CCD vs. DSLR Astrophotography
Film astrophotography began in the late 19th century with photographs of the moon, sun, and bright stars. Film then was thousands of times slower than it is today. While film became faster and faster, the technology itself changed very little. Then, in the 1970s, professional astronomers began using CCD cameras instead of film. Eventually, CCDs became more readily available for amateur astronomers. As CCDs become less expensive, they are becoming more popular and have all but completely overtaken film for astrophotography. Digital cameras for regular photography have also supplanted film use, so the natural question to ask is whether astrophotography can be done with a regular digital camera. The answer is yes, but there are advantages to both types of imaging.
Note: Since only digital SLRs are practical for deep-sky imaging, this discussion is limited to that type of camera. See the Digital Cameras section for a comparison of point-and-shoot digicams and DSLRs.
The Advantages of CCD Imaging
The primary advantage of CCD imaging is sensitivity. The more sensitive the camera, the shorter the exposure required to detect faint detail.
Above: The Veil Nebula imaged with a CCD camera from light-polluted suburban skies. Six 5-minute exposures with a HyperStar C14.
- CCD cameras are up to 50 times more sensitive than standard digital SLRs
- CCD cameras have a greater dynamic range than digital SLRs, meaning they can more easily capture both faint and bright detail in a single exposure
- Most (but not all) CCDs have a linear response, which means they can be used for photometry--studying the brightness of objects such as variable stars or asteroids
- The shorter exposure times required by CCDs means learning is easier
- The fast exposures also mean that most of the time less equipment and hassle will be required
- Less hardware may be required for CCD imaging, depending on the setup (see below)
- Greater sensitivity makes imaging through filters (narrowband or light pollution filters) easier
The Advantages of Digital SLRs
The biggest advantage of using a digital SLR is cost (with some notable exceptions discussed below). The larger the chip is in a CCD or DSLR, the more the camera will cost (all else being equal). For a given size chip, a DSLR will cost less than a CCD. This is due to several reasons. CCD chips are much more sensitive, they have fewer pixel defects, they have less noise, and the cameras often feature thermoelectric cooling to reduce noise even further. DSLRs are mass produced, so economies of scale come into play. Canon has sold over 1 million of its EOS Digital Rebel XT cameras. So the camera itself is less expensive, although with some setups, more extraneous hardware may be required.
Above: Image of the Lagoon Nebula region with a Canon EOS 20Da (enhanced DSLR). Single 20-minute exposure through a 300mm f/2.8 lens. Image by James McGaha.
- Cameras are less expensive for a given size chip and number of megapixels
- A digital SLR can be used for other purposes such as photographing your vacation, your cat, your new telescope, your cat's new telescope, etc.
- A larger chip means a larger field of view, and more pixels means larger printed photos
While a digital SLR may cost less than a comparable (in terms of chip size and pixel count) CCD camera, the overall cost may not be less for a DSLR. The best example is HyperStar imaging. A very common setup would use an 8" Schmidt-Cassegrain telescope. For DSLR imaging of deep-sky objects, this setup will also require an equatorial wedge, focal reducer, guidescope, guide camera, mounting rings and dovetail plate, and counterweights. This extra hardware would cost around $1500. For CCD imaging the only extra hardware needed would be the HyperStar lens. The savings is nearly $1000, which will likely make up for the difference in cost of the cameras. Not to mention an equivalent exposure would be around 50 times shorter!
On a 9.25" or larger telescope with a HyperStar lens, a DSLR can be used. However, the obstruction due to the camera body is most often larger, causing light loss and diffraction effects. And a CCD will still have greater sensitivity, lower noise, and more dynamic range (bit depth).