Astro Glossary

On this page I've collected some topics which seem relevant for astro photography. Most entries contain links to Wikipedia with much more detailed content than I am able to provide here.

Reflector Telescope
Mirror based telescopes provide a longer focal length at moderate cost. Classic constructions based on Newton's design produce the well known cross shape on bright stars. For photographic use other variants with a slightly different design exist like Schmidt-Cassegrain telescopes. Using additional optical components these sometimes do not require additional corrector optics and may even be more compact.
More on reflecting telescopes on Wikipedia
Refractor Telescope
Lens based telescopes in their most simple form only contain two lenses. Due to the required high accuracy of the lenses, the price increases quickly with longer focal length and higher aperture. For photographic use some correcting optics is still required, which increase cost further.
See refracting Telescopes on Wikipedia
Equatorial Mount
An equatorial mount's task is to compensate the rotation of the earth so that the telescope stays aligned to the target object. For this to work correctly, one axis needs to be aligned parallel to the rotation axis of the earth. Then a motor can be used to perform one full rotation on that axis within 23 hours and about 56 minutes (the siderial day), which is about 15 degrees per hour. For astro photography with larger focal lengths a pure mechanical solution is not always feasible (cost, weight, perfection of alignment) so it is often paired with an optical correction system (see Guiding).
See equatorial mount on Wikipedia
OSC / One Shot Color camera
One Shot Color cameras directly generate a color image per exposure. These usually have a tiny color filter on each physical pixel and are grouped in a two by two matrix. One pixel is sensible for red, one for blue and two for green. Consumer digital cameras fall into this class.
Seeing
Seeing is the astronomer's name for atmospheric turbulences which result in refraction of light. This is recognized as flickering stars in the night sky and reduces sharpness in astro photography. This primarily affects planetary and moon photography.
Magnitude
The brightness of celestial bodies is specified in so called magnitudes.. This is a logarithmic, unit-less scale. A difference of one in magnitude equals about 2.5 times the brightness (fifth root of 100). The value decreases with higher brightness and will go negative as well. Our local star, the sun, has a magnitude of -27, the full moon about -13 and the brightest star (Sirius) scores a -1. The star polaris has a magnitude of two and the planet Neptune with its magnitude of about 8 is not observable with naked eyes even under best conditions. Regular binoculars may show stars up to a magnitude of 10, medium sized telescopes may reveal stars up to a magnitude of 14.
See Magnitude on Wikipedia
Stacking
Stacking refers to combining multiple exposures from the same target in order to maximize the signal level over the omnipresent background noise (signal to noise ratio). This allows recovery of very faint objects which can not be differentiated from background noise in a single exposure.
To compensate for negative characteristics from the capture process (vignetting, dust within the optical patch, readout noise) additional reference frames like Darks and Flats are taken into account. The result is a linear image which is suitable for the stretching stage to carve out finer details.
Stretching
To reveal the fainter details of a deep sky image which are often hidden in tiny differences in brightness, a steep curve is applied to the linear image from the stacking process. This, compared to regular photography, steep curve effectively stretches a fraction from the brightness range contained in the source image into the final image.
Light Frame
Light frames are the regular images of the target object. These contain the light which will form the later image.
Flat Frame
Flat frames are used to compensate for shortcomings within the optical system, like vignetting or dust within the visible area. These are captured from a uniform illuminated surface using a regular exposure time. It is highly critical that parameters like focus, focal length and camera rotation are identical to the light frames. It is a good practise to create a set of light frames after each target sequence. For best results specialized light panels are available.
Dark Frame
Dark Frames are used to compensate for shortcomings within the digital domain like amp glow or systematic pattern which some sensors generate during readout. They are taken with the telescope covered but identical camera parameters like exposure time, sensitivity (Gain/ISO) and sensor temperature. If you use a regular camera, then the dark frames need to be taken during the session (to match the temperature) so they will cost some observation time. When using a temperature controlled astro camera, these dark frames may be taken later (or before) and are session independent.
Bias Frame
Bias frames measure the base offset which is generated by almost any camera sensor. These are taken using a very short exposure time with the telescope covered.
Dark Flat Frame
Dark Flat Frames are used to calibrate the flat frames. Similar to regular dark frames these are taken with the same camera parameters used when capturing the flat frames, but with the telescope covered.
Noise
The term noise is used for a variety of artefacts, which are sporadic in nature and reveal different across exposures. Serveral sources of noise may be differentiated, like:
- Quantization or readout noise
  This caused by the limited bit width of the analog to digital converter inside the camera sensor.
- Dark current noise
  Head causes noise inside the sensore due to electrons moving around, reference is absolute zero. Specialized astro cameras use cooling to reduce this source of noise.
- Light pollution
  For astro photography light pollution is a kind of noise as well, which easily covers the faint light of our target objects. Similar to a conversation right before a waterfall.
Regardless of its source, you can not compensate noise within an image. But we may reduce it when integrating color values from multiple exposures.
Amp-Glow
Most camera sensors generate some gradient areas when using longer exposure times. These may originate from non-uniform heat distribution across the sensor which is recorded as infrared radiation. In astro photography this effect is compensated using dark frames and is less relevant in most cases.
Lucky Imaging
Lucky imaging is used in photography for constantly moving or changing objects (like actors on a stage). Many exposures are taken and only the best is picked afterwards. A similar approach is used for planetary and moon photography. Due to constantly changing refraction caused by air turbulences it is nearly impossible to capture these with a single exposure only. Since they are quite bright, many exposures can be captured within a small timeframe, namely a video clip. With a rate of about 60 frames per second you'll capture 1800 frames within 30 seconds. These are then analyzed and sorted by specialized software which stacks the final image only from the best.
Guiding
Guiding specifies an active correction, which optically observes the pointing direction of the telescope and send adjustment commands to the mount for optimal tracking. This allows longer exposure times for astro photography while the stars still maintain their round shape. For this purpose a second camera is used, which either has its own small telescope or, as an off-axis-guider, looks along with the main camera towards the same part in the sky.
Lightyear
A lightyear is not a timeframe but the distance which light will travel through empty space within a terrestric year. At a velocity of 300,000km/s resp. 186,000mi/s (speed of light in vacuum) this equals a distance of about 9500 billion km or 5880 billion miles.
More on Wikipedia
Astronomical Unit
One astronomical unit refers to the median distance between earth and the sun, about 150 million km or 93 million miles.
Deep Sky Object (DSO)
Visible objects in the night sky which are not individual objects, like planets or stars, are referred as Deep Sky Objects. These include star clusters, nebulae and, of course, galaxies.
Messier object
On his hunt for comets the french astronomer Charles Messier found several diffuse objects which, in contrast to comets, did not change their positions over time. To avoid spending time on these objects, he made a list of these, nowadays known as the Messier Catalog which holds 110 objects. Since these are kind of easy to spot with modern telescopes, they give a good start into astronomy. You'll find a Normalized Messier Catalog on this site or many others in the internet.
More on Wikipedia
Star
A star is basically a view into a fusion reactor where hydrogen is converted into helium. Our sun, for example, is a lower mass star. If the hydrogen is exhausted in some distant future, it probably will continue burning helium as a white dwarf. Larger stars may evolve into red giants or even explode into a supernova.
Continue reading on Wikipedia
Solar system
Due to its mass a star may collect dust from its region which concentrates in orbits surrounding the star and eventually form one or more planets. This is called a solar system. Our solar system consists of one star and a total of eight planets (officially, sorry Pluto).
Check out Wikipedia for much more info
Planet
A simple classification for planet would describe a spherical object surrounding a star as the only object within a specific orbit. As for the planetary definition of 2006, our solar system hosts a total of eight planets:
- Mercury
- Venus
- Earth
- Mars
- Jupiter
- Saturn
- Uranus
- Neptune
If you need more details and like to know what happened to Pluto, check out the much more precise definition for a planet on Wikipedia.
Nebulae
The name Nebula in astronomy is used for various types of interstellar clouds. Depending on their visual appearance these may be classified as:
- Emission nebula, planetary nebula, supernova remnants
  Due to radiation of nearby stars their matter is excited to emit light.
- Reflection nebula
  These reflect light from nearby stars and do not emit light on their own.
- Dark nebula
  Their matter obscures the light of stars behind.
My guess is, that nebulae are the most interesing objects for astro photography. The observable ones belong to our galaxy, the milky way.
Find more on Wikipedia
Galaxy
Stars, along with their planetary systems, form as groups of some million or even billion along with interstellar clouds a galaxy, like our milky way. The distance between galaxies may be a few million lightyears but galaxies still form clusters of some thousands. This alone result in a quite impressive number of stars contained in a single galaxy cluster. However, with current technology we are able to spot about 50 billion galaxies from earth.
More on Wikipedia
Milky way
Our home galaxy, the milky way, contains about 100-400 billions of stars and has a diameter of about 200,000 lightyears. All nebulae we may observe from earth belong to the milky way as well, so they are not that distant, in astronomical terms...
More info on Milky Way at Wikipedia
Hubble Palette
When imaging deep sky objects we often use narrowband filters which only pass through very specific wavelengths of light to the sensor. That way you get an image of the distribution of particular ionized elements within a sky region. The most frequent used filters focus on Hydrogen (H-Alpha), Sulphur (SII) and Oxygen (OIII). To combine these into a color image which represents our visual reception is quite difficult, since the spectral colors of SII and H-Alpha are very similar. The Hubble space telescope simply assigns these three filters to the three color channels, so SII is put into the red channel, OIII into blue and the normally reddish H-Alpha is assigned to green.
The result will be a high contrast false color image.