Understanding Common Astrophotography Terms

If you are new to astrophotography, you might come across some unfamiliar terms and concepts that can be confusing. When I first got into this hobby, I found myself constantly searching on Google. Unfortunately, this only added to my frustration because there were so many search results to sift through, most of which were written in language that was too technical for beginners. If you are just starting astrophotography these definitions will help you understand the basic terminology used in astrophotography and astronomy.

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Aperture

Aperture refers to the amount of light that can pass through an optical or photographic instrument. Think of it like a pupil. When it is dark, your pupil expands, allowing more light to hit the retina. On a bright sunny day, your pupil shrinks to reduce the light entering your eyes.

Aperture of a camera lens

Pictures of nightscapes or the Milky Way are often captured using a camera and lens. In this scenario, the aperture refers to the opening in the lens that lets light into the camera. In most lenses, this is adjustable. The aperture of a lens is measured in f-stops. A higher f-number indicates a smaller aperture opening. For example, an aperture of f/1.4 allows a lot of light to pass through, while an aperture of f/32 allows very little light.

The aperture setting has a significant impact on your photographs in two main ways:

• Exposure: The size of the aperture determines the amount of light that reaches the camera sensor, affecting the brightness of your image.

• Depth of Field: The aperture can add dimension to your photos by controlling the depth of field. A wide aperture (small f-number) gives a shallow focus effect with a blurred background, which is popular for portrait photography. A small aperture (large f-number) gives sharp photos from the nearby foreground to the distant horizon, often used in landscape photography.

Aperture of a telescope

In a telescope, the aperture is determined by the size of the main light-collecting part. In a refracting telescope, this is the objective lens, and in a reflecting telescope, it’s the primary mirror. Just like a bigger bucket can collect more rain, a larger aperture can collect more starlight, making the image you see through the telescope brighter and clearer.

The aperture of a telescope is expressed in millimetres based on the size of the primary mirror or objective lens. For example, Skywatchers 150 (aperture in millimetres) | 750 (focal length in millimetres) Black Diamond Newtonian (reflecting telescope). The 'f-stop' number can be calculated by dividing the focal length by the diameter of the main light-collecting part. For example, a telescope with a focal length of 750 millimetres and a primary mirror that is 150 millimetres in diameter, has an f-stop number of F4. Learn more about refracting and reflecting telescopes.

Back focus

Back focus is the distance between the last optical element of your telescope, such as a corrector or a reducer, and the focal plane of your camera sensor.

Incorrect back focus affects the quality and sharpness of your images, especially at the edges and corners. If the back focus is too short or long, you may see elongated or distorted stars, vignetting, or coma.

To set the correct back focus, you need to know the required or recommended back focus of your optical accessories, i.e. field flatteners, focal reducers, or coma correctors and the back focus of your camera, which is the distance from the front flange of the camera to the sensor. This distance may vary depending on the type and model of your camera.

You may need to use spacers, adapters, or extension tubes to adjust the distance between your camera and your telescope. The total length of these accessories should match the difference between the required back focus of your optical accessories and the back focus of your camera. For example, if your optical accessory requires 55mm of back focus, and your camera has 17.5mm of back focus, you need to use 37.5mm of spacers to reach the correct back focus. The ZWO website includes diagrams to help determine the best combination of adapters, filter wheels and extensions to achieve correct back focus.

Binning

Binning involves grouping adjacent pixels to form a larger pixel. Binning reduces noise, boosts signal and improves camera sensitivity. The most common binning mode is 2x2, where four pixels combine into one.

Binning reduces the image resolution but doubles the signal-to-noise ratio.

For example, if your camera sensor is 3000 pixels by 3000 pixels, 2x2 binning will reduce this to 1500 pixels by 1500 pixels, the image will be sharper, but there will be more noise. Binning results in faster data transfer times and eases the strain on computer processors during stacking and editing.

An example of binning. Each square reflects a pixel. The first is bin 2x2, then 3x3 and finally 4x4.

Calibration frames (Darks, flats, dark flats, and bias)

Calibration frames are used during integration (staking) to reduce noise and correct optical issues such as vignetting and dust spots. There are four main types, darks, flats, bias and dark flats.

Dark frames and how to take them

Dark frames are used to subtract sensor flaws such as noise and amp glow from your light frames. Noise is the random variation of brightness or colour in your images caused by various factors such as temperature, exposure time, and gain or ISO. It can make your images look grainy or blurry.

A dark calibration frame records the noise your camera sensor produces during exposures and any stuck pixels (pixels that are always too bright or too dark). By subtracting the dark calibration frame from the light frame during stacking, you can remove the noise and the stuck pixels from your image.

Dark calibration frames are taken with the same settings (exposure, ISO or gain, offset, temperature, binning etc) as your light frame (the actual image of your target), but with no light reaching the sensor. To stop light from reaching the sensor, you need to cover the camera or telescope with a cap and place the camera or telescope in a dark place such as a box. As a rule of thumb, aim for around 30 dark frames.

It is common practice to have a dark frame library for each camera, and your most commonly used exposure settings. Libraries should be updated every six months on average. Unlike flat frames, you don't need to capture dark frames with each astrophotography session as they are not dependent on the type of filter used or the focus point.

Flat frames and how to take them

Flat frames are pictures of a light source that is evenly lit. These frames correct uneven illumination in your imaging train, such as vignetting, dust, scratches, or anything that appears darker than the evenly lit image.

Dividing your light frames by your flat frames removes any darker areas or patches from your light frame.

Most astrophotographers use the sky, a white t-shirt, or a flat field panel as their evenly lit source of light.

To take good flat frames, you need to follow some guidelines, such as:

• Use the same focus, orientation, optical configuration, and gain or ISO as your light frames.

• Adjust the exposure time so that the histogram of your flat frames peaks at around 50% (in the middle).

Most acquisition software includes a flat frame wizard to help determine the correct exposure times. If you are automating your flat frame process you will need to input a target ADU (Analog-to-Digital Unit). There is no set ADU. This value can be altered to achieve an evenly lit field. Try starting at 32K and adjusting up or down from there.

Take at least 30 flat frames for each filter. Make sure you record the exposure times so that you can take dark flats to remove noise from the flat frames.

Dark flats and how to take them

Dark flats are used to remove noise and amp glow from your flat frames. Dark flats can be used instead of bias frames if your camera has:

• amp glow

• thermal noise

• a high dark current.

Dark flats record the noise and bias level of your sensor without any light, so you can subtract them from your flat frames. Dark frames must be taken using the same exposure, temperature, and gain or ISO as your flat frames.

Dark flats do not need to be done for each filter.

Bias frames and how to take them

Bias frames are common for astrophotographers who use a DSLR camera.

Bias frames are used to remove the electronic noise that is generated by your camera's sensor (readout noise). By taking bias frames you can capture this noise and subtract it from your light images.

To take bias frames, you need to cover the camera lens or telescope with a cap so that no light can hit the camera sensor. Then, take an image with the shortest possible exposure time but at the same ISO or gain as your light frames. For DSLR this is generally an exposure of 1/5000th of a second.

Chrominance or chroma noise

Chroma noise, or colour noise, is a type of noise that appears as random colour speckles on an image. It’s particularly noticeable in astrophotography, where images are taken in low light conditions with high ISO or gain settings.

The main cause of chroma noise is random fluctuations in the signal received by the camera’s sensor. These fluctuations can be due to various factors, including the:

• sensor’s sensitivity to light

• length of the exposure

• temperature of the sensor.

There are several ways to reduce chroma noise. A common method is by using noise reduction software that can selectively reduce both chroma and luminance noise. Another is to stack the light frames, which can help to reduce the noise by averaging out the random fluctuations. Additionally, converting the image to CIE Lab* colour space, applying a Gaussian blur to the colour channels, and then converting it back to RGB can also help to reduce chroma noise.

Chroma noise can affect images taken with any digital camera, but the amount and appearance of the noise can vary from one model to another. Cameras with larger sensors, such as full-frame cameras, are generally better in low light conditions, resulting in less chroma noise. Additionally, dedicated astrophotography cameras that can cool the camera sensor using a fan produce less chroma noise as well.

Collimation

Collimation is the process of aligning the optical elements of a telescope—such as mirrors and lenses—so that light rays converge at the correct focal points, producing sharp images. Telescopes that need collimation are Newtonian reflectors, Schmidt-Cassegrain and Makutov-Cassegrain telescopes.

Many great videos show you how to collimate a telescope. Check out YouTube.

Deconvolution

Deconvolution is a process used to improve the clarity and detail of images by reversing the effects of distortion or blurring. This mathematical technique essentially sharpens the image, allowing finer details to be seen more clearly.

Dither

Dithering is a technique used in astrophotography to reduce the appearance of visual artifacts, such as banding, by adding small amounts of noise to the image. This helps create a smoother gradient and makes transitions between colors appear more natural. Dithering involves slightly shifting the position of the telescope between exposures to help average sensor noise, and improve the quality of the image when the images are combined.

Drizzle

Drizzle is an image processing technique primarily used in astrophotography to enhance the resolution of images. It involves combining multiple slightly offset exposures of the same object. Through this combination process, Drizzle uses the small shifts between images to effectively boost the final image's resolution and details. This method is especially valuable for maximising the amount of detail in images.

FITS, TIFF, and JPEG

FITS, TIFF, and JPEG are types of files.

FITS stands for Flexible Image Transport System

They are primarily used in astronomy for storing, transmitting, and processing scientific data.

Features:

• Can store multi-dimensional arrays (For example 2D images, tables).

• Includes metadata for photometric and spatial calibration.

• Designed for long-term archival storage.

• Supports various data types, including images, spectra, and data cubes.

TIFF stands for Tagged Image File Format

This file type is popular among graphic designers, photographers, and the publishing industry for high-quality images.

Features:

• Lossless compression, retaining image quality.

• Supports multiple layers and tags for additional image information.

• Compatible with various operating systems and editing software.

• Ideal for high-resolution scans and professional photography.

JPEG stands for Joint Photographic Experts Group

They are widely used for everyday image storage and display, especially on the web.

Features:

• Uses lossy compression to reduce file size while maintaining reasonable image quality.

• Supports up to 24-bit color.

• Commonly used for digital photographs and web images.

• Stores metadata for information like camera settings and location.

Focal Length

The focal length of a lens or telescope is the distance between the lens (or primary mirror) and the point where it brings light to focus, known as the focal point. It is typically measured in millimeters (mm).

Key Points About Focal Length

Determines Magnification

In telescopes, a longer focal length provides higher magnification, making distant objects appear larger. While, a shorter focal length offers a wider field of view, which is useful for observing larger areas of the sky, such as nebulae regions.

Field of View

A shorter focal length gives a wider field of view, allowing you to capture more of the sky in a single image. This is particularly useful for wide-field astrophotography, such as capturing the Milky Way.

Image Scale

The focal length affects the image scale, which is the size of the image projected onto the camera sensor. A longer focal length results in a larger image scale, making it easier to capture fine details of celestial objects.

Gain and unity gain

Gain refers to the amplification of the signal from the camera sensor. It's essentially the sensitivity setting of your camera. Increasing the gain makes the camera more sensitive to light, which can be useful for capturing faint objects. However, higher gain also increases noise, which can degrade image quality.

Unity gain is a specific gain setting where the camera's analog-to-digital converter (ADC) converts the signal from the sensor to digital values without any amplification or attenuation. At unity gain, the number of electrons collected by the sensor is directly proportional to the digital output value. This setting is often considered optimal because it balances sensitivity and noise, providing a good signal-to-noise ratio.

Gradients

Gradients refer to the gradual change in brightness or color across an image, often caused by light pollution, vignetting, or other uneven illumination. Gradients can be corrected during post-processing to achieve a more uniform background.

Guiding (auto-guiding)

Guiding involves using a secondary camera and software to track a star and make real-time adjustments to the telescope’s position. This helps to counteract the Earth’s rotation and any mechanical imperfections in the mount, ensuring sharp, long-exposure images.

ISO

ISO is a measure of a camera sensor’s sensitivity to light. Higher ISO settings increase sensitivity, allowing for shorter exposure times, but also introduce more noise. Lower ISO settings produce cleaner images but require longer exposures.

Linear and non-linear

In astrophotography, a linear image is one where the pixel values are directly proportional to the light intensity captured by the sensor. This is the raw data from the camera. A non-linear image has been processed to enhance contrast and brightness, making celestial objects more visible.

Luminance Noise

Luminance noise appears as random variations in brightness in an image, often seen as graininess. It is more noticeable in low-light conditions and at higher ISO settings

Meridian flip

A meridian flip is a maneuver performed by an equatorial mount when tracking an object that crosses the local meridian (the imaginary line running from north to south). The mount flips the telescope to the other side to continue tracking the object without obstruction.

Monochrome and One-Shot-Colour (OSC) cameras

Monochrome cameras capture images in black and white. They are often used with filters to capture different wavelengths of light. OSC (One-Shot Color) Cameras capture colour images in a single exposure, using a Bayer matrix to filter light into red, green, and blue components.

Narrowband (filters)

Narrowband filters isolate specific wavelengths of light, such as those emitted by hydrogen, oxygen, and sulfur in nebulae. They are used to capture detailed images of emission nebulae and reduce the impact of light pollution.

OAG

An OAG (Off-Axis Guider) is a device that allows a guide camera to look through the same optical path as the main imaging camera. This helps to reduce differential flexure and improve guiding accuracy.

Offset

Offset refers to a constant value added to the pixel values of an image to prevent negative values and ensure all pixel values are positive. This is important for accurate image calibration and processing.

Over-sampling and Under-sampling

Over-samping occurs when the camera’s pixel size is too small relative to the telescope’s resolution, leading to excessive data without additional detail.

Under-sampling occurs when the camera’s pixel size is too large, resulting in a loss of detail and a blocky appearance in the image.

Polar align

Polar alignment is the process of aligning a telescope’s mount with the Earth’s rotational axis. Accurate polar alignment is crucial for long-exposure astrophotography to minimize star trails and ensure precise tracking.

Slew

Slewing refers to the movement of a telescope from one position to another. This is typically done using the mount’s motors and is controlled by a hand controller or computer software.

SNR (Signal-to-Noise Ratio)

SNR is a measure of the strength of the desired signal (such as light from a celestial object) relative to the background noise. A higher SNR indicates a clearer and more detailed image.

Stretching

Stretching is a post-processing technique used to enhance the contrast and brightness of an image. It involves adjusting the histogram to increase exposure and make faint details more visible.

Transparency and Seeing

Transparency refers to the clarity of the sky, affected by factors like humidity, dust, and light pollution. Good transparency means less atmospheric interference.

Seeing refers to the steadiness of the atmosphere. Good seeing conditions mean less atmospheric turbulence, resulting in sharper images.

Full Well Depth/Capacity

Full well depth is the maximum number of electrons a camera sensor’s pixel can hold before becoming saturated. Higher full well depth allows for capturing a greater dynamic range in images.

Quantum Efficiency (QE)

QE is the measure of a camera sensor’s ability to convert incoming photons into electrons. Higher QE means the sensor is more efficient at capturing light, resulting in better sensitivity and lower noise.

Vignetting

Vignetting is the effect of the edges or corners of your images being darker than the center, which can be caused by various factors such as the aperture, the focal length, the filters, or the dust in your optical system.