When I started out in the astrophotography hobby, there were many terms I had to learn. One of the first real purchases I made was a tracking telescope. It was a Meade LX200, which was a real workhorse in the hobby. When the salesperson told me it was a catadioptric telescope, of course I had no idea what she was talking about. She explained that it was a telescope that was constructed with both lenses and mirrors. Having no knowledge of optics, that definition really didn’t mean a whole lot, but I understood that it was able to create good optics with a long focal length in a “short” package. I learned later that this type of telescope was ideal for small astronomical objects like planets and small galaxies, both of which needed a lot of magnification. And this was best achieved with a long focal length. This particular telescope had a focal length of 1600mm, which was good for planetary images, but not perfect, since the design had a corrector lens that obstructed the central part of the image, reducing the fine detail that planets could offer. But the equivalent refractor telescope would have to be 1.6 meters long to achieve that focal length! The LX200 was a nice compromise.
In the years that have passed, technology has advanced exponentially. And so has the terminology. What follows is glossary of terms that I think are some of the most commonly used in astrophotography. If you are just starting out in this hobby, it will put you on a solid foundation for understanding articles on this topic.
Glossary of Common Astrophotography Terms
A
• Alt-azimuth mount: A mount primarily meant for land-based photography. The two major axes are altitude (angular elevation) and azimuth (degrees deviation from north in the northern hemisphere).
• Aperture: The opening in a lens through which light passes to enter the camera. It is measured in f-stops (e.g., f/2.8, f/4).
• Astroimaging: Another term for astrophotography, the practice of photographing celestial objects and phenomena.
B
• Bahtinov Mask: A focusing aid for telescopes and cameras used in astrophotography to achieve precise focus on stars.
• Binning: A technique used to increase the signal-to-noise ratio in digital images by combining pixels, often used in CCD imaging.
C
• CCD (Charge-Coupled Device): A type of digital imaging sensor used in astrophotography cameras, known for high sensitivity and low noise.
• Chromatic Aberration: A type of distortion in which there is a failure of a lens to focus all colors to the same point, resulting in color fringing.
• Collimation: The process of aligning the optical elements of a telescope to ensure they are correctly focused and aligned.
• Coma: Irregular star images at the corners of an image caused by uneven focus across a curved lens or mirror.
• Compound Telescope: Also known as catadioptric or hybrid telescopes, these combine lenses and mirrors to gather and focus light.
D
• Dark Frame: An image taken with the camera’s sensor covered to capture the inherent noise of the sensor, which is then subtracted from light frames to remove the noise generated by the camera sensor.
• Deep-Sky Object (DSO): Celestial objects such as galaxies, nebulae, and star clusters that are located outside our solar system.
• Dobsonian Telescope: A type of reflector telescope mounted on a simple, yet sturdy alt-azimuth base.
• Dynamic Range: The ratio between the brightest and darkest parts of an image that a camera sensor can capture simultaneously without losing detail.
E
• Equatorial Mount: A mount designed for astrophotography because it is aligned to the polar axis. This means that it tracks with the earth’s rotation. The two major axes are declination and right ascension, which are similar to latitude and longitude for celestial objects
• Exposure Time: The length of time the camera’s sensor is exposed to light. Longer exposure times allow more light to be captured, revealing fainter objects.
F
• Field of View (FOV): The extent of the observable world seen at any given moment through a telescope or camera.
• FITS Format: astronomy-based file format that is the standard of the astronomy community.
• Flat Frame: An image taken to correct for variations in the camera sensor’s sensitivity and any dust or imperfections on the lens or sensor.
• Focal Length: The distance between the lens and the image sensor when the subject is in focus, typically measured in millimeters (mm).
• F/Stop: The focal ratio of a camera lens divided by the aperture of that lens. A lower f/stop means faster light gathering because of the higher (or bigger) aperture.
• Focal Ratio: For a telescope it is the focal length of the telescope divided by the aperture. Much like an f/stop, larger aperture means a lower focal ratio and faster light gathering.
G
• Gain: The amplification level applied to the signal from a camera’s sensor, affecting the brightness and noise levels of the image.
• Guiding: The use of an additional camera and software to track stars and correct for any movement or drift during long exposures (also known as autoguiding).
H
• Histogram: A graphical representation of the distribution of light in an image, used to evaluate exposure and contrast.
I
• ISO: A measure of the way a camera sensor displays the light data it collects. Higher ISO settings increase the brightness of the image and decrease the dynamic range.
L
• Light Frame: The main images captured of celestial objects, which are processed and stacked with other frames to produce the final image.
• Light Pollution: Unwanted or excessive artificial light that can interfere with the observation and imaging of celestial objects.
M
• Magnification: The process of enlarging the appearance of an object using a telescope or lens, typically measured in times (e.g., 10x). For visual use of telescope, the magnification is the focal length of the telescope divided by the focal length of the eyepiece.
• Mount: The device that holds and supports the telescope, allowing it to move and track celestial objects. Common types are equatorial and alt-azimuth mounts.
N
• Nebula: A cloud of gas and dust in space, often the birthplace of stars and planets.
O
• Observatory: A facility equipped with telescopes and other instruments for observing celestial events.
• Optical Tube Assembly (OTA): The main body of a telescope, excluding the mount and tripod.
P
• Pixel: The smallest unit of a digital image sensor, representing a single point in the image.
• Prime Focus: A method of astrophotography where the camera is attached directly to the telescope, using the telescope’s optics as the camera lens.
R
• RAW Format: An uncompressed image format that retains all the data captured by the camera sensor, allowing for greater flexibility in post-processing.
• Reflector Telescope: A type of telescope that uses mirrors to gather and focus light.
• Refractor Telescope: A type of telescope that uses lenses to gather and focus light.
S
• Sensor: The part of the camera that captures light and converts it into a digital image. Common types include CCD and CMOS sensors.
• Stacking: The process of combining multiple images of the same object to reduce noise and enhance details.
T
• Tracking: The movement of a telescope mount to follow the apparent motion of stars caused by the Earth’s rotation.
• T-Ring: An adapter that allows a camera to be attached to a telescope.
U
• UHC Filter (Ultra High Contrast): A type of filter used in astrophotography to enhance the contrast of deep-sky objects by blocking certain wavelengths of light.
V
• Vignetting: A reduction in brightness at the edges of an image, often caused by the optical design of the lens or telescope.
W
• Wide-Field Astrophotography: Capturing large areas of the sky using wide-angle lenses, typically to photograph constellations, the Milky Way, or meteor showers.
Z
• Zenith: The point in the sky directly above an observer.