Astrophotography is a hobby rapidly gaining popularity thanks to the fast advancing CMOS sensor technology. Over a decade ago, the light recording material employed in astrophotography was primarily chemical emulsion. Its low sensitivity makes it very hard to record the weak signal from deep space. In addition, the lack of real-time feedback is a huge source of frustration for beginners. Operational errors such as out-of-focus can only be realized after several nights of hard work after the film is developed. In the mid 90s, the advent of cooled CCD cameras provided solutions to both the sensitivity and real-time feedback problems. However, their high prices and miserably small sensor areas limited their uses to only a few kinds of astrophotography and to very enthusiastic astrophotographers. While CCDs revolutionized astronomical research, this technology has never really changed the landscape of amateur astrophotography. The true turning point took place in 2002. After Fujifilm announced its FinePix S2Pro DSLR and showcased amazing astronomical pictures taken by this camera, people started to seriously explore DSLRs for astrophotography. DSLRs can provide real-time feedback, which is very important for beginners. They have sensitivities not much worse than CCDs, and DSLRs with large sensors (APS-C) are quite affordable nowadays. Today’s landscape in astrophotography is shaped by a series of CMOS-based DSLRs from Canon, but DSLRs and mirrorless cameras based on Sony sensors are gaining popularity very quickly.
Because of my job, I have opportunities to use a broad range of imaging instruments, from multi-million dollar CCD cameras on large professional telescopes to amateur CCD cameras and DSLRs. My training in astronomical research also provides me toolsets to quantitatively evaluate the performance of sensors and to know their true limits. This helps not only my research, but also my lifetime hobby, astrophotography. On the hobby side, I mostly use DSLRs (Canon 5D Mark II and Nikon D800) for their high performance and affordable prices. To get the best astrophoto results, the DSLRs’ internal filters are modified to have higher throughput in the deep red, so they can be more efficient in recording the red light from ionized hydrogen gas in the universe. Other than this filter modification, DSLRs used for astrophotography are no different from DSLRs we use daily.
One very common worry about using DSLRs on astrophoto is the thermal noise generated by the sensors. CCD cameras cooled to -20 or even -40 degrees C do not have such problems. However, CMOS sensors produced in recent five years all have very low thermal noise. Under the same sensor temperature, their thermal noise is actually much lower than common CCDs in astronomical cameras. Another important factor that many people overlook is noise sources other than heat in the sensor, one of which is the photon noise generated by the sky itself. With latest DLSRs under many circumstances, the sky photon noise often overwhelms the thermal noise, making cooling unnecessary. Only in places that are both hot and dark (such as the deserts in the south-west US), cooling is needed to fully exploit the dark sky.
This is the imaging setup I often use. The DSLR is attached to the end of the primary telescope, which acts as a giant telephoto lens (1100mm, f/7.3). It is an APO refractor, with a large corrector lens in front of the focal plane to correct the field curvature and astigmatism. The corrected field is large enough to cover a 67 format sensor. The telescope sits on an equatorial mount, which is motor-driven and can track the stars’ east-west motion on the sky to allow for long exposures. Above the primary scope is another smaller scope with a small CCD camera attached to it. This small scope and camera system can monitor the tracking of the equatorial mount when the primary scope takes exposures. It automatically guides the mount to correct for its tracking errors in real time. The whole system (equatorial mount, DSLR, and guiding system) is controlled by a laptop.This is my setup when I just want to shoot wide-angle images. This looks more like what a beginner may use. The camera and lens are attached to an equatorial mount through a ball head. For wide-angle shots, the mount’s tracking does not need to be super accurate, so a real-time guiding system is not required. As a rule of thumb, when the focal length is shorter than 200mm, it is relatively easy to take long-exposure pictures without using a fancy equatorial mount and guiding system. Things start to become difficult when the focal length is longer than 300mm.
General Procedure
The workflow in astrophotography is quite different from that in daylight photography. Because our targets are very faint, we need to expose for a few minutes or even a few hours, to collect enough photo-signal from our targets. However, the sky background is usually so high that it will saturate the image when exposure is longer than 10 minutes or so (this is especially true under a light-polluted sky). Therefore, what we do is to break the long exposure into many shorter (a few to 10 minutes) ones to avoid saturation, and then stack (average) the short-exposure images in post-processing to combine their signal. This gives a result that is equivalent to a very long exposure.
On the telescope, once the equatorial mount is set up and aligned to the Polaris, what we usually do is to first use a bright star to focus. This used to be a very challenging task, but now it is very easy with DSLR’s live view function. Then we move our telescope/lens to point at our target. We can usually very easily see our target constellation through the camera’s viewfinder if we use a wide-angle or short telephoto lens. On the other hand, if we use a long telephoto lens or a telescope to shoot deep-sky objects, the targets are usually too faint to be seen directly. Some test short exposures with very high ISO can help to verify our framing. Once this is done, we just fire away many long bulb exposures through a computer or a timer shutter release. As mentioned above, typical exposure times range from a few to 10 minutes, depending on how fast our lens is and how dark the sky is. A very commonly used ISO is 1600. However, with recent DSLRs with Sony sensors, it is possible to use ISO 800 or even 400 and still get very good results after post processing. The advantage of lower ISOs is of course their higher dynamical range. It goes without saying that we always shoot RAW.
In addition to the on-sky exposures, we also take many “calibration” images to remove the unwanted signal from the sky, the optics, and the camera. For example, we take exposures on objects with uniform brightness (such as a cloudless day-time or twilight sky, or a large LED panel) afterward. Such images (called “flat field”) can be used to correct for the vignetting caused by the lens/telescope in the on-sky images, to restore the uniform background brightness. In the beginning or the end of the night, we fully cover the lens/telescope and take “dark” exposures when the camera is under the same temperature as the on-sky shots. Such dark images can be used to remove the thermal signal in the on-sky images. This is essentially the same as most DSLRs’ in-camera long-exposure noise reduction, but we do this manually to avoid wasting the precious night time. We also take extremely short (1/8000 sec) exposures (called “bias”) when the lens is fully covered, to account for whatever signal the camera generates when there is no light and also no time for thermal signal to accumulate. Like the on-sky exposures, we take multiple (from a few to several tens) flat, dark, and bias exposures and average them to beat down any random noise in the images to improve the signal quality. There are many software packages (such as DeepSkyStacker, which is free) that can process the on-sky, flat-field, dark, and bias images, and stack the calibrated on-sky images to form a very deep, clean, and high dynamical range image. All these have to be done from RAW files, as JPEG images are not linear and do not allow for accurate removal of those unwanted signal.
(a) is a raw file directly converted in Photoshop and with some contrast stretch. Here we see hints of red nebulas in the image, but the most prominent feature of this image is the vignetting pattern caused by the telescope and the camera. (b) is a “flat field” image taken with the same telescope toward the twilight sky. It is an image that contains nothing but the vignetting pattern. Mathematically, we divide (a) with (b) to remove the vignetting pattern and this calculation is called “flat-field correction.” (c) is the result of such a correction, plus strong contrast and saturation stretches. We can see that without the flat-field correction, there is no hope to bring out the faint nebulas everywhere in the image from (a). BTW, the vignetting correction built in most non-astronomical image processing software (such as Photoshop or Lightroom) is not accurate enough for astrophotography, even if our lens is in the software database. This is why we have to carry out flat-field correction by ourselves using software designed for astrophotography.
After the basic calibration and image stacking, we use software such as Photoshop to further process the stacked images. It usually takes very strong curve and saturation stretch to bring up the faint details in a stacked astronomical image. It also requires a lot of skills and experience to achieve this while still maintaining accurate color and a natural look of an image. It is essentially like manually processing a RAW image from scratch, without relying on any raw processing engines. It is not uncommon for us to spend more time on processing an image than its exposure time, and post-processing is often what separates a top-notch astrophotophotographers from average ones.
Wide-Field Examples
This picture of Orion is taken with the Sigma 50mm f/1.4 Art lens and Nikon D800. It is a composite of more than 60 4-minute exposures at ISO 800 and f/3.2 to f/4.0. The more than 4 hours of total exposure time here is rather extreme. For constellation shots like this, we usually spend only 0.5 to 1.5 hour. However, the extremely long exposure here does lead to better image quality and allows for detecting very faint nebulas around Orion. To efficiently capture the red nebulas in Orion, a modified DSLR is needed. However, with an unmodified one, we can still get the beautiful color of stars in the constellations. So wide-field constellations are great targets for beginners who are not ready to send their cameras for the surgery.This image of the summer Milky Way is taken with a 500mm f/2.8 telescope and Canon 5D Mark II. It is a mosaic of 110 images, so its field of view is comparable to that of a 50mm lens. I am a big fan of mosaic images. I often call it poor people’s large format camera. A crazy mosaic panorama like this contains rich details that far exceed what can be captured with the most high-end medium format digital back. The price is that it takes a very long time to shot and to process the images.This is an expanded version of the Orion image. It shows the Great Winter Triangle and the Milky Way that goes through the triangle. It is taken with Nikon 28-70mm f/2.8D at 50mm f/4 and Nikon D800. It is a four-image mosaic, so the field of view is four times larger than a 50mm field of view. Each of the mosaic frame contains 16 5-minute exposures at ISO 400.This is a two-image mosaic taken with a Mamiya 645 45mm f/2.8 lens at f/4.0 and Canon 5D Mark II. The two-image mosaic allows to capture not only the constellation Cygnus, but also the large-scale Milky Way. Each individual mosaic frame contains 16 4-minute exposures at ISO 1600. In post processing, I applied a layer to blur the light from bright stars so the shape of the constellation is more apparent. The same effect can be achieved with a diffuse filter in front of the lens. Filters commonly used for this purpose include Kenko Softon A and Cokin P830.
Deep-Sky Examples
This wide-field image around the star cluster Pleiades (Meissier 45) is taken with a 500mm f/2.8 telescope and Nikon D800. It is a four-frame mosaic, and each frame contains more than 1 hour of total exposure. The dust and gas clouds around the Pleiades are actually very faint. It does not only require very long exposures to detect them, but also very dark and clean sky. The image calibration also needs to be done with a very high accuracy, otherwise the sky background plus the vignetting of the optics will totally wash out the faint nebulosity. On the other hand, blue gas clouds like this do not require a modified DSLR to record them. The core of the clouds around the Pleiades can be very good targets for people who do not have a modified DSLR.The Andromeda galaxy (Meissier 31) is a target never missed by any astrophotographer. This is taken by the telescope with my first setup and Canon 5D Mark II. It is a two-frame mosaic. Each frame contains about 40 5-minute exposures at ISO 1600. Unmodified DSLRs can take decent pictures of galaxy targets like this. However, if we look at the image carefully, we can see many small red objects along the spiral arms of the Andromeda galaxy. These are the giant gas nebulas that contain ionized hydrogen. To efficiently capture the red light from these nebulas, a modified DSLR is still required.The Horse Head Nebula sits right next to Orion’s belt and is part of the image of Orion presented earlier. It can be seen through moderately large telescopes under dark sky. This image took more than 4 hours of exposure on Canon 5D Mark II on the telescope from my first setup. The red color in the image comes from ionized hydrogen. It requires a modified DSLR to efficiently record the red light.The North American Nebula is in Cygnus, and is part of the Cygnus image shown above. It is a fairly big nebula and it fits in the field of view of a 400mm lens (FF) nicely. This enlarged image was taken with the telescope from my first setup and Canon 5D Mark II. It is a 4-frame mosaic, and the total exposure of each frame is 2.5 hours. The nebula is not completely red. There are also blue components embedded in the red light, which comes from ionized oxygen. If an unmodified DSLR is used, the nebula would appear purple or pink.Meissier 22 is a globular cluster in Sagittarius. It contains roughly 300 thousands of stars. It sits against the summer Milky Way, so there are also numerous stars in the background of this image. This image is taken with the telescope from my first setup and Nikon D800. The total exposure time is 1.5 hours. For the cluster itself, this exposure time is unnecessarily long, as the cluster is relatively bright. I spent extra time in this field to capture the large number of faint background stars that belong to the Milky Way. Stellar targets like this do not require a modified DSLR. An unmodified one can do equally well.The Pinwheel Galaxy (Meissier 101) is a nearby galaxy and therefore appears relatively large on the sky comparing to most other galaxies. However, it is still very small. Its brighter part has a size that is roughly half a full moon. This picture is taken with the telescope from my first setup and Canon 5D Mark II. It is cropped, and the cropped field of view is equivalent to that of a 3000mm lens. It contains a total of 8.5 hours of normal exposures, plus another 3 hours of exposures under an hydrogen alpha (656.3 nm) narrow-band filter. The narrow-band filter image is to enhance the small patches of red nebulas along the spiral arms. Unfortunately, this is not a very efficient way to use a DSLR, as only one quarter of the pixels are actively receiving photons under such a deep red filter. In the background of this image, we can see many small yellow dots. Those are numerous very distant galaxies. Some of the galaxies are so far away that the time it takes for light to travel from those galaxies to us is longer than the age of our Sun.
This guest post was contributed by Wei-Hao Wang, an astronomer working in a national research institute of Taiwan, and is currently visiting the Canada-France-Hawaii Telescope on the Big Island of Hawaii. He is also an astrophotographer and started this hobby in 1990. A collection of his recent astrophotos can be found right here.
