What is Astrophotography? Astrophotography, or astronomical photography, is basically 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 up to what the creator feels is a nice-looking photograph.

Apart from photographing our closest star, the Sun, which by definition of what is daytime, can only be photographed during the day, astrophotography is mostly the photography of the night sky, or objects in the night sky. An object could be a moon, a planet, a comet, galaxies, nebula etc.

Photo by Greg McCall

The objects are still in the sky during the day, but the bright sunlight just overshadows almost any other object in the sky. 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.

Photo by Greg McCall

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 being time-lapse, High Dynamic Range (HDR), Panorama and Light Painting. The types of astro photographs include wide angle landscape (or nightscape), smaller parts of the night sky, or a very small magnified part of the night sky or an object.

Photo by Greg McCall

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 could be framed by foreground objects on one or more sides adding interest or referencing the nightscape to the horizon. Foreground might be silhouettes of mountains, people, houses cars, farm machinery etc.

Photo by Greg McCall

Light painting foreground objects 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, or using beams of light from a lighthouse. 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, a galaxy or nebula become possible. The night landscape options are endless.

Photo by Greg McCall

Photographing objects that are outside our solar system is often referred to as Deep Sky Astrophotography. In summary, the types of astrophotos can be grouped into Nightscape, Planetary and Deep Sky categories. The remainder of this article will concentrate on Deep Sky Astrophotography.

Imaging the Deep Sky

Depending on the equipment used, some special workflows need to be used to capture images with very low light levels, often too low to be perceived by the naked eye. A deep sky object is often so dim, that just one exposure from a camera, using our current amateur equipment technology, will not be satisfactory.

Photo by Greg McCall

To compensate for not being able to gather enough light photons or data from an object in one image, we adopt a strategy of taking many images and adding the images together, which is called stacking. This enables the photons collected in each image to be aggregated to create a single image that has the sum of all those photons from the individual images. The more images captured, the more photons collected and the brighter and clearer the object will appear in the final astrophotograph.

Photo by Greg McCall

Imaging the sky requires the camera to be exposed to the light. The length of the camera exposure will depend on many considerations. Considerations such as the risk of having an interrupted exposure, the capabilities of the camera, the amount of light gathered by the OTA, the brightness of the object, or the brightness of the night sky. Light photons for each image can be gathered by either combining fewer images with each having a longer exposure or by combining more images of shorter exposures. The combined, or stacked, single image will simulate a longer exposure image. Longer exposures contain more light but run the risk of overexposing the brighter areas by collecting too much light for a pixel on the sensor.

Photo by Greg McCall

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 passing plane, someone shining a torch close by, brake lights, distant headlights… In say a 10-minute period, better to ditch a 3-minute bad exposure and keep another 2 good 3-minute ones than ditch a 10-minute exposure due to unforeseen events interrupting an exposure.

Photo by Greg McCall

So, to summarise, to create a deep sky astrophotograph, depending on the brightness of the objects, you would need to capture data from one to hundreds of images, which are later stacked to form the final photograph. The capabilities of camera equipment vary greatly and this will impact the quality of the images. Cameras range from simple colour cameras to high resolution mono cameras.

Various filters can also be placed in front of the camera to restrict the light entering the sensor. These filters can be designed to reduce light pollution or to select a specific colour or part of the light spectrum. The photograph may also consist of groups of images taken with special filters. Mono images could be exposed through standard red, green or blue filters, or with filters that can select particular wavelengths of light.

Some special filters are narrowband, with common types selecting Ha, OIII or SII emission lines. Other types of special filters can be used to reduce pollution from old style lighting, although this is becoming increasingly less effective with broadband LED lights used in street lighting.

Images taken with different filters can be later blended to create a pleasing photo. The combination of images taken for a deep sky object with different filters is up to the individual creating the astrophoto.

Some cameras have built in electronic coolers. This is to reduce the thermal noise that is created in a digital sensor. These cameras can have their sensor cooled to sub zero temperatures in order to reduce the noise generated in the electronics of the sensor. Reducing this noise greatly helps get the best image.

The Trifid Nebula (also catalogued as M20 or as NGC 6514). Photo by Greg McCall

A mono camera was used to get the above image. It provided a higher resolution than an equivalent colour camera and to facilitated taking images with special filters to capture the particular frequencies of light peculiar to this nebula.
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 stacked to enhance the data collected behind that particular filter.
These enhanced red, green, blue filtered mono images were combined to produce the base colour (RGB) image. To increase some of the contrast, an additional 18 mono images taken with a luminance filter in front of the sensor were then merged.

The Trifid nebula is rich in hydrogen, so in addition to the mono images taken with LRGB filters, 50 mono 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 to provide the contrast in the red part of the photo.

The camera sensor was cooled to -20°C to reduce the amount of noise. Note the number of images taken was a balance between the amount of time available each night over a few months and the type of object.

That was a very simplified version of the creation of the Deep Sky astrophotograph of a nebula. It was basically created in a way that looked nice to me. As such, it satisfies my definitions of an astrophoto and a of successful astrophotography session.

Focusing the light

In order to create a nice clear photograph, you need to collect as much signal, or good data, as possible with the least amount of light pollution, noise or bad data. The higher the ratio of good to bad data, the better the image.

Selecting the observation site, planning the night and selecting the target suited to the equipment, location and time of year is a separate topic.

A lens system that concentrate and focus the light onto the camera’s sensor is needed. The focal length of a lens will affect the object’s apparent size and the amount of sky seen by the camera.

The tube holding the lens, together with the lens(es) and the focuser, are collectively called the Optical Tube Assembly or OTA for short. Telescope is common name for the OTA and associated equipment.

The camera’s sensor size, together with the focal length of the OTA, will provide a certain Field of View (FOV). The field of view is the amount of sky you can see with the camera and OTA combination used.

The Orion Nebula (also known as Messier 42, M42, or NGC 1976). Photo by Greg McCall

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

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

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

Capturing an object’s light for an image in a camera is called the camera exposure. The exposure time controls the amount of light that is collected by the sensor for each image. The maximum exposure time for the night sky is basically governed by how much a sensor can be exposed to light before you notice blurring due to the earth rotating in the sky and/or when its individual dots (or pixels) are saturated with too much light.

Comet 46P/Wirtanen. Photo by Greg McCall

A simple maximum exposure time guide for the night sky that allows for the moving stars not to have noticeable smudges, particularly at the edges as the Earth rotates is called the ‘500 Rule’. The ‘500 Rule’ says that the longest exposure in seconds for a full frame sensor is equal to 500 divided by the focal length.

Sensors that are smaller than a 36mm x 24mm (diagonal of ~43.3mm) of a 35mm full frame are considered cropped.
For these sensors, the formula becomes 500 / (focal length x crop factor), the crop factor, being the ratio of the camera’s sensor to the size of a 35mm film frame.

For example, the crop factor for a full frame DSLR, such as the Canon 6D, is 1. The crop factor for a the ZWO ASI1600mm, a common Astronomy 4/3” (diagonal of 22.2mm) camera sensor is about 2. So, using a full frame 35mm camera for a wide angle night scape of say the Milky Way, with say a lens focal length of 17mm, the maximum exposure would be 500/(17×1)=29.4 seconds, which is close enough to 30 seconds. I would be about 15 seconds for the ASI1600 camera.

For Deep Sky objects, the starting focal length might be around 400mm, making the longest exposure of about 1.2 seconds for a full frame sensor or about 0.6 seconds for the ASI1600mm camera. A common OTA focal length for deep sky objects is around 800mm. This would require the longest exposure to be 0.6 seconds or about 0.3 seconds for the ZWO ASI1600mm.

As well as the lens focal length, another important attribute is the aperture and F ratio.
For cameras, an important characteristic is its gain, either the amplification multiplier for astro cameras or the ISO standard setting for the gain in consumer cameras.

Following the light

For an OTA, with say a focal length of 800mm, the exposure time using the 500 Rule is 0.3 seconds. This exposure would be too short to capture enough light in an image. To be able to use a longer exposure to capture more light from an object, the camera needs to point to the exact same spot in the sky as the Earth rotates.

By pointing to the same spot in the sky the object remains in the same position on the sensor thus avoiding blurring the object during an exposure that exceeds the 500 rule and allowing enough light to be gathered for an image. To this effect, a motorised mount is used to move the OTA to follow the object across the sky.

Southern Pinwheel Galaxy (also known as Messier 83, M83, or NGC 5236). Photo by Greg McCall

A mount is also what supports the OTA, the camera and all the accessories. For support it can be placed, or mounted, on a tripod or pier. The rotations of the motors in the mount, can be computer controlled, which also facilitates pointing the OTA to the selected object (GOTO systems). To control their movements mounts can utilise a built in astronomy computer with a hand controller or interface to astronomy applications on an external computer.

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. Therefore, the OTA and the image in the camera remain constantly aligned with the target as the mount moves with the sky. This rotation around the equatorial axis prevents blurring parts of the image, particularly around the edges, due to the field rotation of the image.

Long exposure of the night sky on an altitude-azimuth, Alt-Az, mount would have this field rotation issue. However, an Alt-Az mount is a common choice for visual observations of the night sky, where the observer’s eye can compensate for the field rotation.

A motorised equatorial mount will turn at the same rate as the Earth is turning, which is called the sidereal rate. The sidereal rate is the rate at which the Earth spins, which is also the rate that stars appear to move across the sky. It is about 360 degrees every 23.9344696 hours or about 23 hours, 56 minutes and 4.0905 seconds. The OTA position about the equatorial axis as measured eastward is called Right Ascension or RA. As well as moving in RA, east/west direction, a mount can move in the north/ south direction. The north/south position is called the Dec, short for declination. RA and Dec are also used to specify a location in the equatorial co-ordinate system for any object in the sky. 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 apparent movement of the deep sky objects due to the Earth rotation.

Mounts come in various sizes with various capabilities. It can be as simple as just matching the Earth’s rotation with a lightweight camera and lens. It can also be larger, and more complicated, in order to not only match the Earth rotation but also to point to any part of the sky when directed. Most of them are also able to execute small pointing corrections as needed. Such an auto guiding technique is based on a closed loop system that sends small orders in RA and/or Dec to the mount.

More expensive mounts are more accurate in pointing, 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 budget when purchasing your astrophotography gear.

So, in summary, a mount is needed to keep the camera and OTA on target, moving in synchronisation with the sky and thus allowing the target to appear stationary on the sensor, which enables much longer exposures, capturing more light to provide the base data needed to create a nice astrophotograph.

Greg McCall

Notes:
All images have been taken by myself.

For the landscape/nightscape images, I used a Canon 5D Mk4 with Canon, Rokinon, Tamron and Sigma lenses, a Canon Intervalometer, and Really Right Stuff tripod. Lightroom & Photoshop were used to process the landscape photos.

For 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. The camera is a ZWO ASI1600mm Pro, a ZWO electronic 7 position filter wheel with 36mm ZWO filters (L,R,G,B,Ha,O3,S2). PixInsight, Photoshop and Lightroom were used to process the deep sky photo.