How is an astrophotograph made?
We have all seen the amazing pictures that are captured by the Hubble Space Telescope and released to the public. The public relations department of the Space Telescope Science Institute maintains a collection of them at the Hubble Heritage Project. What most of us don't know, though, is that with recent advances in CCD cameras, computer guided telescopes, and digital SLR's it's actually possible to capture your own pictures of deep sky astronomical objects even from suburban locations. The pictures on this web site, for example, were all captured with comparatively modest equipment from a heavily light polluted location.
So what's the catch? Why can't I just grab a DSLR, a telephoto lens, and a tripod and snap away? First, let's describe a few misconceptions about astrophotography in particular and telescopes in general:
- The main purpose of a telescope is not to magnify—it's main purpose is to gather light. The dark adapted human eye has a typical diameter of seven millimeters. Even a small telescope such as those used by most amateur astronomers will gather several hundred times as much light as the naked eye. A telescope is judged by its diameter (light gathering ability), not its magnification.
- Most astronomical objects are extemely dim—tens or even hundreds of times dimmer than the faintest object you can see with the naked eye. As a result, you not only need lots of light gathering ability (a telescope), but you also need very long exposures. One of the most impressive astrophotographs in the past thirty years is the Hubble Deep Field. This image involved nearly ten days of exposure time and captured objects as much as four billion times fainter than the dimmest star you can see. Getting lots of exposure time is critical for most subjects.
- In order to capture long exposures, your telescope will need to track the night sky as the earth turns. The more accurately it can track, the sharper your images will be. A good telescope mount that is properly aligned will be able to keep a telescope pointed at the same point in the night sky within 1/1000 of a degree for several minutes. This is probably the most important and most expensive part of capturing technically pleasing astrophotos. If you have a budget of $2,000 for your astronomy equipment and you have astrophotography in mind, expect to spend half the money on the mount.
- The pictures you see on this website or on any other astrophotographic web site involve large amounts of post processing. You are not seeing what came straight out of the camera. This does not mean the pictures are "fake." Post processing is a simple fact of life for astronomical photos, even if your intent is to show the subject as accurately as you can. You need to eliminate the noise from light pollution, read noise from your imager, balance inconsistencies in the sensitivity of individual pixels in your camera, etc. In many cases, the images straight out of the camera aren't even in color. Here is an example of a "raw" image before any adjustments, after a simple levels adjustment, and after careful processing:
Unlike in many forms of photography, the objective in Astrophotography is to maximize resolution and signal to noise ratio, not necessarily to have the best possible result straight out of the camera.
So how do you go about prime focus astrophotography? Here is my technique as it has evolved for the particular equipment I use. It's not an instruction sheet on "how to become an astrophotographer", but it should give you a good idea of just what is involved in the craft.
- During daytime, pick out the object you wish to capture; you'll want to match some of your equipment to what you intend to photograph. Make sure that the object you are going to photograph will be high in the sky at the hours you will be out. If you choose something that will be just above the treetops, the images will be of poor quality due to atmospheric refraction, turbulence, and water vapor.
- Choose your equipment carefully for the object you are trying to capture. For example, if you are going to take sweeping pictures of a large expense of sky, you won't want a telescope at all--you'll want to use a wide angle lens. If you are going to capture images of a particular constellation, you'll want a digital SLR and a short focal length telescope or a short telephoto lens. For most galaxies, you want the longest focal length your telescope mount is capable of driving accurately.
- Setup the tripod, mount, telescope, and camera at the darkest location you can find within a reasonable drive. Make sure that the location is reasonably safe from animals and people alike. If you plan on photographing a dim object--pretty much everything except the moon and planets--make sure that the moon will not be up when you are planning to take you picture.
- Ensure the mount is as close to level as possible using bubble levels.
- Carefully balance your scope and camera on the mount. If the balance is off by more than just a couple of ounces, the accuracy of your drive will be affected.
- Using the dedicated polar alignment scope in the tripod mount, carefully align your telescope mount with the north celestial pole. This will ensure that as the telescope turns to compensate for the Earth's rotation, stars will track accurately. Since I typically shoot multiple, shorter exposures and add them together, perfect polar alignment is not critical to my photography. The adaptive optics unit I use is capable of compensating for slight imperfections.
- Take the camera off and replace it with a star diagonal and eyepiece for visual alignment of the telescope. Turn the telescope mount on and carefully align the computer with the night sky. The chips on most CCD cameras are quite small, so if you want the telescope to be able to place an object within the field of view of your scope/camera it is important to perform this step carefully.
- Replace the star diagonal and eyepiece with the camera and slew to a bright star. Turn on the camera and notebook computer and start the cooling process on the camera. Most astronomical CCD cameras are cooled to a temperature of at least 30 degrees celsius below ambient temperature. This reduces the amount of electronic noise in the camera.
- By taking repetitive, short exposures of the bright star, slowly start to focus the telescope and camera. Due to the long focal length in most telescopes and the variability in Earth's atmosphere, this is actually one of the hardest parts of astrophotography. It can easily take thirty minutes or more to achieve good focus. Don't skimp here since it will noticeably affect the resolution in your photographs. There are many software and hardware tools available to ease this process.
- Once focus is achieved, slew to your subject and take a (fairly) short exposure--perhaps one minute--to determine whether your subject is properly centered in the field of view. Using the hand controller, move the telescope in small increments until you are satisfied with the composition.
- Determine the exposure duration for your particular subject. While there are several competing mathematical forumulae for determining the "perfect" subexposure duration, it really comes down to balancing the following: ensure that your subject is not overexposed (just as in traditional photography, you don't want to clip the highlights, though some clipping on bright stars in inevitable); ensure that your drive is able to accurately track for the exposure duration; ensure that your exposure duration is not so long that sky glow/light pollution will saturate your exposure; ensure that you have picked the longest exposure that meets the above criteria since that will maximize signal to noise ratio/picture quality. I have settled on five minute subexposures as being optimal for my equipment and location for most subjects.
- Using a moderately bright star in the field of view, callibrate the autoguider unit to its particular orientation in the telescope. Since I have a relatively light weight (portable) and inexpensive mount, my drive is not particularly accurate. I compensate for this with the use of an autoguider unit. autoguider takes repeated exposures of a single star and sends corrections to the motors in the mount to account for inaccuracies in the gearing and misalignment of the telescope. This maximizes resolution in the final image and reduces the number of subexposures that need to be thrown out due to poor tracking accuracy.
- Start taking subexposures and saving them to your hard drive. Between subexposures, ensure that the autoguider is continuing to track accurately. Take as many subexposures as you can of your subject. This will allow you to both weed out the lowest quality pictures and allow you to capture additional detail in your image while reducing noise.
- Change to the next color filter, re-confirm focus (since filters are not quite parfocal), re-set the autoguider, and continue to take subexposures.
- Once you are done taking subexposures, capture several "dark" frames for your current temperature and exposure. Dark frames are just what they sound like--images taken with the shutter closed. These dark frames will later be averaged and subtracted from your subexposures. They will account for variability in pixel response.
- Pack it all up and go home for some rest.
- Now that you have taken your subexposures, it's time to start processing them in the computer. The first step is to callibrate your exposures by subtracting the dark frames and applying a flat frame (optional). Since I run a Macintosh notebook for all of my photography work, I have comparatively few choices for image processing. I use a product called Equinox Image for performing callibration on my images. Each subexposure needs to be independently callibrated.
- Next, the images need to be carefully aligned (registered) to sub-pixel accuracy. There are automated tools to perform this alignment, but better results can be achieved by hand. The basic process is to overlay images on top of each other and shift/rotate them by tiny increments until the stars line up as perfectly as possible. Each aligned image is saved separately.
- The next step is to combine or "stack" the subexposures. There are many different algorithms for performing this stacking, and the particular one to choose depends on the subject and the exposure duration. The basic choices come down to either adding, averaging, or median combining the pictures. The objective in stacking exposures is to bring out resolution by averaging out atmospheric variation and to reduce noise.
- The SBIG STL-11000 camera I use is a monochrome camera. That is, it contains no Bayer matrix for capturing color in a single exposure. Traditionally, astrophotographers have used monochrome cameras to take separate exposures through colored filters in order to capture red/green/blue/luminance exposures and combine them in the computer. Monochrome cameras have the advantages of greater sensitivity, greater resolution, and the ability to image in colors other than the standard red, green, and blue.
- The different channels are imported into Photoshop as separate layers and adjusted for black point, white point, color balance, curves, noise, levels, and sharpness.
- Careful post processing can require a significant time commitment--up to several hours per picture--to bring out the maximum detail in the image without increasing noise
Sound like a lot of work? It is. Most astronomers never take up astrophotography, and the primary reason is the steep learning curve. Capturing good results requires patience, skill, and dedication. There is nothing more frustrating than spending two hours driving out to your dark sky site, setting up your equipment, adjusting and aligning your camera and telescope only to find you forgot to charge a battery, or to have the clouds roll in. Still, there is nothing like having that first image pop up on the computer screen showing detail you had never seen through the telescope visually--being able to share your views through photographs with your family and friends without dragging them out on a cold night to look at a fuzzy blob through a dim eyepiece.
- Jared Willson