New to astrophotography and overwhelmed by strange acronyms like OTA, OAG, or PEC? This A to Z guide explains the most common astrophotography acronyms - including gear types, software, filters, and tracking terms - in plain English.
Whether you're reading forums, watching tutorials, or setting up your first deep-sky session, this reference will help you understand what all those abbreviations really mean.
Let's decode the universe of astro terms, one letter at a time.
A
ADC - Atmospheric Dispersion Corrector
An optical element that reduces color fringing caused by the atmosphere, especially near the horizon.
AF - Auto Focus
Autofocus system used in lenses or cameras. Often not used in astrophotography due to manual focus precision.
Alt - Altitude
Angle of an object above the horizon.
Alt/Az - Altitude/Azimuth Mount
A type of mount that moves up/down (altitude) and left/right (azimuth). Easy to use but not ideal for long exposures due to field rotation.
AP - Astrophotography
APO - Apochromatic Refractor
A telescope design that minimizes chromatic aberration using special lens elements.
APP - Astro Pixel Processor
Software for calibrating and stacking astrophotography images.
APS-C - Advanced Photo System - Type C
A common crop sensor size used in many DSLR and mirrorless cameras.
APT - Astro Photography Tool
Popular control software for DSLR and CMOS cameras during astrophotography sessions.
AS/AS!3 - Autostakkert! / Autostakkert! 3
Software for stacking planetary and lunar images.
ASI - ZWO ASI Cameras
A series of astronomy cameras made by ZWO, widely used in both planetary and deep-sky imaging.
AVX - Advanced VX
A type of equatorial mount made by Celestron.
Az - Azimuth
Angle of an object along the horizon, measured from north.
B
BG - Background
Often used to refer to background noise or brightness in an image, especially during post-processing or stacking.
BPM - Bad Pixel Map
A file or process that helps identify and correct hot or dead pixels in an image sensor during calibration.
BXT - BlurXTerminator
An advanced image deconvolution tool used in PixInsight to reduce star bloating and sharpen details.
C
CA - Chromatic Aberration
A type of image distortion where colors don't focus at the same point, creating color fringing around bright stars.
CCD - Charge-Coupled Device
An older but still respected type of camera sensor known for low noise and high-quality imaging. Often compared with CMOS.
CGEM - Computerized German Equatorial Mount
A class of motorized equatorial mounts with GoTo capability, commonly used in deep-sky astrophotography.
CLS - City Light Suppression
A light pollution filter from Astronomik. Blocks common artificial light wavelengths to improve contrast under bright skies.
CMOS - Complementary Metal-Oxide Semiconductor
The dominant type of modern camera sensor - known for fast readouts and low power use.
CN - Cloudy Nights
One of the most active online forums for amateur astronomers and astrophotographers.
CST - Clear Sky Tool
Short for services like Clear Sky Chart or Clear Outside, used to forecast sky conditions and transparency.
D
DARV - Drift Alignment by Robert Vice
A manual method for achieving polar alignment by observing star drift while slewing in declination.
DEC - Declination
The celestial equivalent of latitude, used in telescope mounts to locate objects in the sky.
DOB - Dobsonian Telescope
A type of simple, alt-azimuth mounted Newtonian reflector. Excellent for visual observing, not ideal for long-exposure astrophotography.
DR - Dynamic Range
The range between the darkest and brightest signals a camera sensor can capture without losing detail.
DSLR - Digital Single-Lens Reflex
A type of camera widely used in beginner astrophotography setups - especially with star trackers.
DSO - Deep Sky Object
Includes galaxies, nebulae, and star clusters - targets outside our solar system, usually faint and ideal for long exposures.
DSS - DeepSkyStacker
A free software tool used for calibrating, aligning, and stacking astrophotography images.
DARK - Dark Frame
An image taken with the same settings as a light frame, but with the lens cap on - used to subtract sensor noise and hot pixels.
E
EAA - Electronically Assisted Astronomy
Real-time or near-real-time observation using sensitive cameras and screens, rather than direct eyepiece viewing.
ED - Extra-Low Dispersion Glass
A type of optical glass used to reduce chromatic aberration, especially in apochromatic refractors.
EFL - Effective Focal Length
Refers to the apparent focal length when accounting for sensor crop factor (e.g., APS-C vs. full frame).
EN - Emission Nebula
A cloud of ionized gas emitting its own light, often imaged using narrowband filters like Hα.
EP - Eyepiece
An optical element used to view the image formed by the telescope. Interchangeable and available in various focal lengths.
EQ - Equatorial Mount
A mount aligned with Earth's axis, allowing smooth tracking of celestial objects - essential for long exposures.
EVF - Electronic Viewfinder
A digital display simulating an optical viewfinder. Not typically used in astrophotography, but present in some mirrorless cameras.
F
f/ - F-Ratio (Aperture Ratio)
The ratio of a telescope or lens's focal length to its aperture. Lower f/ numbers mean faster systems and shorter exposure times.
FF - Full Frame (1)
A camera sensor size equivalent to traditional 35mm film. Offers a wider field of view compared to APS-C or MFT.
FF - Field Flattener (2)
An optical accessory that corrects edge distortion in images, especially when using refractors.
FL - Focal Length
The distance from the lens/mirror to the focal plane. Determines magnification and field of view.
FLO - First Light Optics
Popular UK-based astronomy and astrophotography gear retailer.
FOV - Field of View
How much of the sky is visible through your setup. Affected by sensor size and focal length.
FPL-53 - Type of ED Glass
A high-end extra-low dispersion glass used in apochromatic telescopes to minimize chromatic aberration.
FPN - Fixed Pattern Noise
Sensor noise that repeats in a fixed pattern. Often corrected with calibration frames.
FPS - Frames Per Second
Important in planetary and lunar imaging where high-speed video is stacked to improve image quality.
FR - Focal Reducer
An optical element that shortens focal length (increasing field of view and speed). Common in SCT and refractor setups.
FRAC - Refractor Telescope
General shorthand for any refracting telescope (uses lenses, not mirrors). Many are ideal for astrophotography, especially APO models.
G
GEM - German Equatorial Mount
A common equatorial mount design ideal for long-exposure astrophotography. Offers stability and precise tracking.
GT - GoTo Mount
Mounts that can automatically point to and track celestial objects via a hand controller or software.
GUIDE - Guide Camera or Guiding Setup
Used in long-exposure imaging to correct minor tracking errors. Works with a guide scope or off-axis guider (OAG).
GOTO - GoTo System
Another term for GoTo mounts - useful for automated slewing and tracking.
GN - Guide Number (less common)
A flash-related term from photography, not typically relevant to astrophotography.
H
Ha / Hα - Hydrogen Alpha
A specific deep-red wavelength of light emitted by ionized hydrogen. Commonly captured with narrowband filters when imaging emission nebulae.
Hb / Hβ - Hydrogen Beta
Another hydrogen emission line in the blue part of the spectrum. Less commonly used than Ha, but still relevant in narrowband imaging.
HDR - High Dynamic Range
A post-processing technique where multiple exposures are combined to capture both bright and faint areas - used for the Moon, Sun, and some DSOs.
HEQ5 / EQ6-R / AZ-GTi - Mount Model Names
Popular equatorial (or hybrid) mounts from SkyWatcher. HEQ5 and EQ6-R are German equatorial mounts suitable for astrophotography; AZ-GTi is a portable alt-az mount (not ideal for long exposures unless modified).
HFR - Half Flux Radius
A measurement used during focusing - describes how tightly starlight is concentrated. Lower HFR = better focus.
HV - High Voltage
Sometimes found in power supply specs - relevant if you're powering a mount or cooled camera from battery sources.
I
IDAS - Astrophotography Filter Brand
Produces light pollution filters and narrowband options for DSLRs and astro-cameras.
IR - Infrared
Light beyond the visible spectrum. Some astro cameras are IR-sensitive; many DSLRs have IR-cut filters by default.
IS - Image Stabilization
A feature in some lenses and camera bodies - useful for handheld photos, but not relevant for long-exposure astrophotography (often disabled).
ISO - Sensor Gain Setting (originally film speed)
In astrophotography, ISO affects how the camera amplifies signal (and noise). It doesn't actually make the sensor more sensitive to light.
ISS - International Space Station
A popular fast-moving object to image - can be photographed during visible passes or transits across the Sun or Moon.
iOptron - Telescope & Mount Manufacturer
Known for portable and lightweight tracking mounts like SkyGuider Pro and CEM40.
IC### - Index Catalogue Objects
Similar to Messier or NGC - e.g., IC 1396 = Elephant Trunk Nebula.
J
JPEG - Joint Photographic Experts Group (Image Format)
Compressed image format. Not ideal for astrophotography due to lossy compression - use RAW or FITS instead when capturing or processing.
JNOW / J2000 - Julian Epochs
Coordinate systems used in astronomy. J2000 is a fixed reference frame (epoch January 1, 2000), often used in catalogs. JNOW adjusts for current precession.
K
K - Kelvin (Color Temperature)
Used in setting white balance. For astrophotography, it's common to use custom white balance or shoot in RAW and adjust later.
L
LENR - Long Exposure Noise Reduction
An in-camera process where the system takes a dark frame after each long exposure. Often disabled by astrophotographers, who prefer manual dark calibration.
LP - Light Pollution
Unwanted artificial light that brightens the sky and reduces contrast - especially in urban and suburban areas.
LPR / LPS - Light Pollution Reduction / Suppression
Types of filters designed to block common city light wavelengths and improve contrast in images.
LR - Lightroom
Adobe photo editing software. Widely used by astrophotographers for quick edits, especially on Milky Way and landscape shots.
LRGB - Luminance, Red, Green, Blue
A method of combining monochrome images with different filters to produce a full-color photo. L channel carries detail; RGB adds color.
LV - Live View
The camera's LCD preview. Helpful for focusing on bright stars, especially in DSLR setups.
L-eNhance / L-eXtreme - Popular Dual-Band Filters
Light pollution filters from Optolong designed for OSC (one-shot color) astrophotography. Block unwanted light while passing key emission lines.
LS - Line Spectrum
Refers to specific wavelengths emitted by excited atoms - key to understanding narrowband filters.
M
MAK - Maksutov-Cassegrain Telescope
A compact catadioptric telescope design. Great for lunar and planetary viewing; less common in deep-sky astrophotography due to narrow field of view and long focal ratio.
MC - Multi-Coated
Refers to anti-reflective coatings on lenses and optics to improve contrast and reduce flare.
MF - Manual Focus
Preferred focusing method in astrophotography - autofocus is typically unreliable at night on faint objects.
MFT / M4/3 - Micro Four Thirds
A sensor format smaller than APS-C. Used in some mirrorless cameras - offers high portability, but with reduced sensitivity and narrower field of view.
ML - Mirrorless
Camera design without a reflex mirror. Popular in modern astrophotography due to electronic viewfinder, low weight, and live-view focusing.
MPCC - Multi-Purpose Coma Corrector
An optical corrector used with fast Newtonian telescopes to reduce coma - distortion of stars near the edge of the image.
MR - Mirror Lockup
A DSLR feature that helps prevent vibrations caused by the mirror flip. Useful for sharper images in long exposures.
MSL - Mean Sea Level
Sometimes mentioned in altitude/elevation data when planning observing sites.
MW - Milky Way
Our home galaxy, often photographed as a wide-field target in nightscapes and tracked astro-landscape shots.
M# - Messier Object
A catalog of 110 bright deep-sky objects compiled by Charles Messier. Examples: M31 (Andromeda Galaxy), M42 (Orion Nebula), M45 (Pleiades).
N
NB - Narrowband
Imaging method using filters that isolate specific emission lines (Ha, OIII, SII). Ideal for imaging nebulae from light-polluted areas.
NEWT - Newtonian Telescope
A type of reflector telescope using mirrors. Popular among astrophotographers for deep-sky imaging due to fast focal ratios and large apertures.
NGC# - New General Catalogue
A deep-sky catalog containing over 7,000 objects - widely used for object identification in imaging software and star charts.
NR - Noise Reduction
Applies to both in-camera settings and post-processing techniques. Manual calibration frames (dark, bias) are preferred over automatic noise reduction for deep-sky imaging.
NCP / SCP - North/South Celestial Pole
Reference points in the sky aligned with Earth's rotational axis - used in polar alignment for equatorial mounts.
NINA - Nighttime Imaging 'N' Astronomy
Open-source astrophotography control software with growing popularity for deep-sky automation and sequencing.
NPF Rule - Night Photography Formula Rule
A more accurate version of the 500 rule for determining max shutter speed without star trails on wide-field shots.
O
OAG - Off-Axis Guider
A device that uses a small prism to pick off a portion of light from the telescope's optical path for guiding. Helps eliminate flexure - ideal for long focal lengths.
OIII - Oxygen III
A specific emission line (500.7 nm) from doubly ionized oxygen. Captured with narrowband filters - shows up as teal or blue-green in nebula images.
OPT - Optical Telescope Products
Popular US-based astronomy gear retailer. Often appears in forums, product links, and YouTube reviews.
OSC - One-Shot Color (Camera)
A color astro camera that captures RGB in a single exposure - convenient for beginners, but with lower color control compared to mono + filters.
OTA - Optical Tube Assembly
The telescope tube itself - includes the optics (lenses or mirrors) but not the mount or tripod.
OAGL - Off-Axis Guiding Lens
Less common term, sometimes used to describe the small guide scope in an OAG system.
P
PA - Polar Alignment
The process of aligning the mount's right ascension axis with the Earth's rotational axis. Crucial for accurate tracking and long exposures.
PEC - Periodic Error Correction
A mount feature that compensates for small mechanical inaccuracies in the gears over time. Helps smooth tracking.
PE - Periodic Error
The repeating tracking error found in all equatorial mounts due to gear imperfections. Affects long-exposure accuracy.
PHD / PHD2 - Push Here Dummy
Popular autoguiding software used with guide scopes or OAGs to correct tracking during imaging.
PI - PixInsight
Professional-level astrophotography processing software. Known for its steep learning curve and powerful tools.
PIPP - Planetary Imaging Pre-Processor
Free tool for sorting and converting planetary/lunar imaging videos before stacking. Often used with Autostakkert.
PS - Photoshop
Widely used for final tweaks in astrophotography - especially star color, curves, levels, and cosmetic corrections.
PS - Polar Scope
A small optical finder built into equatorial mounts, used for manual polar alignment using Polaris or the Southern Cross.
PNG / TIFF / FITS - File Formats
- PNG - compressed but lossless format, OK for sharing
- TIFF - high bit-depth, good for editing
- FITS - standard in astrophotography, holds image + metadata + calibration data
Q
QE - Quantum Efficiency
Describes how efficiently a camera sensor converts incoming photons into electrons (i.e., signal). Expressed as a percentage - the higher, the better for low-light sensitivity.
QHY - QHYCCD (Brand)
Manufacturer of dedicated astrophotography cameras, known for high-performance cooled CCD and CMOS models.
R
RA - Right Ascension
The celestial equivalent of longitude - used with declination to locate objects in the sky. Key axis for tracking in equatorial mounts.
RGB - Red, Green, Blue
Standard color channels used in photography. In astrophotography, RGB filters (or OSC sensors) are used to build full-color images.
RF - Reflection Nebula
A type of nebula that reflects light from nearby stars rather than emitting its own. Often appears blue in astro images.
RN - Read Noise
Noise introduced by the sensor's electronics when reading pixel values. Lower read noise = cleaner data, especially in faint objects.
RMS - Root Mean Square (Error)
A statistical measure of guiding accuracy (in arcseconds). Lower RMS generally means better tracking performance during exposures.
ROI - Region of Interest
Used to crop or limit the active area of the sensor - helps increase FPS and reduce file sizes during planetary imaging.
RAW - Uncompressed Image Format
Contains full sensor data with no processing. Essential format for capturing astrophotography data.
S
SA - Sky-Watcher Star Adventurer
Popular portable star tracker for DSLR astrophotography. Ideal for wide-field images and beginner setups.
SII - Sulfur II
A deep-red emission line at 672.4 nm. Commonly used in narrowband imaging to capture specific nebula structures.
SC / SCT - Schmidt-Cassegrain Telescope
A compact telescope design combining lenses and mirrors. Great for planetary and lunar work. Not ideal for beginners in deep-sky due to long focal length and narrow field of view.
SGP - Sequence Generator Pro
Advanced automation software for deep-sky astrophotography. Used for planning, focusing, plate-solving, and imaging sequences.
SQM - Sky Quality Meter
A device that measures sky brightness in magnitudes per arcsecond². Used to objectively assess light pollution levels.
SS - Shutter Speed
The duration the sensor is exposed to light. In astrophotography, typically ranges from 10s to several minutes (for deep-sky), or milliseconds (for planetary).
ST - StarTools
An image processing software suite designed specifically for astrophotography. Focuses on simplicity and structured workflow.
SUB / Subframe - Individual Exposure Frame
Each single image taken during a session. Multiple subs are stacked together to form the final image.
SW - Sky-Watcher
Popular telescope and mount manufacturer. Known for HEQ5, EQ6-R, and Star Adventurer.
STACK - Stacking
The process of combining multiple exposures (subs) to reduce noise and improve signal-to-noise ratio.
T
T-Ring / T2 Adapter - Camera Adapter for Telescopes
Connects a DSLR or mirrorless camera to a telescope. Specific to camera mount type (e.g. Canon EF, Sony E, Nikon F).
TS - Teleskop-Service (Brand)
European vendor and manufacturer of astronomy gear, especially refractors and accessories.
TILT / Tilt Adapter - Image Plane Correction
Accessory used to correct sensor tilt - especially in fast optics or large sensors where corners show distortion.
TA - Tracking Accuracy
Refers to how precisely a mount follows the sky. Affects star sharpness in long exposures.
TPO - Third-Party Optics (Brand)
A brand for budget-friendly astro gear, especially Newtonians and accessories.
TV - Tele Vue (Brand)
High-end manufacturer of eyepieces and refractors - less common in imaging, more in visual astronomy.
U
UHC - Ultra High Contrast Filter
A type of light pollution filter designed to improve the contrast of emission nebulae by blocking broadband light and passing key wavelengths like OIII and Ha.
USB3 - USB 3.0 Interface
Used by modern astro cameras and filter wheels for high-speed data transfer. Essential for planetary and video-based imaging.
V
VB - Vibration Blur
Motion blur caused by mechanical movement (mirror slap, shutter, or wind). Can be minimized with mirror lockup and remote shutter release.
VDB### - Van den Bergh Catalogue
Catalog of reflection nebulae (e.g. VDB 152). Sometimes used as target IDs in astro images.
W
WB - White Balance
Color temperature adjustment used mostly in DSLR/mirrorless photography. In astrophotography, RAW capture allows WB to be set in post-processing.
WO - William Optics
Popular manufacturer of apochromatic refractors and astro accessories - especially known for RedCat series and guiding gear.
WCS - World Coordinate System
Astrometry metadata in FITS images that maps pixel positions to sky coordinates.
Z
ZWO - Zhen Wang Optics
One of the most popular astrophotography camera brands. Known for ASI line of CMOS cameras, filter wheels, and guiding accessories.
Zenith - Point in the Sky Directly Overhead
Used in describing object altitude and telescope pointing.
#
1.25" / 2" - Filter or Eyepiece Size
Standard diameters for astro accessories. 2" allows for wider field of view, especially in full-frame or wide sensors.
"/px - Arcseconds per Pixel
A measure of resolution in an imaging system - how much of the sky is covered by each pixel. Important in assessing sampling and guiding.
8-bit / 16-bit - Bit Depth
Defines how much tonal range is captured per channel. Higher bit depth = more color information and better editing flexibility.
Summary - From OTA to ZWO, You've Got This
Astrophotography can feel like learning a whole new language - and in a way, it is. But once you start recognizing these acronyms and what they stand for, things begin to click. Whether you're setting up your first mount or fine-tuning your image stacking workflow, this guide is here to help you navigate the world of astro-imaging jargon with confidence.
I've been deep in this hobby for over a decade, and I still refer back to lists like this now and then - because there's always something new to learn or a term to refresh.
Bookmark this page, share it with a fellow stargazer, or come back whenever a weird abbreviation pops up on a forum or tutorial. The sky's full of stars - and now, you're fluent in the language we use to capture them.
FAQ - Astrophotography Acronyms
APO refers to apochromatic refractors that reduce color fringing using multiple lens elements. ED glass is one of those elements, but not all ED scopes are APO.
OTA stands for Optical Tube Assembly - basically, the telescope without a mount or tripod.
PEC = Periodic Error Correction. It reduces small mechanical errors in tracking mounts to improve long-exposure accuracy.
Yes - many great astro images are taken with APS-C cameras. They offer a good balance between size, price, and field of view.
This list! And just keep practicing - you'll pick them up naturally as you read, watch, and shoot more.
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