Astrophotography Techniques with the Canon EOS System
August 14, 2012
Astrophotography is the imaging of celestial objects such as the Sun, Moon, planets, comets, stars, clusters, nebulae and galaxies. Astrophotography captures the beauty and majesty of the night sky and wonders of the universe.
Canon DSLRs are amazingly versatile for astrophotography. We can shoot long exposures of deep-sky objects at high ISOs because Canon CMOS sensors are remarkably sensitive and have exceptionally low noise. We can also use them to do time-lapse photography of the heavens, and even use the high-definition video modes for high-resolution planetary photography. Unlike specialized astronomical CCD cameras, DSLRs don't require a computer for use in the field for focusing and recording of the images.
We can get started in astrophotography with really simple equipment – just a camera and lens on a tripod. We can take shots of constellations and asterisms like the Big and Little Dippers and Iridium Satellite flares. From a dark-sky location, we can even shoot the Milky Way – our own galaxy as we see it from inside.
With longer focal lengths on an equatorial tracking mount, we can piggyback a 50mm lens with a ball-and-socket head on top of a telescope and shoot large areas of star clouds and dark nebulae. Lenses with longer focal lengths, such as Canon's 200mm f/2.8 L USM L, can be used to shoot close-ups of deep-sky objects such as the fascinating Antares-Rho Ophiuchi complex.
Celestial objects, from our perspective down here on Earth, can range in size from gigantic – such as the Milky Way stretching 180 degrees from horizon to horizon – to extremely small, such as galaxies and planets which require thousands of millimeters of focal length to shoot. Astronomical subjects also come in all shapes, sizes and brightness. Some can be shot with exposures of just a couple of minutes, but others require hours of total exposure. For smaller objects we will need longer focal lengths such as with the shots of M13, the Great Globular Cluster in Hercules, and M20, The Trifid Nebula in Sagittarius, which were shot with a telescope with 1,000mm of focal length on an equatorial mount. Saturn, was shot with 5,600mm of focal length and 400 stacked video frame grabs from the EOS 60D's High-Definition video mode.
Special Considerations for Deep-Sky Astrophotograph
Celestial objects in the sky all appear to move because of the Earth's rotation. This is why the Sun seems to rise in the east, move across the sky during the day, and set in the west. It is not because the Sun is moving, it is because the Earth is spinning on its axis like a top, taking about 24 hours to make one complete rotation.
Because many objects in the sky, such as nebulae and galaxies, are faint, they require long exposures. This becomes a problem when shooting on a fixed tripod. Even the stars will trail during a long time exposure. How much they will trail will depend on the length of the exposure, the focal length of the lens, and the location of the object in the sky. You will get more trailing with longer exposures, and longer focal lengths, and with objects in the sky near the celestial equator.
For longer exposures without trailing, and at longer focal lengths for smaller objects, you will need a special equatorial mount to track the sky and objects as they move to compensate for the Earth's rotation. At focal lengths longer than about 500mm for long exposures you will also need to guide, which requires a separate computerized CCD guiding camera.
The Darkness of the Night Sky
The darkness, or brightness, of the night sky at your observing location will have a tremendous impact on the quality of your images. Nights when a bright Moon is up can also adversely impact astrophotography of faint deep-sky objects.
From downtown in a light-polluted city, you will only be able to see the brightest stars and planets in the night sky, and you will have a very hard time photographing anything faint.
As with other forms of nature photography, you really have to go to where the subjects are best seen. In the case of astrophotography of the night sky, this usually means a truly dark location, far away from the light pollution found in cities and suburbs, if you want to shoot faint deep-sky objects.
For those that can't travel far enough away to find those ideal conditions, there are some special filters that will help by filtering out some of the light pollution, such as Astronomik CLS filter. Using these filters from a dark-sky site will help even more, because they also filter out natural airglow, which is present in the atmosphere, everywhere, all the time. These filters allow longer exposures and increase the contrast between the object and sky background and make them more visible.
Shooting daylight white balance will almost always produce an ugly red-brown sky background. Unfortunately, this is the true color of the night sky. It may be too faint to be seen with your eyes, but the camera is sensitive enough to record it. Under urban or suburban skies, this color comes from light pollution. Under true dark skies, it comes from natural airglow.
To correctly record the color of a deep-sky object we need to remove this "near-sky" color from the Earth's atmosphere. Auto White Balance will sometimes work, but the best results are usually obtained by using a custom white balance set on the sky background itself. This is especially useful if we are shooting in-camera JPEGs. Our other option is to shoot raw and use Canon's Digital Photo Professional (DPP) software and use the eyedropper and "click white balance" in a blank area of the sky background.
Astrophotography requires that you focus your camera on infinity, and this focus is critical. The high resolution of Canon's digital sensors requires exact focus. In the old days of manual focus lenses, you could simply rack the lens to the hard stop at the infinity mark and be good to go. But today's modern autofocus lenses need to focus past infinity so the internal lens focus mechanism isn't damaged as the lens seeks focus and also to compensate for temperature-induced focus change at longer focal lengths and fast apertures. This means that you can't just use the infinity mark on the focus scale of the lens. And you definitely can't just turn the focus ring in the direction of infinity, until it stops.
The best way to focus on infinity is to use Live View and 10x magnification on a bright star or planet. The vari-angle LCD screen on Canon's latest DSLRs makes this easy, especially when the camera is pointed at a normally inconvenient angle, such as overhead when used at the bottom end of a long refracting telescope.
Here are a several important hints to remember when using Live View to focus:
Use the brightest star or planet in the sky to focus on. This is especially important the wider the angle of the lens that you use.
Set the lens to manual focus. Some of the longer, faster Canon lenses, such as the 200mm f/2.8 USM L II will autofocus on a bright star, but it is very difficult to get a star exactly on one of the small autofocus areas.
Manually set the focus scale to infinity to start. It is important to be at least close to focus to begin. If you are grossly out of focus, you won't see anything on Live View.
Turn on Live View exposure simulation in the menus (with most EOS Rebel models, this is always active when you're using Live View.)
Turn off Image Stabilization if the lens has this feature.
Set the camera to manual exposure mode.
Set the camera lens to its widest aperture.
Set the ISO to 1600 and the shutter speed to Bulb. This will ensure that you get the brightest Live View display.
Realize that Live View is not sensitive enough to show a faint nebula or galaxy, even through a telescope. All you will be able to see are the very brightest stars.
Faster (wider) lens apertures and higher ISO speeds can mitigate the effects of trailing on a fixed tripod. Note however that photographing a star field is the single most difficult test of a lens. Every aberration will be revealed by the point sources of stars across the entire field. Many fast lenses need to be stopped two stops or more to get the best compromise between lens speed and optical quality. No lens is perfect, especially wide-open. Generally, the faster the lens, the more you will need to stop it down to correct aberrations. Some lenses, such as Canon's L series, can be used wide open, but test them for your specific requirements.
The correct exposure for the night sky will be influenced by many different factors, but can be easily be determined by simply taking test shots and examining the results on the LCD on the back of the camera. The histogram is particularly helpful. The sky background will make up the bulk of the "mountain" of the histogram in an astrophoto, and the exposure should usually be long enough so that this "mountain" is completely separated from the left wall of the histogram box.
Note that the sky should not be black in your long-exposure astrophotos. It should have some tone. The sky is not black even at the darkest location on Earth. Once you are dark adapted, your eyes are sensitive enough to easily distinguish the sky from the horizon. If you underexpose to make the sky black, the faintest details in the subject will be difficult to separate from the noise in the image.
Exposure for an astrophoto is a compromise between image noise at high ISOs and long exposures, and dynamic range for holding detail and color in the brightest objects in the image. As you increase the exposure, the "mountain" of the histogram will move to the right. This normally means the image will have a higher signal-to-noise ratio, because longer exposures collect more photons which make up the signal in the image. But as you expose longer, you also saturate more stars and lose star colors and lose detail at the top end of the dynamic range. You also get more trailing on a fixed tripod with longer exposures.
Canon DSLRs have excellent noise characteristics. But if you are shooting long exposures under high ambient temperatures, the sensor is so sensitive it also starts recording electrons from heat in the camera. This will look like hot pixels and red, green and blue blobs of noise in the image. These are technically not noise, but rather a thermal signal. Luckily this thermal signal can easily be removed by using Long-Exposure Noise Reduction. This function takes a dark frame, an exposure as the same length as your light frame, but with the shutter closed, in the camera immediately after your light exposure has ended. Note that you will not be able to take another image until after this dark frame exposure has ended. After both the light frame and dark frame are taken, the camera subtracts the dark frame from the light frame to remove the thermal signal.
The best ISO setting for astronomical objects will also depend on many different factors. A good rule of thumb for longer tracked exposures is use a moderately high ISO, like ISO 800 or 1600 for faint nebulae and galaxies, and a low ISO for constellations, stars and star clusters. On a fixed tripod you may need to use a very high ISO if you want to minimize star trailing, but there are ways to improve the final image even at high ISOs, such as by using High-ISO Noise Reduction and more advanced techniques such as image stacking.
Tips for Beginner Astrophotography
Start out simple – Learn how to focus on a star and how to determine the correct exposure by using the histogram. Here are some subjects you can shoot with just a camera on a tripod:
Twilight scenes with the crescent Moon with Earthshine and bright planets nearby like Venus or Jupiter
Asterisms like the Big and Little Dipper, and constellations like Cassiopeia and Orion
Satellite passes such as those by the International Space Station, and Iridium satellite flares
Circumpolar star trails
Meteors and meteor showers
The phases of the Moon
Tips for Intermediate Astrophotography
Longer exposures are needed for fainter objects. This requires a motorized mount to track the sky as it turns due to the Earth's rotation. There are two basic mount types. One is an equatorial design that tracks the sky with the movement of one of its two axes. The other is an altazimuth design that tracks by moving both axes. You will be seriously limited in the length of exposures you can take with an altazimuth mounting because the field will rotate around its center due to the design of the mount. For serious long-exposure deep-sky astrophotography, you need an equatorial mount.
Longer focal lengths in the range of 300 to 3,000mm are also needed to shoot smaller objects. These can be Canon Super Telephotos or telescopes. There are several different telescope designs. Refractors use lenses; reflectors use mirrors; and compound designs, such as Schmidt-Cassegrains, use a combination of mirrors and lenses. Apochromatic refractors are usually an excellent choice for deep-sky astrophotography at focal lengths more than 600mm.
To hook your camera up to a telescope, you will need a 2-inch adapter to go into the back of the focuser on the scope, and a T-mount Canon adapter. The telescope takes the place of a camera lens.
One difference in nomenclature between telescopes and camera lenses is that telescopes are usually specified by their aperture, and camera lenses by their focal length. This is because telescopes usually work at a fixed aperture and focal ratio, and camera lenses have a variable aperture when they stop down to different focal ratios. A 300mm telescope means it has an aperture of 300mm. A 300mm camera lens means it has a focal length of 300mm.
The Canon 60Da Astrophotographic DSLR Camera
Canon's EOS 20Da was the first DSLR camera specifically designed for astrophotography. The 60Da carries on this tradition in a camera that has even more sensitivity and lower noise.
Sensors in DSLR cameras are more sensitive to deep red and infrared wavelengths than the eye, so ordinary digital SLRs use a long-wavelength filter to block most of these wavelengths to produce natural colors that more closely match the eye's spectral response.
Red nebulae emit most of their light at the hydrogen-alpha emission wavelength in the far red at 656.28 nanometers. The normal long-wavelength filter in a standard DSLR filters out much of this light.
Canon has modified the transmission of this filter in the 20Da and 60Da to pass more of the hydrogen-alpha wavelength to produce better pictures of red emission nebula. The 60Da filter transmits 3 times more hydrogen-alpha light than the regular 60D, and 1.5 times more than the 20Da. This makes the 60Da an excellent choice for astrophotography of the many beautiful red emission nebulae, which are also some of the largest objects in the night sky. M8, the Lagoon emission nebula shows a comparison between the 60D and 60Da hydrogen-alpha sensitivity. The pastel blue-green color in the 60D shot comes from Oxygen III emissions in the nebula which are not filtered by the long-wavelength filter.
This doesn't mean that you can't shoot emission nebulae with a normal Canon DSLR camera. You certainly can, but you will usually need a very dark sky and longer total exposures to do it. Special filters such as the Astronomik CLS will also help produce dramatically better images with a normal Canon DSLR. This comparison image shows the difference between the red North America emission nebula shot with a Canon 60D with and without a CLS filter. Astronomik even makes a CLS filter that snaps into Canon APS-C DSLR camera bodies in front of the mirror. This way the filter can be used not only with a telescope, but also with almost any Canon lens except the EF-S models.
Another exceptionally convenient feature of Canon's 60D and 60Da DSLRs is Vari-angle 3.0-inch Clear View LCD monitor. Frequently the camera will end up at an awkward angle at the bottom of a telescope that is pointing overhead. The vari-angle LCD screen can be tilted to make focusing, composition and exposure determination easy.
The Canon TC80-N3 Remote Interval Timer
For intermediate astrophotography we will frequently be taking many long exposures and averaging them together. Canon's TC80-N3 is an indispensable tool for this. It is a remote release that incorporates an intervalometer.
It can be programmed for a delay before the first frame is taken, and then for any number of frames at any exposure, with a user-specified pause between frames if desired.
Note that the camera must be set to Bulb exposure to use the TC80-N3 for exposures longer than 30 seconds. The shooter then can program in a specific length of time on the remote controller’s LCD panel for the shutter to stay open, ranging up to many hours. An adapter supplied with the 60Da must be used with the TC80-N3 to convert the Canon N3 proprietary plug to a single-pin E3 plug, so it can be used with the EOS 60Da.
See Canon's excellent quick guide to the TC80-N3 for more information and tips on usage.
Dealing with Dew
When you are out shooting astrophotos, you will very probably run into problems with your lenses dewing up when the temperature drops at night (unless you are shooting in the desert).
The best way to combat dew is to prevent it from forming by applying a small amount of heat to your lens. This is best done by using an anti-dewer. This is a heated strip of material that you wrap around your lens that is powered by a 12-volt battery. You can purchase these from DewNot or Kendricks.
Go someplace dark to shoot! Get as far away from light pollution as possible – this means driving well out into the country!
Use a red flashlight. After a half-hour or so in the dark, your eyes will become dark adapted and you will be able to see surprisingly well. A red light that is not too bright will help preserve your dark adaptation.
Use a light fog filter to bring out bright stars in constellation shots.
Tape down the focus and zoom rings after achieving infinity focus.
Refocus if you change the focal length on a zoom lens, as many zooms are not parfocal.
Turn down in-camera sharpening all the way, it will exaggerate noise at high ISOs and long exposure times.
Set a custom white balance on sky background.
At high ambient temps use in-camera long-exposure noise reduction.
Shoot JPEG and Raw concurrently (RAW + JPEG). This is helpful, as you can approximate what images will look like with a non-linear curve applied to the linear data, particularly if you plan to process the RAW photos in a specialty astronomical image processing program which treats the images are linear raw files (which appear extremely dark, low contrast, and not color-corrected).
Use Manual Exposure Mode.
Put an interesting foreground in wide-angle shots and paint foreground objects with red light from your red flash light.
Turn off the image display on the LCD monitor after each shot; it will heat the camera up and increase the thermal signal/noise.
Carry extra camera batteries, or use a 12-volt adapter and 12-volt deep-cycle battery – it can also power your anti-dewers.
Look up and observe. Enjoy the beauty and wonder of the night sky! It is awe inspiring to stand out under a dark sky far from today's man-made light pollution and realize photons of light from these objects travel incredible distances across the universe and interact with our own eyes and consciousness. And best of all, we can record these photons for posterity with our Canon DSLR cameras!
If you are going out observing at a remote location, let someone know where you are going and when you plan to be back. Your cell phone might not work and they will know to come looking for you if you have car trouble.
Be aware of any potential hazards from wild animals and humans at your observing site. The general rule of thumb, in both cases, is that if you don't mess with them, they won't mess with you. However, if you are in bear country, observe common sense rules such as not leaving any food out. It may make sense to have a portable, battery-powered radio playing, allowing animals like bears will hear you before they see you, so they won't be startled by your presence.
In mosquito and tick-prone areas, use an insect repellant with DEET. It is the only scientifically proven insect repellant. This is important because of illnesses like the mosquito-borne West Nile Virus and tick-borne Lyme disease.
Utilizing the EOS 60Da or any of Canon's digital SLR cameras and some of the basic tips and techniques discussed above is a great way to begin capturing the vast universe that we live in. Below is a list of resources to help plan out the best times and locations for the best chances of a successful shoot.
Clear Sky Chart - Excellent cloud cover and weather predictions specifically for astronomers.
Sky and Telescope - Current celestial events, star maps, equipment reviews, observing tips and lots more.
Heaven's Above - Satellite visibility and Iridium Flare predictions for your specific observing location.
SpaceWeather - Find out what's happening on the Sun and the chances of seeing an aurora.
FREE DOWNLOAD: Click the link below to download our PDF quick reference guide "DSLR Camera Settings for Astrophotography". This is an ideal field guide for EOS shooters looking to get started in astrophotography.
All Canon contributors are compensated and actual users of Canon products promoted.