Advanced CCD Cameras

When starting in CCD imaging, price often determines what camera you buy. Relatively inexpensive CCDs typically have smaller chips, lowerquantum efficiency, and lack certain features such as self-guiding and compatibility with certain accessories. Advanced CCDs take up where the smaller camera leave off, offering larger chips and more features (although inexpensive CCDs get better and better all the time).

CCD Chip Size

Larger chips mean wider fields of view. To capture more sky at once, using a given telescope, a larger chip is required. It is possible to use a short-focal-length telescope or camera lens to capture a wider field of view, but a longer-focal-length scope provides more image scale. For example, a telescope with a 1400mm focal length using a CCD with a Kodak KAF-401 chip has the same field of view as a 3000mm focal-length telescope using a KAF-3200 CCD. However, the 3000mm scope provides more than twice the image scale.

Above: A comparison of the sizes of some common CCD chips. The TC-237 and KAF-0401 are used in smaller CCDs usually costing under $2000. The larger chips are used in higher-end CCDs often costing $6000-$9000.

Number of Pixels

If the goal of your imaging is to get pretty pictures, the number of pixels on the CCD chip can be an important factor. More pixels means a larger image on screen and the possibility of creating larger prints. Cameras with a large number of pixels will often have small pixel sizes, meaning the resolution is higher than a chip with large pixels, which is also often an advantage.

For example, the Kodak KAF-1001 chip is much larger than the KAF-3200. The KAF-1001 is 24mm x 24mm, whereas the KAF-3200 is 15mm x 10mm. However, there are only about 1 million pixels on the KAF-1001 chip, compared to about 3 million on the KAF-3200. This means the pixels on the KAF-1001 are very large (24 microns), while those on the KAF-3200 are very small (6.8 microns).

The advantage of larger pixels is increased sensitivity and a better match to longer focal length telescopes. On short focal length scopes, large pixels can produce undersampled images where the resolution is not as high as is possible. While the disadvantage to small pixels on a long focal length scope is a decrease in sensitivity, this is more than made up for by the advantages that the sheer number of pixels provides for image processing and display. A quick look through the astronomical magazines and websites of the tops CCD imagers will show that CCDs with lots of small pixels matched to long-focus scopes are producing very impressive pictures!

Typically "beginner" CCDs (i.e., under $1000-2000) have less than 2 million pixels, although 3- and 4-million pixels cameras are becoming available for under $2000. Very good quality 8x10 prints can be made with a 3 million pixel CCD. The current state-of-the-art in amateur CCDs is the 11-million-pixel STL-11000M camera from SBIG. This camera offers a huge field of view and tons of pixels for extremely high-resolution images.

Quantum Efficiency

If the goal of your imaging is more scientific in nature--imaging extremely faint galaxies, hunting for asteroids and comets, etc.--a CCD with a high quantum efficiency (QE) may be desirable. Usually high QE is matched with large pixel sizes (since large pixels tend to be more sensitive). The trade off is resolution and the number of pixels (unless money is no object). For pretty pictures, the advantages of smaller and more pixels outweigh the advantages of higher QE. For capturing data, higher QE has the edge. Keep in mind that many of the most spectacular images being taken by amateur astronomers currently are being done with the SBIG STL-11000, a camera with a relatively low peak QE of only 42%. This is still 20 times more sensitive than film was!

What is High QE?

The table below gives examples of different cameras and their peak quantum efficiencies.

The combination of large pixels and high QE allows pictures to be taken rapidly. Many more pictures can be taken during the night if the exposure times are very short. This is a definite advantage for search programs where much of the sky must be covered during one night.

Many camera, however, now offer very high QEs with small pixel sizes. Many of the Kodak sensors used in many popular CCD models use microlenses to increase the sensitivity of the CCD. Microlenses are placed over the photosites in a CCD array and concentrate light that would normally be lost to the edges of the photosite. Cameras with QEs as high as 85% are now common, even in the $1000 price range.

Self-Guiding CCD Cameras

SBIG offers several cameras with a feature called self-guiding. In these cameras a second, smaller autoguiding CCD is built into the camera just above the main CCD chip. While a long exposure is taken with the main CCD, the autoguider takes short exposures (usually a few seconds in duration) and sends corrections to the mount to keep the telescope tracking properly. This eliminates tracking errors inherent in the drive motors of all telescopes.

Other cameras may be guided using separate CCD cameras and off-axis guiders or guidescopes. However, off-axis guiders tend to be difficult to use. Despite the potential for flexure, guidescopes are by far the preferred method for guiding with a second camera.

Compatibility

Some of the advanced accessories for CCD cameras (adaptive optics units, spectrographs, certain filter wheels) are often only compatible with higher-end CCD cameras. If these features are useful to the type of imaging you wish to do, a more advanced CCD camera may be appropriate.