What is Astrophotography?

Astrophotography, or astronomical photography, is photography of the sky. The aim is usually to create “nice” photos, rather than photos for scientific purposes. The “niceness” of the photo is open to interpretation and what the creator feels is a nice-looking photograph.

Astrophotography is mostly the photography of the night sky or objects in the night sky. Photographing the sun, our closest star in the sky, is a daytime exercise but still Astrophotography.

Photographing the Sun is a unique category of astrophotography requiring a specially designed Telescoped that can handle both the harsh conditions of the sunlight and protect the user from the extremely bright light being magnified by the Telescope. The design allows a very fine narrow band of light (typically light given off by excited Hydrogen) through to the sensor or eyepiece as well as getting rid of the intense heat focused within the Telescope from the Sunlight.

Do not try to look at or image the Sun with anything other than the specially designed telescope, as it will instantly blind you with no chance of recovery, or if imaging, it will burn your equipment.
Night-time filters are NOT SUITABLE for filtering solar light. No matter the brand or type of night-time Ha filter, the bandwidth is way too large to reduce the light and they also have no heat rejection. You will either go blind if viewing or cook your equipment if imaging

The image below of the Sun was taken with an 80mm Lunt Solar telescope with a 2.5x Tele Vue Powermate.

Examples of objects in the sky that could be photographed are the moon, planets, a comet, galaxies and nebula. The objects are still in the sky during the day, but the bright sunlight mostly overshadows almost any other object.

The exception to not being visible during the day would be the Moon and some of the planets such as Venus as the sun is setting. If imaging the moon during the day, the contrast in your image is lost as the sun tends to wash out the shadows on the Moon’s surface. A danger with imaging during the day is that you risk accidentally pointing to the sun with the associated dangers of permanently damaging the equipment.

The dim light from the rising or setting sun can also be used to create nice photos. While not normally considered astrophotography, nice landscape photos can be created using the light from our closest star.

The various types of Astrophotography can be divided up by how much of the sky is included in the photograph. This division, also roughly aligns, with the types of equipment needed to collect the images to produce the final photograph.
 
Astrophotography could involve many aspects of photography techniques used for normal photography. Examples are time-lapse, High Dynamic Range (HDR), Panorama and Light Painting.

The types of astrophotographs include wide-angle landscape (or nightscape), smaller parts of the night sky, or a very small, magnified part of the night sky.

The night sky landscape images might also include the Milky Way as the main subject, or as a background in an image. The star background can be framed by foreground objects on one or more sides adding interest or reference the nightscape to the horizon.

Foreground examples might be silhouettes of mountains, people, houses, cars, farm machinery etc.

Light painting foreground objects, (Fig 4, 9, 10 & 11) can also be effective with the night sky in the background. Light can be used in other creative ways such as people holding a torch providing a beam of light to a point of interest in the scene (Fig 10), or by utilising beams of light from a lighthouse (Fig 5).

Longer exposures creating star trails can also provide an interesting background to a night landscape photo. This can be particularly effective if the south celestial pole is close to an interesting foreground subject.

The magnified image of parts of the sky might highlight the moon or craters on the moon.

With more magnification, images of astronomical objects such as a distant planet, a comet (Fig 6), a nebula (Fig 7), or a galaxy (Fig 8).

The night landscape options are endless.

Photographing objects that are outside our solar system, is often referred to as Deep Sky Astrophotography.

In summary, the types of Astrophotography can be grouped into Solar, Nightscape, Planetary and Deep Sky categories.

The remainder of this article will concentrate on Deep Sky Astrophotography.

Solar, Nightscape and Planetary Astrophotography require different equipment, software and workflows.

To create an astrophotograph, you need at least a Telescope to collect and focus the light, a camera attached to the Telescope to capture raw images (or data) from the Telescope, something to stabilise and hold the Telescope for the length of time the camera shutter is exposed to the sky and a way of pointing and tracking the Telescope as it moves across the sky.

Imaging the Deep Sky

Special workflows need to be used to capture images, with very low light levels, often too low to perceive with the naked eye.

Imaging the sky requires a camera attached to the Telescope to capture the light from the sky. A deep-sky object is often so dim, that just one single exposure from a camera, will not satisfactorily display the image. To compensate for not being able to gather enough light photons or data in just one image, a process of taking many images and using computer software to add the small detail from each image together to form a single image. This process is called Image Stacking.

Image Stacking enables the light photons collected in each of the many images out of the camera to be aggregated together, using computer software, to create a single image that has the sum of all those photons that have been travelling from distant galaxies for millions of years. The more individual images captured and stacked, the brighter and clearer the object will appear in the final astrophotograph. The length of the camera exposure for each individual image will depend on many things.

Collecting enough light to create an astrophoto of the distant nebula, stars and galaxies can be achieved by either, image stacking fewer images with individual images having a longer exposure, or by image stacking more images of shorter exposures. The total light for either way is roughly the same. The difference in individual exposure depends on the technology of the camera and the local sky conditions. The combined (image stacked) single image will simulate a longer exposure image. Longer exposures contain more light but run the risk of overexposing the brighter points by collecting too much light for a pixel on the sensor. In addition, the longer the exposure of a single image, the greater the risk of having to discard the image due to “incidents” outside your control. For example, a plane, meteors, satellites, someone shining a torch close by, someone’s brake lights, distant headlights, etc. For example, if you were imaging for 10 minutes, it would be better to ditch a three-minute bad exposure image and keep another two good three-minute exposures, than ditch a ten-minute exposure due to unforeseen aircraft interrupting an exposure.

A summary of camera exposure length considerations:

• The risk of having an interrupted exposure from aircraft flying across the view.
• The specifications of the camera used.
• The amount of light the Telescope can send to the camera.
• The brightness of the object in the sky being photographed.
• Atmospheric conditions such as the local air turbulence.
• How low the object it is to the horizon. The lower, the more atmosphere that the light passes through so the more the image quality is reduced.
• The background brightness of the night sky from light pollution.

So, to summarise, to create a Deep Sky astrophotograph, depending on the brightness of the target objects being imaged, you would need to capture data from just a few images to hundreds of images. All these individual images are later combined (or imaged stacked) to concentrate all the data and also reduce background noise to form the final astrophotography.

The camera equipment available for astrophotography varies greatly in capability. The choice of camera, as well as the number and exposure length of images, will have an impact on the quality of the resultant astrophoto. Camera types range from simple colour cameras to high-resolution mono cameras. Various filters can also be placed in front of the camera to condition the light hitting the sensor within the camera.

A high-resolution astrophotograph may also consist of groups of images taken with special filters. Mono (Monochromatic or black and white) images can be exposed with standard red, green or blue filters in front of the camera, or with filters in front of the camera that can select particular very small wavelengths of light. Common types of filters that select the very small light wavelengths within the visible light spectrum are Ha (Hydrogen Alpha), OIII (Oxygen 3) or SII (Sulphur 2).

Nebular are often bright in Ha and O3 (and sometimes S2) so images capturing these filters in front of the camera can enhance your final Astro photograph.

Another type of filter available can be used to reduce light pollution from old-style fluorescent or sodium vapour street lighting from overwhelming the camera sensor which can swamp (or override) with the Deep-Sky light. Nowadays, the old-style street lighting is being replaced with LED (Light Emitting Diode) lights which cannot simply be filtered out with a light pollution filter.
Imaging through external filters with Monochrome cameras are much more efficient at capturing the light from deep-sky objects, than colour cameras (or One-Shot Colour (OSC) cameras)

Images taken with different filters can be later blended to create a more dramatic colour photo. The combination of images taken for a deep sky object with different colour filters or assigning colours to images taken with very small wavelength filters is common in an Astrophotograph. It is up to the individual creating the astrophoto to decide on how to create their image using the data obtained from many hours of imaging.

Camera sensors are not perfect and introduce stray electrical signals into their images. These signals are unwanted and appear in the images being captured by the camera and are called noise. Noise mostly appears as varying levels of speckles in the image looking a bit like looking at the scenery through a snowstorm.

Much of the noise in the image is caused by the hot camera sensor. The warmer the sensor is in the camera, the more noise appears in the image captured. Camera sensors warm up when being used to take an image.

Deep Sky cameras have built-in electronic sensor coolers to cool the sensor to sub-zero temperatures to reduce the noise produced by a warm camera sensor. Continually taking long exposures warms the sensor and with no cooling, the camera sensors stay warm which adds to the background noise in the image. This is particularly an issue with consumer cameras such as DSLRs, Mirrorless etc. when used to take the long exposures needed for Astrophotography.

Cooled camera sensors are particularly important in the warm nights of our temperate Sydney summer climate.

Another advantage of a cooled camera is consistency. Being able to cool to a set temperature, with varying ambient temperatures during the night and of different nights the images are taken, greatly simplifies the processing of the images later. Dark calibration images are needed as part of the final image Astrophotography creation process. These dark calibration images, need to be taken at the same temperature as the original deep sky images from the telescope. Using a cooled Astro camera will allow these dark calibration images to be done later without wasting hours of imaging time at night out in the field. The exposure length of dark calibration images is the same as the original images that need calibration so this can take many hours. This consistency with the camera temperature will greatly simplify the astrophotography creation workflow.

It’s the lower camera sensor noise and simplifying with image processing time is the biggest advantage of cooled Astro cameras over alternatives such as DSLRs or Mirrorless consumer cameras.

An example of a deep sky object is shown in Figure 12. It is commonly known as the Triffid Nebula (also catalogued as M20 or as NGC 6514). This photograph was developed from many images on a mono camera.

A mono camera was used to provide a higher resolution than an equivalent colour camera, and to more easily capture the Ha light coming from this nebula.

Before we jump into the Triffid Nebula, I’ll explain why a mono camera provides a higher resolution than an equivalent colour camera. First off, because colour cameras can take the three colours in one image, they are often called by astronomers as OSC or One-Shot Colour cameras.

Camera sensors can only record light levels on a pixel (or dot) on the sensor. No colour information is recorded, just levels of light from black to maximum brightness. Another way of looking at it is levels of grey from black to white or a black and white camera. This type of camera is called a monochromatic camera (or one colour). This is often shortened to monochrome or just mono camera.

You can create many colours of light by mixing the three primary additive colours of Red, Green and Blue. If all three are added equally, you get white. By adding mixtures of individual light levels of these three additive primary colours, almost any colour can be created. This is essentially how your computer monitor, TVs etc. would display colour.

Now thinking of this in reverse, to capture any colour, you could capture how much red, blue and green light is received and record those intensity values.

Cameras work in a similar way to producing coloured light but in reverse. Coloured filters are used to reject all coloured light except the colour of the filter. If three pixels in a sensor, each have either a red, green or blue filter over them, then the mono pixel under each can be assigned a colour. In that way, measuring the light intensity under each filter, the actual colour received on those pixels can be represented as an RGB (Red, Green, Blue) colour.

Most colour sensors are set up in a 4 x 4 matrix of coloured filters on top of the individual sensor pixels (called a Bayer matrix) consisting of a Red, a Blue and two Green pixels.

Referring back to the earlier statement of “A mono camera was used to provide a higher resolution than an equivalent colour camera,” each pixel in a mono camera represents colour data for the resolution of that single pixel. The colour represented is that of the colour of the filter in front of the sensor at the time the image was taken. The resultant high-resolution mono image is known to contain the light intensity for a single colour. The final colour photograph consists of these mono images taken with different filters, being combined later in software. For colour, four pixels are needed to get a single piece of colour information and thus, a smaller number of colour dots effectively produce a lower resolution final image.

For the more knowledgeable reader, some detail around figure 12.

Most images were 3-minute exposures. The images taken were 38 with the red filter in front of the image, 56 with a green filter and 55 with a blue filter. Each group of filtered images were aligned and combined (stacked) to enhance the data collected behind each filter. The stacked red, green, blue filtered, mono images were combined to produce the base colour (i.e. RGB) image. To enhance some of the RGB contrast of the photo, an additional 18 mono images using a luminance (L) filter, were then merged.

The Trifid nebula is rich in Hydrogen, so in addition to the LRGB image created, 50 images of 4 minutes each, were taken with a Ha (Hydrogen-alpha) filter in front of the sensor. These were aligned and merged, then added to the LRGB image to both enhance the red brightness, as well as provide the contrast & definition in the red nebula part of the photo.

The camera sensor was also cooled to minus 20 deg C to reduce the amount of noise added to each image.

Note the number of images taken was a balance between the amount of time available each night over a few months, the atmosphere, and the object.

That was a very simplified version of the creation of the Deep Sky Astro photograph of a Nebula. It was created in a way that I thought looked nice. As such, it satisfies my definition of an astrophoto and a successful astrophotography session.

To create a nice clear photograph, you need to collect as much signal or as much deep-sky data as possible, with the least amount of unwanted background light (light pollution, noise, or bad data). The more of the good image data collected compared to unavoidable noise that appears, the cleaner the image or the better the Signal to Noise Ratio (SNR). Selecting the observation site, planning the night, and the selection of an object to image suited to the equipment, location and time of year, is another complex topic. The selection process can vary in complexity from a simple guess to a complex set of calculations.

Focusing the light

A lens system is needed to concentrate and focus the light onto the camera’s sensor. The focal length of a lens is a key component in the deep sky object’s apparent size and the amount of sky seen by the camera.

The container holding the lens(s) and the focuser, are collectively called the Optical Tube Assembly (or OTA). A telescope is a generalised name for the OTA and associated equipment for either viewing or imaging the sky. The OTA on general consumer cameras is just called a lens. The lens is available in either a fixed focal length (called a prime lens) or variable focal length (called a zoom lens). Fixed focal length lens provides a better-quality image.

The Field Of View (FOV) is the amount of sky the camera sensor can see taking into account the Camera sensor size and OTA’s focal length.

As a guide, the following are OTA focal lengths and their common use:

• For landscape (or nightscape) photos, a lens, with small focal lengths of 14 to 24mm, usually in the form of a quality consumer camera lens is used.
• For some of the larger deep sky objects in the night sky, such as the Andromeda galaxy or Carina Nebula, an OTA with a focal length of around 400mm might be a suitable size.
• For many of the common deep-sky objects, an OTA of around 600mm to 1000mm would be a suitable size.
• The more distant and smaller galaxies require much longer focal lengths (or more magnification).

Generally, the selection of an OTA’s focal length is a compromise between the size of the object being imaged, the focal lengths available for the OTAs, and the pixel size of the camera’s sensor.

Uncovering the camera’s sensor to capture the light from a deep sky object focused through the OTA pointing to the sky is called the camera exposure. The exposure time (of the sensor to the deep sky object), will control the amount of light that is collected by the sensor for each image. The maximum exposure time is governed by how much a sensor can be exposed to light before you notice blurring due to the earth rotating in the sky, or you saturate the individual dots (or pixels) on the sensor with too much light.

A relatively simple guide/rule, used by nightscape imagers using consumer cameras to take images, is the 500 rule. It provides a guide to the maximum exposure time when imaging the night sky, which allows for the moving stars not to have noticeable smudges in the image as the earth rotates. Other more complicated and accurate calculations exist, but the 500 rule is one of the simplest.

The '500 Rule' says the longest exposure (in seconds), equals 500 divided by the focal length of the lens being used on a Full Frame camera. The equation has a slight adjustment if it’s not a Full Frame camera.

The label “Full Frame” refers to consumer cameras that have their historical roots back to 35mm film photography. Astronomy cameras sometimes refer to these traditional photographic sizes so that customers can relate to the sensor sizes. Astronomy cameras tend to have all different sensor sizes, and the Full Frame terminology is sometimes referred to in sales literature or general talk, but the real sensor size in mm is needed by Astro imaging applications or Astro calculations. A more important factor in Astro imaging is the pixel size used in the sensor rather than the sensor size.

In the photographic world, cameras with sensors that are smaller than the Full Frame size of 36mm x 24mm, are considered cropped. For these cropped sensors, the formula becomes 500 / (lens focal length x crop factor). The crop factor, being the ratio of the camera’s sensor to the size of a full-frame camera. For example, the crop factor for a full-frame DSLR is 1, for an APS-C size sensor, it is 1.6 or about 2 for a 4/3” camera sensor.

Example consumer cameras for each sensor size:

• Full Frame - Canon 6D or Nikon D850
• APS-C - such as a Nikon D3500 or Canon 7D

As an example, using a Full Frame camera for a wide-angle nightscape of the Milky Way, with a lens focal length of 17mm, the maximum exposure would be about 500/(17x1) and equal about 30 seconds.

In comparison, the exposure length for imaging Deep Sky objects, with a starting lens focal length might be around 400mm, would make the longest exposure of about 1.2 seconds if using a full-frame sensor (or about 0.6 seconds for the ZWO ASI1600MM astronomy camera with a 4/3” sensor).

A common OTA focal length for deep sky objects is around 800mm. This would require the longest exposure to be 0.6 seconds on a full-frame camera. (or about 0.3 seconds for the ZWO ASI1600MM Astro camera)

Other considerations in focusing the light onto the camera sensor are the OTA F ratio and camera settings. The ratio between the lens Aperture and focal length is called the F ratio. The lower the F ratio, the more light will reach the camera sensor. The camera gain setting (the amplification multiplier for Astro cameras, or an ISO standard setting for the gain or amplification in consumer cameras) will control how much light is passed from the sensor to the image. Better quality cameras that can pass more light at lower gain settings to the image obtained from the camera.

Following the light.

Following on from the previous section, while a 17mm wide-angle lens on a full-frame camera might allow a usable exposure time of about 30 seconds if we want to focus a bit more on deep sky objects, we need to use something like a 200m focal length or greater. So, a simple application of the 500 rule to a Full Frame camera with an OTA (or lens) Focal Length of 200mm, would give a camera exposure length of 2.5 seconds (500/200x1 = 2.5). For an OTA, with say a focal length of 800mm, the exposure time of 0.3 seconds using the 500 Rule. Both these exposure lengths would be too short to capture enough light in a Deep Sky image.

To be able to use a longer exposure to capture more light from an object, the camera needs to point to the same spot, in the sky, as the earth rotates. By pointing to the same spot, the object remains in the same position on the sensor and avoids blurring the object during an exposure that exceeds the 500 rule and thus, the 500 rule would no longer apply to the exposure time.

Moving the camera across the sky, at the same speed as the earth rotates, will enable the longer exposures needed for deep sky objects to allow enough light to be gathered for an image.

A mount is used to support the OTA, point it to an object in the sky, and keep it pointed to that object as the earth rotates. The mount is placed, (or mounted) on some form of support such as a tripod.

For astrophotography, the movement of the OTA by the mount is controlled by two motors. One motor for each moving axis on the mount. The Mount usually also contains the electronics to control the motors and provide an interface to an external computer controller. The computer controller can be in the form of a “Hand Controller” (optionally) supplied with the mount, or the computer controller can be an external (personal) computer running the appropriate applications.

The most appropriate type of mount for Astrophotography is an equatorial mount. This type of mount has a rotational axis that can be aligned with the rotational axis of the earth. The position of stars etc. in an image taken by a camera on an equatorial mount remains constant as the stars seem to move as the mount moves with the sky.

An Alt-Az mount is a common choice for visual observations of the night sky. The observer’s eye can compensate for the field rotation. Long exposure imaging of the night sky, using an Altitude-Azimuth (Alt-Az) mount, would suffer from this field rotation issue showing up as smudged stars towards the edge of the image.

A motorised equatorial mount will rotate the Telescope at the same rate as the earth is turning, which is called the Sidereal rate. The Sidereal rate is the rate that stars appears to move across the sky. It is about 360 degrees every 23.9344696 hours (or about 23 hours, 56 minutes, 4.0905 seconds)

The measurement of the OTA position about the equatorial axis is called Right Ascension (or RA)

As well as an Equatorial Mount rotating in RA, east to west direction at the Sidereal rate, the mount can move in the south/north direction. The south/north movement direction is called the Declination (or Dec).

The RA and Dec movements can also be used to specify a location in the equatorial coordinate system for any object in the sky. The mount can be commanded to move to a part of the sky using the RA & Dec coordinates.

So, in summary, a Mount moves in an east to west direction, at the Sidereal rate, and thus, moves the OTA and camera across the sky, at the same rate as the movement of the deep sky objects due to the earth rotation.

Mounts come in various sizes, quality levels and capabilities. The mount can be as simple as just matching the earth’s rotation with a lightweight camera and lens. (usually called a Star Tracker) It can also be larger, and more complicated, to not only match the earth rotation but point to any part of the sky when commanded, as well as respond to small corrections as needed. A closed-loop system of sending small pointing corrections to the mount can be employed and is called guiding.

The more expensive mounts can be more accurate in pointing, will need less guiding and can carry heavier camera/OTA configurations. The mount is probably the most important component of the astrophotography setup and should be allocated the highest spend when purchasing your astrophotography gear.

So, in summary, a Mount is needed to keep the camera & OTA on target, moving as the sky moves, and thus, the object in the sky appears stationary on the sensor, which enables much longer exposure times, capturing more light to provide the base data to create a nice astrophoto.

Note: Re Images

• All images have been taken by myself.
• For the landscape/nightscape images, I used a Canon 5D Mk4 with Canon, Rokinon, Tamron or Sigma lenses, a Canon Intervalometer, and a ‘Really Right Stuff’ tripod. Lightroom & Photoshop were used to process the landscape photos.
• The Deep Sky images, the OTA used was a Sky-Watcher ESPRIT 120 ED Super APO (840mm @ F7) with a Moonlite Electronic Focuser on a Sky-Watcher AZ-EQ6 Pro (EQ mode) mount & tripod.
• Other equipment was a ZWO ASI1600mm Pro camera, a ZWO electronic 7 position filter wheel with 36mm ZWO filters (L, R, G, B, Ha, O3, S2).
• The Software used to process the Deep Sky photo was PixInsight, Photoshop and Lightroom.
• Solar Image, the OTA used was a Lunt LS80MT/B1800FT with an ASI174MM camera & Tele Vue 2.5x Powermate.

Greg McCall © 2021

Back to Home Page.

Jump top of this page.