Digital Cameras, Webcams and Video Cameras
Digital cameras, digital video cameras, and webcams all use CCD chips (or similar CMOS chips), just like an astronomical CCD camera. They are also incredibly popular, with millions being sold every year. So the obvious question is whether these types of imaging devices can be used for astronomical imaging as well. Fortunately, the answer is yes, although they have certain limitations as well as certain advantages.
There are two basic types of digital cameras. There are the point-and-shoot type, called digicams, and there are the digital single lens reflex type, or DSLR (or the mirrorless equivalents). The essential difference is that digicams do not have removable lenses while DSLRs do. This limits the type of astrophotography that can be done with digicams. Digicam sales have diminished with the advent of smartphone cameras, so they are much less common now.
Above: Typical digicam, the Canon PowerShot SD850
Since digicams and smartphone cameras do not have removable lenses, images must be taken afocally, that is, through both the camera lens and the telescope eyepiece. This increases the focal ratio and reduces the speed of the imaging system. This is exactly the opposite of what is desired for deep-sky imaging -- the exposures become prohibitively long. However, it is ideal for planetary imaging, as the planets are small and bright. This makes digicams suited to solar system imaging. In fact, you can take a pretty decent photo of the moon by simply holding a smartphone camera up to a telescope eyepiece!
Above: Typical DSLR, the Canon EOS 20D
DSLRs are particularly well suited to deep-sky imaging and have become very popular. While not nearly as sensitive as a true astronomical CCD, DSLRs have the advantage of cost and versatility -- they can be used equally well for regular photography. Some DSLRs have been produced that are specialized for astrophotography, such as the Canon EOS 60Da and Nikon D800a. These cameras are closer to CCDs in sensitivity and provide a large-format sensor for less money than a comparable CCD.
The main sensitivity limitation with DSLRs is that they have poor red sensitivity, making it difficult to image faint nebulas like the Horsehead and Rosette. It turns out the sensor in a DSLR is actually red sensitive, but the window placed over the sensor to protect it blocks much of the red light in order to make the sensitivity of the camera more closely match that of the human eye for capturing daytime photos. Many DSLRs can be "modified" by replacing the original window with one that allows red light through. This is what has been done in the 60Da and D800a cameras by the manufacturers, but many aftermarket options exist for modded DSLRs. A modified DSLR performs more closely to an astronomical CCD, but still has more noise and lower dynamic range.
The removable lens of a DSLR makes it ideal for attaching to a telescope for deep-sky imaging. Fast focal ratios are possible with DSLRs, unlike digicams. Also, the interchangeable lenses themselves are ideal for imaging as well. Wide-angle lenses can be used for images of the Milky Way or meteor showers, and fast telephoto lenses are available for imaging larger celestial targets such as the Andromeda Galaxy or Pleiades star cluster.
Above: Image of the Andromeda Galaxy with a Canon EOS 20Da. Three 5-minute exposures through a 180mm f/2.8 astrograph. Image by James McGaha.
Since a digital camera stores data to a removable memory card, and since the images may be reviewed using the camera itself, it is possible to use the camera without a computer. Astronomical CCD cameras require the use of a computer to take and store and view the images. However, it is highly recommended to use a computer to control a DSLR for astronomical imaging, in the same manner that it would be used for a CCD camera. The primary advantage is that focusing using only the camera can be very difficult; focusing using a computer program such as MaxIM DL, Nebulosity or BackyardEOS to control the camera is vastly easier and more precise. Also, automatic sequences of images can be taken using the computer software and then stored directly to the hard drive.
Pros and Cons of DSLRs
Less expensive than a comparably sized CCD
Less sensitive overall than a CCD
Can be used for regular photography
Typically not very red sensitive
Not absolutely necessary to use a laptop in the field (although highly recommended)
Webcams and Planetary Cameras
A decade or so ago, amateur astronomers started using webcams as ideal planetary cameras. The lens of the webcam was removed, and the little camera inserted like an eyepiece into the telescope. Picking up on the trend, astronomical camera manufacturers started producing their own optimized planetary cameras. These started out basically as lensless webcams, but they have matured into low noise, high-speed video cameras that are perfect for planetary imaging. Why are these cameras so ideal?
The most important factor in imaging the planets is seeing conditions, or the steadiness of the atmosphere. You can have the best telescope in the world, but if the seeing is poor, all those optics won't do you any good. However, the atmosphere is constantly changing. Anyone who has done much planetary viewing at high magnifications knows that you have to watch and wait for moments of steady seeing to catch the finest details.
The same is true for imaging. If you try to take a still image (with a CCD or digital camera) of a planet, odds are the atmosphere will blur the picture. If you take enough pictures, eventually you will end up with a sharp one. As with most astronomical imaging, stacking multiple frames helps reduce noise and increase detail. So you might want to stack many planetary images to get the most detail. Now, if only 1 image in 5 is sufficiently sharp, and you decide you want to stack 100 frames, you will have to take 500 images. This is a nightmare using a CCD or digital camera.
Enter the webcam. Webcams and planetary cameras capture video clips, which are typically taken at a rate of 30 frames per second or faster. This means that in under a minute you could capture thousands of images. Then, using software such as Registax, the computer can analyze the images, reject the blurry ones, stack the sharp ones, and enhance the image to give incredible detail. A $10,000 CCD camera can't give better planetary images than a $200 planetary camera!
Above: On the left, a raw frame from a webcam video of Mars. On the right, the processed image, the result of stacking the best 400 images out of 1000.
Be sure to visit the Webcam page for more details on capturing images with webcams.
Deep-Sky Video Cameras
Video systems offer the possibility of real-time viewing of objects on a computer screen or TV monitor. While most video systems are not anywhere near as sensitive as CCD cameras, some offer features to allow viewing of deep-sky objects. By hooking a video camera to a computer, images can be captured and stacked as with a CCD camera, allowing detailed still images of deep-sky objects to be captured.
In general, black and white video cameras are more sensitive than color cameras, so for deep-sky viewing or imaging, black and white cameras give the most detail, but newer color cameras are becoming quite good. Systems such as the Mallincam allow images to be stacked and displayed on a TV monitor without the use of a computer. Using a computer with these systems further enhances their capabilities by allowing images to be combined and enhanced just like a CCD image. Real-time systems like these strongly benefit from a telescope with a very fast focal ratio because you have a very limited exposure time. Systems like the HyperStar at f/2 are perfect for these types of camera.