quality database that works in partnership with institutions such as NASA and the Hubble Space Telescope.
Exoplanets are planets outside the solar system orbiting other stars. Because of their long distance, they cannot be observed directly even with the most powerful instruments. Indirect methods are used. For example, if a planet, orbiting its star, comes between the observer and its star, the star’s brightness barely diminishes. This phenomenon is called a transit and usually lasts about 2-3 hours. As you can guess, stars are much larger than planets, and during transit, the changes in brightness are imperceptible to the eye but can be detected by precise photometry.
The graphic depicts the moment when a planet transit occurred for the star HAT-P-32 in the constellation Andromeda. Over the course of 3 hours, the star’s brightness drops by 0.02 magnitudes. The picture was generated with AstroImageJ. It shows a transit model based on the raw data that takes into account many parameters, including astrophysical ones. HAT-P-32b is a planet orbiting a star about 950 light years from Earth. The planet is thought to be a hot Jupiter, and although it is slightly smaller than Jupiter, it is inflated to nearly twice its size. Its period is 2.2 days, transit lasts 3 h. 10 min.
- Антонов, Н. Астрофотографията – между изкуството и науката. – Светът на физиката, 2022, № 2, 137 – 141