Afocal - Using a camera with its lens attached to take a picture through a telescope's eyepiece.

Alt-azimuth - Altitude-Azimuth. A telescope mount that moves in two axes, one left-to-right around a turntable base, and one up and down. Any part of the sky can be located with movement in these two axes.

Aperture - The size of the opening that lets light into a camera lens, which can usually be adjusted and changed. In a telescope, the aperture is usually fixed, and is defined by the size of the primary mirror or objective lens.

Autoguider - A separate CCD or Webcam that is used to send corrections to the telescope's mounting to guide or follow the stars with high accuracy to compensate for inaccuracies in the mount's tracking.

Averaging - adding together a number of frames and dividing by the total. Averaging many frames together increases the signal-to-noise ratio.

Banding (Posterization) - Visible steps in an area that is supposed to be even-toned or smoothly varying, such as a gradient of brightness in a blank sky that fades from light to dark. Banding is usually caused by an insufficient number of digital steps of tone.

Bulb – The name given to the exposure setting where the shutter stays open for a long time exposure until it is closed by the photographer. Derives from the ancient days of film photography when an air bulb was used to pneumatically open a leaf shutter in a camera lens.

CCD - Charged Coupled Device. A light-sensitive solid-state silicon sensor used in digital cameras to record light intensities.

Composite - In digital photography, to combine different individual images sub frames of the same exposure into a single master image.

CMYK - A subtractive color printing system where the primary colors are comprised of Cyan, Magenta, Yellow and Black components. The pigments in inks of these colors subtract from white light. All of these primary colors mixed together equally in maximum intensity result in black.

Dark Frame - A dark frame is a photo taken at the same exposure, ISO and temperature as a light frame, but with no light reaching the sensor. It is a picture of the camera's thermal current and bias current. Dark frames are used in calibration of digital image data in light frames. By subtracting a dark frame from a light frame, thermal current signal and bias signal are removed.

Dark Frame Subtraction - Removing thermal current signal by subtracting a dark frame. This can be done automatically as a custom function in some DSLR cameras, or manually in later processing.

DPI - Dots Per Inch. A measure of resolution that refers to the number of dots a printer can print in an inch of output. Higher resolution means more dots per inch.

DSLR - Digital Single Lens Reflex. A camera that uses a mirror to intercept the light from the camera's lens and send it to a focusing screen for inspection by the photographer's eye. The reflex mirror swings up and out of the way when the picture is taken, allowing the light to reach the digital sensor.

Equatorial Mount - A telescope mount designed with two axes, one of which (the Polar axis) is made parallel to the Earth's axis of rotation. Movement in this single axis allows celestial objects to be followed to compensate for the Earth's rotation. Any object in the sky can be found by a combination of movements in the two axes. The polar axis corresponds to right ascension, and the other axis to declination.

Exposure - The length of time that the shutter is open and light is hitting the sensor in the camera.

Field of View (FOV) - The amount of a scene that is captured by a given focal-length lens. Wide-angle, short-focal length lenses capture a wide field of view.

File Format - The structure of a specific type of computer file. Different file formats are associated with different file types and programs. For example, JPEG and TIFF file formats are associated with image files.

Focal Length - The distance from the lens or mirror in an optical system and the focal plane where the light is focused.

Focal Ratio (F/ratio) - The ratio between the aperture and focal length of a lens or telescope.

Grayscale - Containing no color, only various shades of gray in a black and white image.

Guiding - Manually or Automatically following a star by making corrections in right ascension and declination to produce higher tracking accuracy.

Highlight - The brightest areas of an image that contain detail.

JPEG - A file format with a .JPG extension that compresses image image data according to a standard algorithm as defined by the Joint Photographic Experts Group (JPEG). JPEG compression is lossy, it throws image data away, but in a method that reduces the visual impact at reasonable compression ratios.

LRGB - A method of creating an image where high-resolution black and white Luminance data is combined with lower-resolution RGB color data to decrease total exposure time. The L in LRGB stands for Luminance.

Noise - Technically, random and non-repeatable signal in an image. In common use in digital photography, any unwanted or undesirable signal that does not convey useful information.

Piggyback - Mounting a camera and lens on top of a telescope on a mounting. Piggyback photography is usually used for long-exposure wide-angle astrophotography where the camera and lens take the picture while riding on top of a telescope on an equatorial mount that is polar aligned and tracking the stars to compensate for the Earth's rotation.

Pixelization - Pixelization occurs when an image is enlarged so much that individual pixels become visible.

Polar Alignment - Making the polar axis of an equatorial mount parallel to the Earth's axis of rotation by pointing it accurately at the North Celestial Pole in the Northern Hemisphere

Raw - Unprocessed data directly from the sensor.

RGB - Red, Green and Blue. These are the three primary colors, out of which all other colors can be created, in the additive color model.

Seeing - The steadiness of the atmosphere that allows fine details to be seen in celestial objects. If the seeing is good, detail is not blurred as much by atmospheric scintillation. Scintillation is what causes the stars to twinkle.

Stack - A generic term that means to combine images by any of several mathematical processes such as averaging, or addition.

Stretching - Redefining the black or white points in an image to increase the contrast.

Telecompressor (Focal Reducer) - An optical component made of a lens or glass elements that decrease the focal length (and focal ratio) of a telescope.

TIFF - Tagged Image File Format. An image file format with the .TIF file extension. The TIFF file format has become a standard for storing uncompressed images.

USB - Universal Serial Bus. A protocol and hardware system for transferring data from peripherals to a computer over cables.

Vignetting - Light falloff in the corners of an image due to optical, geometrical or mechanical reasons.

White Balance - Adjusting the color in an image to compensate for the color temperature of the illumination source.

White Point - The brightest area of an image that is mapped to the highest level available (pure white) based on the bit-depth of the image (Level 255 for an 8-bit image) when setting the dynamic range during a levels adjustment in image processing.