Astrophotography Glossary: Essential Terms Explained
This astrophotography glossary explains the terminology used in deep-sky imaging, from telescope specifications and tracking to calibration frames and image processing. It is designed to help beginners understand the equipment, techniques and processes involved in capturing astronomical images.
The West Coast Cosmos astrophotography glossary explains the key terms used in deep-sky imaging, from exposure times and guiding to filters, calibration frames and image processing. Whether you’re new to astrophotography or looking to understand a particular technique, this glossary provides clear explanations of the terminology.
Telescope & Optical Equipment
Understand telescope specifications, focal length, aperture and the filters used to capture different wavelengths of light.
Cameras & Image Capture
Learn about cameras, exposure settings and the light and calibration frames used to capture accurate images
Mounts, Tracking & Guiding
Understand equatorial mounts, polar alignment, tracking, guide scopes and the corrections that keep stars sharp.
Stacking & Calibration
Discover how multiple exposures and calibration frames are combined to improve detail and reduce unwanted noise
Processing & Image Quality
Explore stretching, gradients, noise reduction, masks and the techniques used to bring out detail in astrophotographs.
The Night Sky
Learn about celestial coordinates, light pollution, atmospheric conditions and the objects we photograph.
Telescope & Optical Equipment
Aperture
Think of aperture as the size of the pupil of your telescope. The wider it is, the more light it collects, allowing you to capture fainter objects and potentially reveal more detail.
Aperture is measured in millimetres (mm). The important thing to remember is that a larger aperture number means a bigger opening and more light collected. For example, a telescope with a 100mm aperture collects approximately four times as much light as a 50mm aperture, assuming similar optical efficiency.
Don’t confuse aperture with focal ratio, where a smaller number can mean a faster optical system.
Focal Length
Think of focal length as how zoomed in your telescope makes the sky appear. The larger the number, the smaller the area of sky you’ll capture, but the larger objects will appear in your image.
Focal length is measured in millimetres (mm). A shorter focal length gives you a wider field of view, allowing you to capture larger areas of the sky. A longer focal length gives you a narrower field of view, making smaller objects appear larger.
For example, a telescope with a focal length of around 400mm is often a good choice for large nebulae, while a longer focal length can be useful for smaller galaxies and planetary nebulae. These are general guidelines rather than strict rules; the size of your camera sensor also affects your field of view.
Before buying a telescope, think about the objects you want to photograph. Large nebulae often benefit from a wide field of view, whereas smaller galaxies may need a longer focal length to show their detail.
There is another important consideration: the longer your focal length, the more demanding accurate tracking and guiding generally become. Small tracking errors become more visible, and stars can appear elongated more easily. This is worth considering when choosing your first astrophotography setup.
Narrowband Filters
Certain targets, particularly emission nebulae, emit light at specific wavelengths. This is known as emission, and some of these wavelengths are especially useful for astrophotography.
Narrowband filters are designed to pass a relatively narrow range of wavelengths while blocking much of the other light. This allows you to emphasise particular emissions from nebulae, such as hydrogen-alpha (H-alpha) and oxygen III (OIII).
Narrowband imaging can be particularly useful under light-polluted skies because the filter reduces much of the unwanted background light. However, it doesn’t eliminate light pollution completely, and results depend on the target, filter and sky conditions.
Broadband Filters
Broadband imaging captures a much wider range of visible wavelengths, producing a more natural representation of the colours emitted or reflected by an object.
This is commonly used for galaxies, star clusters and reflection nebulae. Stars can display a range of natural colours, depending on their temperature and other properties.
Broadband imaging is often a good choice when you want to capture the overall colour and appearance of an object rather than isolate particular emission lines. However, light pollution and the brightness of the Moon can affect the results.
H-alpha (Hydrogen Alpha)
You’ll often see H-alpha mentioned when reading about astrophotography filters and emission nebulae.
H-alpha is a specific wavelength of red light emitted by hydrogen, at approximately 656.3 nanometres. Some emission nebulae produce a strong amount of this light.
A suitable H-alpha filter allows this wavelength through while blocking much of the surrounding light. This can make the hydrogen-rich structures in a nebula stand out more clearly in your image.
H-alpha is particularly important in narrowband astrophotography, where it is often combined with other wavelengths, such as OIII, to reveal different details within the same target.
Cameras & Image Capture.
Sensor
The sensor is the part of your camera that collects light and turns it into digital data. Think of it as the digital equivalent of the film in an old film camera.
Sensor size affects how much of the sky you can capture through your telescope. Resolution and pixel size also influence the detail in your images.
For example, my ZWO ASI2600MC Pro has an APS-C sensor measuring approximately 23.5 × 15.7mm, with a resolution of 6248 × 4176 pixels. That’s a considerably larger sensor than the tiny sensor found in a typical smartphone camera, although phone sensors vary significantly between models.
A larger sensor generally captures a wider field of view when used with the same telescope. This is particularly useful when photographing large nebulae or galaxies.
Cooled Camera
When photographing faint objects in space, we often need to use longer shutter speeds, known as exposures, so the camera collects enough light. Think of it as keeping your eye open for longer to let more light in.
The problem is that camera sensors generate heat, and this produces unwanted signal known as thermal noise. This can make an image look grainy or introduce unwanted patterns, particularly during long exposures.
A cooled astrophotography camera uses active cooling to lower the sensor temperature, helping to reduce thermal noise and produce cleaner images.
Cooling also makes calibration much easier because you can set the camera to a consistent temperature. This allows you to capture dark frames at different exposure lengths and temperatures, then reuse them in future imaging sessions when the settings match.
Gain
Gain is similar to ISO on a conventional camera. It controls how strongly the camera amplifies the signal collected by its sensor.
Increasing gain can help faint details become more distinguishable from certain types of noise, but it can also reduce the amount of bright detail the sensor can record before reaching its maximum value.
The best setting depends on your camera, target and imaging conditions. A higher gain isn’t automatically better.
Offset
Offset, sometimes called the black level, sets a baseline for the camera’s recorded pixel values. It helps prevent the darkest parts of an image from being clipped to zero, which would lose information.
The appropriate setting depends on the camera and capture software. If the offset is too low, shadow detail can be lost. I left this section in because it seemed important, but I genuinely have no idea what this means. I’ve never had to worry about it.
Total Integration Time
Total integration time is the combined length of all the individual exposures (subs) you’ve taken of an object. The longer you spend collecting light, the more faint detail you can generally reveal in your final image.
For example, if you take 120 images, each with an exposure of 180 seconds, your total integration time is 6 hours.
You don’t have to capture all of this in one night. You can photograph the same target over several nights and combine the images, provided your framing and capture settings are compatible.
Generally, more integration time helps reveal fainter structures and makes the final image cleaner. However, the improvement isn’t always immediately noticeable. Going from one hour to two hours can make a meaningful difference, while increasing from four hours to five hours may produce a more subtle improvement.
As a general rule, doubling your total integration time helps you bring out fainter details and reduces the appearance of random graininess. The improvement depends on the target, sky conditions, camera, filters and processing.
Sub-exposure (Subs)
A sub-exposure, often shortened to a sub, is one individual photograph taken during an imaging session. Instead of taking one extremely long exposure, astrophotographers capture many shorter exposures and combine them through stacking.
For example, 60 subs at 120 seconds each give you two hours of total integration time, excluding download and other delays.
The length of each sub will depend on several factors, including the target, your camera, your mount and the quality of your guiding. Longer exposures aren’t always better if tracking errors cause the stars to become elongated.
Light Frame
A light frame is an actual photograph of your astronomical target. It contains the object you’re trying to capture, along with stars, background sky, noise and any optical imperfections.
These are the images you’ll eventually calibrate, align and stack to create your final photograph.
Calibration Frames
Calibration frames are supporting images that help the stacking software correct unwanted patterns and imperfections in your light frames.
Dark frames help correct thermal signal and certain sensor patterns. Flat frames correct uneven illumination, vignetting and dust shadows. Bias frames, or dark-flat frames, help account for the camera’s readout signal when calibrating flats.
These images help the software distinguish the astronomical signal you want to keep from unwanted artefacts you want to reduce. When calibration is done correctly, the result is a cleaner starting point for stacking and processing.
Dark Frame
A dark frame is taken with the camera covered so that no light reaches the sensor. It records unwanted thermal signal and certain sensor patterns that can be corrected during calibration.
With an uncooled camera, it is important to capture dark frames at the same exposure duration and as close as possible to the same sensor temperature as your light frames. This helps the stacking software recognise and subtract matching patterns caused by thermal signal.
In practice, this can mean extending your imaging session to take dark frames immediately after your light frames, while the camera is at a similar temperature. Around 30 dark frames is often a reasonable starting point, although the ideal number depends on your workflow.
With a cooled camera, this process is much easier. You can set the sensor to a specific temperature and build a library of dark frames at different exposure lengths and temperatures. You can then reuse matching dark frames in future sessions without having to capture them every time.
Flat Frame
A flat frame helps correct uneven illumination, vignetting and shadows caused by dust on your telescope, filters or camera sensor window.
For my own setup, I use an old tablet with its brightness turned up to maximum and a plain white image displayed on the screen. I hold it in front of the telescope and use the ASIAIR to capture flat frames, allowing it to determine suitable exposure settings.
The aim is to produce an evenly illuminated image of the optical system, including any dust shadows. During calibration, the stacking software uses the flats to identify and correct these patterns in the light frames.
Flats should be captured with the same optical configuration as your light frames. Avoid moving or cleaning the imaging train between sessions if you intend to reuse flats.
Bias Frame
A bias frame is a very short exposure taken with the camera covered. It records the camera’s electronic readout offset and certain fixed sensor patterns.
Bias frames are often taken using the camera’s shortest practical exposure. They are not normally taken at the same shutter speed as your flat frames.
If your flats require longer exposures, you may instead use dark-flat frames. These are taken with the camera covered, using the same exposure duration and sensor temperature as the flats. They can then account for the sensor’s dark signal during flat calibration.
Dithering
Dithering is when your imaging software moves the telescope’s pointing position by a small amount between exposures. This shifts the target slightly across the camera sensor, so fixed sensor patterns and other unwanted artefacts fall in different places in successive images.
When the images are stacked, these artefacts can be reduced, improving the final image.
I generally set my ASIAIR to dither every three images and leave it to run automatically. Once configured correctly, you don’t need to intervene during every dither.
Signal-to-Noise Ratio (SNR)
Signal-to-noise ratio describes how clearly the light from your target stands out from the unwanted graininess and variation in your image.
Think of it this way: the signal is the light coming from the nebula or galaxy you’re photographing. The noise is the unwanted graininess that makes it harder to see the faint details.
When you photograph a faint nebula, its light can be very weak compared with the noise in your image. This can make the nebula’s fainter structures difficult to see, particularly when you start processing the image.
One of the best ways to improve this is to increase your total integration time by capturing more exposures and stacking them together. This helps the genuine details build up while random noise becomes less noticeable.
This is why an image captured over six hours will often show cleaner backgrounds and fainter detail than one captured over just one hour, assuming similar conditions.
In simple terms, the better the signal-to-noise ratio, the easier it is to see faint details without the image looking excessively grainy.
Plate Solving
Plate solving identifies the telescope’s exact position in the night sky by matching the stars in an image to a star catalogue.
In the ZWO ASIAIR, select a target and use the GoTo function to slew towards it. Once the telescope takes an image, plate solving identifies where it is pointing. The ASIAIR then corrects the telescope’s position to centre the target accurately. You can also use plate solving to refine framing and return to the same composition across multiple imaging sessions.
It is particularly useful because it removes the need to rely solely on the mount’s initial pointing accuracy.
Mounts, Tracking & Guiding
Mount
The mount is what your telescope attaches to. It sits on a sturdy tripod and is responsible for moving your telescope to find and track objects in the sky.
For deep-sky astrophotography, a suitable tracking mount is essential. Without one, the stars will drift across your camera’s view as the Earth rotates, making long exposures impossible without star trails.
Right Ascension (RA)
Your mount moves in two directions: Right Ascension (RA) and Declination (Dec).
Think of RA as the movement that follows the sky as it appears to move from east to west. When you’re looking at your telescope’s view of the sky, this can feel a little like moving the view from side to side, although the exact direction depends on how your telescope and camera are oriented.
The RA axis is the main tracking axis of an equatorial mount. Once your mount is correctly aligned, it rotates around this axis to follow your target as the Earth rotates.
Declination (Dec)
Declination is the other direction in which your equatorial mount moves. You can think of it as moving the telescope towards the celestial north or south.
In a typical setup, this may feel a little like moving the view up or down, although the direction on your camera screen depends on your framing and orientation.
RA and Dec work together to let you point your telescope at different objects in the sky. RA then does most of the work when tracking a target during an exposure.
Polar Alignment (PA)
Polar alignment is the process of aligning your mount’s RA axis with the Earth’s rotational axis. In the Northern Hemisphere, we use Polaris, the North Star, as a nearby reference point.
Polaris appears to stay almost in the same place in the sky while other stars appear to move around it. It isn’t exactly at the north celestial pole, so simply pointing your telescope at Polaris isn’t accurate enough.
Once your mount is correctly polar aligned, it can track objects across the sky much more accurately. Tools such as the ASIAIR can help you make the small adjustments needed to achieve good alignment.
Guiding
Guiding helps your mount keep tracking your target accurately during an imaging session.
A small guide scope and guide camera monitor stars in the sky. After the guiding software has calibrated the system for that session, it measures how those stars move and sends small corrections to the mount when needed.
This helps compensate for small tracking errors, allowing you to take longer exposures while keeping stars round and your target accurately framed.
Guiding doesn’t make the mount perfect, and it can’t correct every possible problem, but it can make a substantial difference to your images.
Backlash
Backlash is a small amount of mechanical play between the gears inside your mount. When the mount changes direction, there can be a slight delay before the gears engage and the movement takes effect.
You don’t need to worry about this too much when starting out. Guiding software can sometimes compensate for backlash, particularly in Declination, depending on the mount and settings. However, excessive backlash can still cause guiding problems, so it’s worth understanding if you start experiencing persistent tracking errors.
Stacking & Calibration
Stacking
Stacking is the process of combining multiple photographs of the same target into one image. Instead of relying on a single exposure, your software combines many individual images to bring out more detail and reduce unwanted graininess.
For example, you might capture 100 images of a nebula, each lasting 180 seconds, and combine them into one image. The software aligns the stars and combines the information from each exposure.
Stacking is one of the most important steps in deep-sky astrophotography because it helps reveal faint structures that would be difficult to see in a single photograph.
Calibration
Calibration is the process of correcting unwanted patterns and imperfections in your images before or during stacking.
Your camera and optical equipment can introduce things you don’t want in the final image, such as dust shadows, uneven illumination and heat-related noise. Calibration frames help your software recognise and correct these issues.
The main types are dark frames, flat frames and bias frames, although dark-flat frames are also commonly used. Each type corrects different problems.
Registration
Registration is the process of aligning your individual images so that the stars and other features line up correctly.
Even if your mount tracks well, the framing can shift slightly between exposures or across different nights. Registration software detects the stars and adjusts the images so they can be combined accurately.
This is especially important when you collect your data over several nights.
Rejection
When stacking images, the software can identify and reject certain unwanted pixels or features that appear in only some exposures.
For example, a satellite may pass through one image, or an aircraft may leave a trail across the frame. With enough suitable exposures, the stacking software can often reject these unwanted features when combining the images.
Rejection works best when the unwanted feature is not present in most of the individual images.
Master Calibration Frame
A master calibration frame is created by combining several calibration images of the same type.
For example, multiple dark frames can be combined into a master dark. Combining them helps reduce random variation in the calibration data, giving the software a more reliable reference when correcting your light frames.
You can then reuse a master calibration frame when the camera, exposure settings and other relevant conditions match.
Integration
Integration is another term used for combining multiple exposures into a single image. In astrophotography software, it often refers to the process of stacking the aligned images to produce the combined result.
The more suitable data you integrate, the more faint detail you can generally reveal. The quality of the original images, sky conditions and calibration also affect the result.
Processing & Image Quality
Image Processing
Image processing is the stage where you turn your stacked image into the finished photograph. The initial stacked image often looks dark, flat and lacking in colour because much of the information is difficult to see at first.
Using software such as PixInsight, you can reveal faint nebulae, improve colour, reduce unwanted graininess and bring out details in galaxies and other objects.
The aim is to reveal the information captured by your camera without introducing unwanted artefacts or making the image look unnatural.
Stretching
Stretching is the process of making faint details in an image more visible. A freshly stacked image often contains a lot of information, but much of it is hidden in the darker tones.
Stretching adjusts the brightness of the image so that faint nebulae, galaxies and dust structures become easier to see.
Too little stretching can leave an image looking dark and faint. Too much can make the background look excessively bright, increase the appearance of noise or cause bright areas to lose detail.
Gradient
A gradient is an uneven change in brightness or colour across an image. For example, one side of your image might look brighter or more orange than the other.
Gradients can be caused by light pollution, moonlight, atmospheric effects or reflections and stray light in your optical setup.
Processing tools can help remove unwanted gradients, making the background more even. However, care is needed because real nebulae and dust clouds can also create large-scale changes in brightness.
Noise Reduction
Noise reduction is used to reduce unwanted graininess and other random variations in an image.
Noise can be particularly noticeable in faint areas of a nebula or galaxy, especially when you stretch the image to reveal more detail.
Processing software can help smooth this noise, but too much noise reduction can remove genuine detail and make the image look unnaturally smooth.
Capturing more good-quality data is often the best starting point for reducing noise.
Masks
A mask allows you to apply a processing adjustment to selected parts of an image while protecting other areas.
For example, you might want to increase the brightness of a faint nebula without making the stars brighter, or adjust the colour of a galaxy without changing the surrounding background.
Masks are useful because they give you more control over individual parts of an image rather than applying every adjustment to the entire photograph.
Curves
Curves is a processing tool that lets you adjust brightness, contrast and colour.
You can use it to make a nebula more visible, increase contrast in a galaxy’s dust lanes or adjust the colours of stars.
Small, controlled adjustments often produce better results than making large changes all at once.
Colour Calibration
Colour calibration is the process of adjusting an image so that its colours are represented more accurately.
Astrophotography cameras, filters and processing techniques can affect the colour balance of an image. Calibration helps correct these differences so that stars and other objects display more natural-looking colours.
In PixInsight, tools such as Spectrophotometric Colour Calibration (SPCC) can use information about the stars in an image to help establish an appropriate colour balance.
Star Reduction
Star reduction is a processing technique used to make stars appear smaller or less dominant in an image.
This can help reveal the surrounding nebula or dust structures that might otherwise be overwhelmed by bright stars.
The aim is to balance the stars with the rest of the target, rather than remove them or make them look unnatural.
Dynamic Range
Dynamic range describes the range of brightness levels an image can represent, from its faintest details to its brightest areas.
In astrophotography, a galaxy may contain a very bright core surrounded by much fainter outer arms. Preserving detail in both areas can be challenging because bright regions can become washed out while faint regions remain difficult to see.
Careful exposure planning and processing can help retain detail across these different brightness levels.
Clipping
Clipping occurs when image brightness or colour values reach the limits of what the image can represent, causing detail to be lost.
For example, if you stretch a bright galaxy core too aggressively, it can become a featureless white area. Similarly, making the background too dark can crush faint details into black.
Careful processing helps preserve detail in both the brightest and darkest parts of your image.
Artefacts
Artefacts are unwanted features introduced during image capture, calibration or processing.
Examples include dark rings around stars, strange colour patches, halos, overly smooth areas or patterns that don’t belong to the astronomical object.
Artefacts can be caused by incorrect calibration, excessive processing or limitations in the original data. If you notice them, it can help to revisit earlier processing steps and make smaller adjustments.
File Types: FITS, TIFF and JPEG
You’ll come across different file types when capturing and processing astrophotographs. Choosing the right one matters because some formats preserve all the image data, while others discard information to reduce file size.
FITS (.fits)
FITS stands for Flexible Image Transport System. It is a format commonly used by astronomy cameras and astrophotography software to store captured images and their data.
FITS files are typically used for your original light frames and calibration frames. They preserve the captured pixel values without the lossy compression used by JPEG, making them suitable for calibration and stacking.
TIFF (.tif or .tiff)
TIFF is commonly used to save processed images while retaining high image quality. It can support high bit depths, allowing it to preserve subtle differences in brightness and colour that are important when editing an astrophotograph.
For example, you might export a finished image from PixInsight as a 16-bit TIFF and open it in another editing application without throwing away as much image information as you would with a JPEG.
JPEG (.jpg or .jpeg)
JPEG is a widely used image format designed to keep file sizes relatively small. It achieves this by discarding some image information, a process known as lossy compression.
JPEG is useful for sharing your finished astrophotographs on websites and social media. However, it is generally a poor choice for your original capture data or for images that you intend to process extensively, because compression can introduce unwanted artefacts and discard subtle detail.
Which should you use?
As a general rule, keep your original capture files in FITS format, save your processed master images in a high-quality format such as TIFF, and export a JPEG when you want to share your finished photograph online.
Keeping your original files means you can return to the data and process it again in the future without having to start from a compressed image.
The Night Sky & Observing Conditions
Bortle Scale
The Bortle scale describes how dark your night sky is, based on the amount of light pollution. It runs from Class 1, representing exceptionally dark skies, to Class 9, representing heavily light-polluted city skies.
Light pollution makes astrophotography more difficult because artificial light brightens the background of your images, making faint nebulae and galaxies harder to distinguish.
I image from skies around Bortle Class 4. If you live in a Bortle 9 city, you may need considerably more total integration time to achieve a comparable result, particularly when photographing galaxies and other objects using broadband imaging.
As a rough illustration, an image that takes 4 hours under Bortle 4 skies might require 20 hours or more under heavily light-polluted skies to reveal a comparable level of faint detail. The actual difference can be smaller or much larger, depending on the target, camera, filters and sky brightness.
This doesn’t mean you can’t take excellent astrophotographs from a city. Emission nebulae can be particularly suitable for urban imaging when using narrowband filters, which block much of the unwanted background light.
The main lesson is that darker skies give you an advantage, but good equipment, suitable filters and patience can still produce impressive results under light-polluted skies.
Sky Quality (SQM)
Sky Quality Meter readings, usually shortened to SQM, provide another way of measuring how dark the sky is. The reading is expressed in magnitudes per square arcsecond.
Unlike the Bortle scale, which uses nine broad classes, SQM provides a numerical measurement. The higher the SQM reading, the darker the sky generally is.
For example, a reading of 21.5 indicates a darker sky than a reading of 19.0. These readings can help you compare observing locations or monitor how conditions change during the night.
My own sky has measured around 19.77 magnitudes per square arcsecond. This gives me a useful reference when comparing different imaging sessions.
Dew Point
The dew point is the temperature at which moisture in the air begins to condense into liquid water.
When your telescope, camera window or lens cools to the dew point, moisture can form on the surface. This is known as dew, and it can spoil your images by reducing contrast or making stars look blurred.
Dew is particularly common on clear nights because equipment can cool below the surrounding air temperature. Dew heaters can help prevent moisture from forming on your optics.
Checking the dew point and air temperature before an imaging session can help you decide whether to use dew heaters.
Seeing
Seeing describes how much the Earth’s atmosphere distorts the light coming from stars and other objects. When seeing is poor, stars can appear to shimmer or blur. Good seeing helps preserve fine detail, particularly when photographing small galaxies or planetary nebulae at longer focal lengths.
What weather conditions give you good seeing?
There isn’t one guaranteed weather pattern, but some conditions can offer better chances:
Light winds and a relatively calm atmosphere: strong air currents and turbulence can distort starlight.
A settled weather pattern: high pressure can sometimes bring calmer conditions, although high pressure doesn’t automatically mean good seeing.
A weaker jet stream overhead: fast-moving winds high in the atmosphere can create turbulence, so checking a jet-stream forecast can be useful.
A telescope that has cooled to the outdoor temperature: warm air moving around inside or near your telescope can blur the image, even when the atmosphere itself is relatively calm.
Avoiding heat sources: roofs, patios and buildings that release heat after sunset can create local air currents that affect image quality.
Clear skies alone don’t guarantee good seeing. A perfectly clear night can still have turbulent air, while a slightly hazy night can sometimes have steadier conditions.
For deep-sky astrophotography, seeing is only one part of the picture. Transparency, light pollution, tracking and total integration time also affect your final image.
Transparency
Transparency describes how clearly light passes through the atmosphere. Haze, mist, thin cloud and airborne particles can reduce transparency, even when the sky appears mostly clear.
Good transparency is important for capturing faint galaxies and nebulae because less of their light is lost or scattered before it reaches your telescope.
A night can have good seeing but poor transparency, or excellent transparency but poor seeing. They describe different aspects of the atmosphere.
Emission Nebulae
Emission nebulae are clouds of gas that glow because their atoms have been energised, often by nearby hot stars. These nebulae emit light at particular wavelengths, including hydrogen-alpha (H-alpha) and oxygen III (OIII).
Examples include the North America Nebula and the Elephant Trunk Nebula
Emission nebulae are popular targets for narrowband astrophotography because suitable filters can emphasise their glowing gas and reveal structures that might otherwise be difficult to capture, especially under light-polluted skies.
Reflection Nebulae
Reflection nebulae are clouds of dust that reflect light from nearby stars rather than producing their own visible glow in the same way as emission nebulae.
They often appear blue because dust scatters shorter wavelengths of visible light more efficiently than longer wavelengths.
The reflection nebulosity surrounding the Pleiades is a well-known example.
These objects are often better suited to broadband imaging because their appearance depends on reflected starlight across a wider range of wavelengths. Narrowband filters can suppress much of this light.
Dark Nebulae
Dark nebulae are dense clouds of gas and dust that block the light from stars or brighter nebulae behind them. They appear as dark shapes against a brighter background.
The Dark Shark Nebula is an example of this type of target.
These objects can be challenging because their detail is often revealed by the contrast between the obscuring dust and the surrounding stars or glowing gas. Broadband imaging and dark skies can be particularly helpful.
Planetary Nebulae
Despite their name, planetary nebulae have nothing to do with planets. They are shells of gas expelled by stars during the later stages of their lives.
They are often relatively small in the sky, which means a longer focal length can help show their structure in greater detail.
Many planetary nebulae emit strongly at specific wavelengths, including OIII, so suitable narrowband filters can be useful for capturing them.
Supernova Remnants
Supernova remnants are the expanding clouds of material left behind after a star explodes as a supernova. They can contain intricate filaments of glowing gas and shocked material.
The Veil Nebula is a well-known example.
Many supernova remnants emit strongly in H-alpha and OIII, making them excellent targets for narrowband imaging. Their faint structures often benefit from several hours of integration time.
Galaxies
Galaxies are enormous collections of stars, gas, dust and other material held together by gravity. Our own Milky Way is a galaxy, and objects such as the Andromeda Galaxy are separate galaxies beyond it.
Unlike many emission nebulae, galaxies often emit light across a broad range of visible wavelengths. Broadband imaging is therefore commonly used to capture their stars, spiral arms, dust lanes and overall colour.
Large galaxies can be photographed with shorter focal lengths, while smaller or more distant galaxies may benefit from longer focal lengths to show more detail.
Star Clusters
Star clusters are groups of stars held together by gravity or sharing a common origin.
Open clusters, such as the Pleiades, contain relatively young stars spread across a wider area of sky. Globular clusters are much denser, often appearing as compact, spherical collections of hundreds of thousands of stars.
Many star clusters are well suited to broadband imaging because the colours and brightnesses of their individual stars are important parts of the final image.
Field of View (FOV)
Field of view describes how much of the sky your camera captures in one image. It depends on both the telescope’s focal length and the size of the camera sensor.
A wider field of view is useful for large objects such as the North America Nebula, while a narrower field of view can be better for smaller targets such as compact galaxies or planetary nebulae.
Before choosing a target, it is worth checking how it will fit into your camera’s field of view. This can help you avoid cropping important parts of the object or leaving too much empty space around it.