Astrophotography Glossary – Definition Of The Most Common Astro Terms

Photography is a field that is (in)famous for the number of technical concepts and words used to describe how things work.

This can be intimidating for beginners and can leave them confused while wondering what all they read really means.

Astrophotography is even worse with an even more difficult vocabulary to get familiar with, and still today, after 5 years spent photographing the stars, I can read words I don’t know what they mean.

For this reason, I wrote this glossary of the terms and acronyms that you will more likely encounter in your journey to the stars.

Glossary Of Terms: Real Life Astrophotography Jargon

I think we all agree that endless lists of words are boring. To keep things interesting, I decided to introduce some of the most common astrophotography terms using real-life examples.

Here are some of the most commonly asked questions from beginners, along with the typical answers, packed with astrophotography jargon.

Q: “I have a tripod and a camera. How can I photograph the starry night?”

A: ”Well, without a tracker, your exposure time will be rather short: stars move! Try using the 500 or NPF rule to determine your maximum exposure, use a fastwide-angle lens, and limit yourself to star fields or starry landscapes. Don’t forget to shoot for stacking.”

Let’s see what all those obscure terms mean. 

(Star) Tracker

A star tracker is a rather compact and lightweight astrophotography mount capable of moving the camera in sync with the stars. This allows you to take long exposures of the night sky.

In this article, you can find what are the best trackers on the market.

500-Rule

The 500-rule is an empirical rule you can use to calculate how long you can expose the starry sky from a fixed tripod before stars will trail noticeably.

For a given focal lens, FL, and a camera sensor with a given crop factor, CP, the maximum exposure time, ET in seconds is: ET = 500 / (FL*CP).

With a 50mm lens on a Canon APS-C camera (CP=1.6), the maximum exposure time will be 500 / (50*1.6) = 6s.

If you still see stars are trailing, you can switch to a more conservative 400-rule, where the number 400 replaces 500 in the formula above.

NPF Rule

The NPF rule is another empirical rule to determine the maximum exposure time to have pin-point stars when photographing without a tracker.

This is a more complex and accurate formula than the 500-rule but is more complicated. Luckily there are handy calculators for your phone, the best of which is probably PhotoPills.

Stacking

Stacking, also known as image stacking, is a technique consisting in taking many images of the same target, so to combine later in post-processing to produce a cleaner and more detailed image. You can read more about stacking in this article.

Q: “ How can I improve my DSLR astrophotography?”

A: “If you don’t have it, get a tracker. If you are happy with your gear, there are some things you can do to get better deep sky images on a tracker: 

1. balance your payload
2. be sure your polar alignment is spot on
3. increase your integration time by taking more subs
4. take calibration frames: darks, flats, and bias.”

Deep Sky

Deep Sky is the astrophotography of everything outside the Solar System: stars, star clusters, star fields, nebulae, and galaxies (including our Milky Way).

Nebulae, galaxies, clusters, and sometimes comets are often called deep sky objects (DSO). 

Payload

Everything mounted on a mount, and which is not part of the mount (like the counterweight), is the payload.

This includes the telescope or the camera, lens, the camera itself, and all the accessories.

All manufacturers specify the maximum mount payload and here is a couple of info about it:

1. For astrophotography, the weight of the payload should not exceed the 60-70% of the maximum payload of the mount.
2. Heavier the payload, the better you need to balance it on the mount. If you are facing North, make sure the balance is shifted slightly to the East (your right), so as to remove any playing between the mount gears. 

Polar Alignment

Polar alignment is the crucial task of aligning an equatorial mount (or star tracker) to the Celestial Pole. This way, the mount can rotate the payload to follow the circular movement of the stars around the celestial pole.

In the Northern Hemisphere, polar alignment is done using Polaris (the bright North Star) as a point of reference in the sky. In the Southern Hemisphere, you have to rely on the much fainter sigma-Octan star in the Octans constellation.

Integration Time

When you do astrophotography, you should shoot for image stacking, meaning you take many exposures to be later combined into a single, cleaner image. 

If you stack, say, 10 images, each 1 minute long, your integration time is 1’ x 10 = 10 minutes.

Longer the integration time, the better the details in the image.

Subs / Lights

Subs, also known as lights or light frames, are the actual images of the night sky. In the example above about integration time, you would have taken 10 subs. 

Calibration Frames

To improve image quality, you should take extra images that are used in post-processing to improve the quality of your subs before they get stacked. These images, called Darks, Flats, and Biases, are calibration frames, and the process is called light frames calibration.

Darks / Dark Frames

These calibration frames take care of suppressing hot pixels and other digital noise. To be effective, they must be taken at the same temperature of the light frames, using the same exposure time. 

It is easier to record dark frames when working with cooled cameras as you can control the camera temperature. If you are using a classic camera, you cannot control the temperature, and your best bet is to record some dark frames at the end of your imaging session.

Bias Frame

Bias frames are used to suppress the readout noise. These frames are not temperature-dependent, so you can build your own bias library in the comfort of your home. Set the camera to the fastest possible shutter speed and shoot 50-100 frames.

Flats / Flat Frames 

With flats, you try to remove vignetting and specks of dust from your light frames. 

Ideally, you don’t want to change anything in your imaging setup: not the focus, not the camera orientation. There are led panels you can use directly in the field. 

If you use photographic lenses, though, you can take flats at home, just be sure you do not change the focus (tape down the focusing ring before packing and don’t remove the lens from the camera).

Put a white t-shirt stretched in front of the lens and aim at the blue sky. Set the camera so that the histogram peaks near the center and take 50 flats or so.

Q: ” I would like to get closer to DSO, but I am on a tracker. What can I do?”

A: “ Trackers have limited max payload, so you do not want to use heavy telephoto lenses or large telescopes on them. Your best bet is to photograph with the longer focal length you can comfortably use on your tracker, and then drizzle in post-processing. 

Remember that for drizzle to be effective, you need to have lots of undersampled subs and dither your images.”

Drizzle

Drizzle (or Variable Pixel Linear Reconstruction) is a method originally developed by NASA for the Hubble Deep Field observations with the Hubble Telescope.

The idea is to enhance the resolution of the stacked image compared to the resolution of a single image. This is possible as each sub is supersampled and projected to a finer pixel grid before being stacked.

This means an object that in each sub was only a few pixels across, in the drizzled image, will take twice or even trice that amount of pixels depending on the settings used.

Undersampling / Oversampling

Each combination of camera and lens (or telescope) has a certain optical resolution. 

Assuming perfect seeing conditions, the diameter of a star in the image is determined by the focal length of the lens or telescope, while the number of pixels used to record the image that stars depends on the sensor (pixel sizes and pixel density).

With undersampling, you are using too few pixels to represent the star, which therefore assumes an angular and squared shape. With Oversampling, you are using more pixels to record the star, which will now appear nicely round.

A good sampling is a must to have the best possible amount of details in the image. Of course, seeing conditions (the ensemble of factors such as air turbulence, wind, haze, etc.) can affect the amount of details one can record.

This website allows you to calculate the optical resolution for your setup.

Dither

Dither is a method to obtain cleaner images than simply stacking images together. 

When dithering, the mount slightly moves the camera in a random way between subs. This has the effect of moving hot pixels and other fixed pattern noises with respect to the stars. 

When the stars are aligned for image stacking, those kinds of noisy pixels are averaged out. 

The SkyWatcher Star Adventurer Mini offers dithering without the need of adding a guiding system and a computer. If you want to know more, you can read our review here.

Q: “ What telescope should I get?”

A: “It depends on what you want to do. If you want to observe only, for the money, your best bet is a Dobson. While not ideal for astrophotography, you may also be able to snap some images of the moon and planets. 

For planetary and lunar imaging, Maksutov telescopes provide the most contrasted and sharp view, but they have a very long focal length and a rather small aperture. Thus, they are rather dark and not suitable for deep sky.

For deep-sky better an apochromatic refractor, fast and very well optically corrected. You probably need to have a flattener/reducer too and mind the back focus.

Then there is the mount you put your telescope on: for observing and doing moon and planetary astrophotography, an Alt/Az mount is fine, but for deep-sky, you need at least a tracker or, better, an equatorial mount.”

Dobson / Maksutov / Cassegrain / Refractor / Newton

These are all types of telescopes that differ for optical design, size, weight, and application. With the exception of refractors, which work on the same principle of photographic lenses, all other telescopes are reflectors, using mirrors to collect light and form the image.

Maksutov are catadioptric telescopes that use a combination of mirrors and lenses and are great for observing/photographing the moon and other bright targets (planets, double stars, and star clusters).

Dobson, are usually newtonian telescopes that are mounted on a rockbox or tabletop alt/az mount: large aperture dobson are common among those into visual observation as they provide a bright view of the sky. 

They are also called light buckets, and all considered they are easy to use and rather affordable.

Newtonian telescopes are usually considered to be all-rounder telescopes suitable for planetary and deep-sky astrophotography, depending on the mount they are mounted on.

Refractors are the most hassle-free telescopes (no collimation issues, negligible cooling time, sharp) but are expensive. The apochromatic ones use a number of low dispersion glass to control purple fringe and chromatic aberration better than achromatic refractors.

Flattener / Reducer

Optical element to be used between the camera to flatten the field of view and have round stars from corner to corner (field flattener) and to reduce the focal length of the lens or telescope (reducer).

Flatteners are most used with refractor telescopes. 

Back Focus

With back focus, we mean how far back the telescope can focus the image, as they are designed to work with diagonals and eyepieces for visual observation.

If the back focus is very short, you cannot focus the image on a camera sensor, and the only (easy) option is to shoot with your telephone at the eyepiece (afocal astrophotography) using an adapter.

If the back focus is too long, you need extension tubes to image with your camera mounted in the place of the eyepiece (direct or prime focus).

Maksutov telescopes have a wide focus range and will let you achieve focus in almost any setup.

Aperture

Loosely speaking, the aperture is the diameter of your telescope. With photographic lenses, things get more technical, but you can read more in this article.

Together with the instrument focal length, aperture determines the brightness of the view (f-ratio) and also the resolution of the image.

As shown in the image below, the bigger the aperture, the brighter the image, and the higher the resolution.

Alt/Az / Equatorial Mounts

In astrophotography, there are two kinds of mounts: Alt/Az and equatorial mounts. 

Alt/Az mounts track the sky by moving the camera in altitude and azimuth, but can’t correct for field rotation. Long exposures can’t be done with them.

An equatorial mount will make the camera revolve around the celestial pole, thus allowing it to track the sky while correcting for the field rotation.

The two schemes below illustrate the difference between the mounts’ working principles.

If you are into deep sky astrophotography, getting an equatorial mount is the way to go. 

List Of Acronyms Used In Astrophotography 

Astrophotography has a rich language, made of technical terms and obscure acronyms. To help you decipher what you read, here is a list of the most frequently used acronyms in astrophotography.

– A –

ADC = Atmospheric Dispersion Corrector (Optical Element)

AF =Auto Focus

Alt = Altitude

Alt/Az = Altitude/Azimuth (Type of Mount)

AP = Astrophotography

APO = Apochromatic Refractor (Telescope Type)

APP = Astro Pixel Processor (Software)

APS-C = Type of Camera or Sensor

APT = Astro Photography Tool (Software)

AS/AS!3 = Autostakkert! / Autostakkert! 3 (Software)

ASI = ZWO camera models

AVX = Advance VX mount (Celestron)

Az = Azimuth

– B –

BG = Background

– C –

CA = Chromatic Aberration

CASS = Cassegrain (Telescope Type)

CATS = Catadioptric (Telescope Type) 

CCD = Charge-Coupled Device (Camera Sensor Type)

CGEM = Computerised German Equatorial Mount (Type of Mount)

CLS = LP filter from Astronomik

CMOS = Complementary Metal Oxide Semiconductor (Camera Sensor Type)

CN = Cloudy Nights (Forum)

– D –

DARV = Drift Alignment by Robert Vice (Alignment Method)

DBE = Dynamic Background Extraction (PixInsight Tool)

DEC = Declination 

DOB = Dobson Telescope (Type of Telescope)

DR = Dynamic Range

DSLR = Digital Single Lens Reflex (Type of Camera)

DSO = Deep Sky Object (Targets such nebulae and galaxies)

DSS = Deep Sky Stacker (Software)

– E –

EAA = Electronically Assisted Astrophotography

ED = Extra-Low Dispersion (Glass Type for Lenses)

EFL = Equivalent Focal Length (refers to the FL needed on FF camera to reproduce the same FOV than a given lens on smaller sensors)

EN = Emission Nebula

EP = EyePiece (Optical Element)

EQ = Equatorial (Type of Mount)

EVF = Electronic Viewfinder

– F –

f / = F-stop or Aperture Ratio

FF = Full Frame (Type of Camera or Sensor) or Field Flattener (Optical Equipment)

FL = Focal Length 

FLO = First Light Optics (Web Shop Site)

FOV = Field Of View

FPL-53 = Type of ED glass

FPN = Fix Pattern Noise

FPS = Frame Per Second 

FR = Focal Reducer (optical Equipment)

FRAC = Refractor (Type of Telescope)

– G –

GEM = German Equatorial Mount (Type of Mount)

GoTo = Automatic aiming system available on computerized mounts. It is based on a celestial coordinates database.

– H –

Ha/Hb = Hydrogen Alpha / Hydrogen Beta (Narrowband Filter)

HDR = High Dynamic Range

– I –

IDAS = Filters Brand

IR = Infrared

IS = Image Stabilisation 

ISO = Something related to the old film speed. In digital sensors, it does not increase sensor sensitivity to light, just electronically amplifies the signal (both noise and data)

ISS = International Space Station

– L –

LENR = Long Exposure Noise Reduction (In-camera noise reduction by dark frame subtraction after each shot) 

LP = Light Pollution

LPR = Light Pollution Reduction (Filter)

LPS = Light Pollution Suppression (Filter)

LR = Lightroom (Software)

LRGB = Luminance, Red, Green, Blue

LV = Live VIew

– M –

MAK = Maksutov (Telescope type)

MC = Multi Coated

MF = Manual Focus

MFT/M43 = Micro Four Thirds (Type of Camera or Sensor)

ML = Mirrorless (Type of Camera)

MW = Milky Way

M# = Messier Object (e.g., M45 = Pleiades)

– N –

NB = Narrowband (typically referred to imaging with NB filters)

NEWT = Newtonian Telescope (Type of Telescope)

NGC# = New General Catalogue of Nebulae and Clusters of Stars (Target designation alternative to the Messier catalogue)

NR = Noise Reduction

– O –

OAG = Off Axis Guide

OIII = Oxygen III (Narrowband Filter)

OPT = Web Shop Site

OSC = One Shot Camera (Typically a colour, RGB, camera)

OTA = Optical Tube Assembly (Telescope Tube)

– P –

PA = Polar Alignment

PE = Periodic Error (Mount/Tracking)

PF = Polar Finder

PEC = Periodic Error Correction 

PHD = Push Here Dummy (Guiding Software)

PI = PixInsight (Software)

PIPP = Planetary Imaging PreProcessor (Software)

PS = Photoshop (Software) or Polar Scope (Optical Device)

– Q –

QE = Quantum Efficiency % (Camera Sensor)

– R –

RA = Right Ascension

RC = Ritchey Chrétien (Telescope Type)

RGB = Red, Green , Blue

RF = Reflection Nebula

RN = Read Noise (Camera Sensor)

– S –

SA = SkyWatcher Star Adventurer (Tracking Mount)

SII = Sulfur II (Narrowband Filter)

SC/SCT = Schmidt–Cassegrain / Schmidt–Cassegrain Telescope (Telescope Type)

SGC =Sequence Generator Pro (Software)

SQM = Sky Quality Meter (Measure for Sky Darkness)

SS = Shutter Speed

ST = Star Tools (Software)

SW = SkyWatcher (Brand)

– T –

T2 = Type Of Connector

TS = Telescope Service (Brand)

– U –

UHC = Ultra High Contrast (Filter)

– W –

WB = White Balance

WO = William Optics (Brand)

– Z –

ZWO = ZWO (Brand)

– # –

1.25″ = Diameter (in inches) for Filter or Optical Device 

2″ = Diameter (in inches) for Filter or Optical Device

“/px = arcsec/pixel, it refers to optical resolution

Conclusion

Astrophotography is a thought field to play in, filled with technicalities and obscure vocabulary. 

In this article, I presented a comprehensive list of astrophotography terms and acronyms you will likely read when researching how to do things to improve your astrophotography, or when seeking shopping advice for your new upgrade.

I hope this will help you to get a better understanding of the field of astrophotography.