There are many options to consider when choosing a CCD camera, but usually there are a couple majors factors which can make the decision easier. Admittedly cost is one of the most important factors, but there can be a variety of cameras available within your price range. Matching a CCD camera to a particular telescope can be a factor as well. The following section gives you some ideas how to select the right camera for you. If you have any questions concerning the choice of an appropriate CCD camera, please feel free to contact us.
Types of CCD Cameras
Just as there are various types of telescopes, there are also many different types of CCD cameras. The basic difference between models is the CCD sensor itself, just like the mirror or lens is the basic difference between telescope models. And like telescopes, there are other features available which may make a certain camera more desirable.
CCD Sensor Size
A bigger CCD sensor will, all other things being equal, image a bigger part of the sky. However, all things are not necessarily equal, and a short focal length telescope with a smaller CCD may have a wider field of view than a long-focal-length scope with a larger CCD. Depending on what you want to image, a small-format CCD may suffice, but it is usually desirable to have as large a CCD sensor as possible. Back to the telescope analogy, figure that everyone wants a bigger telescope, just like everyone wants a bigger CCD, but the real limitations are budget and practicality.
The image above shows common CCD sensors to scale. The actual sizes are about half what they appear on a typical monitor. The table below lists specifications for the sensors above.
|Sensor||Number of Pixels||Pixel Size||Width x Height||Example Camera Models|
|KAF-16803||16 million||9 microns||36.8 x 36.8 mm||Apogee U16M, SBIG STX-16803|
|KAI-11000||11 million||9 microns||36 x 24.7 mm||SBIG STL-11000|
|APS-C Format||6-18 million||4.3-7.8 microns||23.4 x 15.6 mm||Canon EOS 7D, Starlight Xpress M26C|
|KAF-8300||8.3 million||5.4 microns||18 x 13.5 mm||Atik 383, SBIG ST-8300, Starlight Xpress H18|
|KAI-4000||4 million||7.4 microns||15 x 15 mm||Atik 4000, SBIG ST-4000|
|ICX694||6 million||4.54 microns||12.5 x 10 mm||Starlight Xpress H694|
|KAI-2000||2 million||7.4 microns||11.8 x 8.9 mm||SBIG ST-2000|
|ICX674||2.8 million||4.54 microns||8.8 x 6.6 mm||Atik 428EX, Starlight Xpress H674|
|KAF-0400||400,000||9 microns||6.9 x 4.6 mm||SBIG ST-7, SBIG ST-402|
The size of the CCD chip is one of the most important factors in imaging. The size of the chip largely determines the field of view, and thus which objects you can take pictures of. Different telescopes can change the field of view as well, but since most people own just one telescope, choosing a CCD with a sufficient field for that telescope is a good idea.
CCD Pixel Size
Here’s a touchy subject. One school of thought is that there is an ideal match between CCD pixel size and telescope focal length. The idea is you want to properly “sample” the stars in your image, which are usually limited by seeing conditions to around 2 arcseconds. Thus you nominally want around 2-3 pixels per 2 arcseconds. If your goal is the highest resolution images possible, especially for scientific applications, then this idea is valid. However, if pretty pictures are your goal, then the second school of thought is probably more appropriate: pixel size doesn’t matter.
On a given telescope, smaller pixels give higher resolution. Larger pixels tend to be more sensitive. There is always a trade-off. Figure the number of pixels, and the size of the chip itself, are more important than pixel size for most purposes.
Above: Standard CCD theory holds that a resolution of about 1 arcsecond/pixel is ideal. Above is an image taken with a resolution of 3.7 arcseconds/pixel. Pixel size is really no big deal for most imaging.
Number of Pixels
Again, more is better… usually. The only potential drawback to a greater number of pixels is a slower download time from the camera and longer processing times while manipulating the image later. Many CCD cameras now have high-speed USB 3.0 connections, and computers are always getting faster, so these disadvantages are becoming less and less important. The advantages of more pixels include larger images on a computer monitor, the ability to make larger prints, and the images tend to be more aesthetically pleasing, even when reduced significantly to be viewed on the web.
One-Shot Color vs. Tri-Color Imaging
A major advance in CCD technology in recent years has been the advent of one-shot color cameras. Many CCD chips are inherently black and white (monochrome). These CCDs require colored filters (red, green, and blue), and a minimum of 3 exposures (one through each color filter) to capture a color image. Many new CCDs incorporate one-shot color chips. These chips divide the sensor into separate red, green, and blue sensitive pixels. There is no need for individual color filters, so a single exposure can capture a color image. While these chips are sometimes less sensitive than their monochrome counterparts, they require only a single exposure instead of three, so the total exposure times tend to be similar or even less. Also, newer one-shot color cameras tend to be much more sensitive than their predecessors. The pixels in a one-shot color CCD are typically divided into 1/2 green, 1/4 red and 1/4 blue pixels. This means each color has to be interpolated to create an RGB color for each pixel. This reduces the resolution versus a monochrome camera, though thanks to the algorithms used to interpolate the pixels, it is only around 25% less resolution.
Monochrome CCDs tend to have more versatility than one-shot color cameras. They are better suited to narrowband imaging and scientific applications such as asteroid searches or photometric studies. However, for taking pretty pictures, one-shot color cameras work very well and offer a very easy learning curve for beginners.
The Reality of the Situation
In an ideal world, every CCD imager would have the biggest, most sensitive CCD camera available; just like every amateur astronomer would have the biggest telescope! But like telescopes, price is always a factor, and almost any quality CCD camera will work fine with any capable telescope. Technique is a big part of it. (Everyone with an inexpensive DLSR has a better camera for normal photography than Ansel Adams ever had, but he still took better pictures than any of us!) Great images have been taken with small telescopes and relatively inexpensive CCD cameras from suburban backyards, many of which are better than some images taken with high-end, sophisticated telescopes, monster CCDs, and permanent observatories at dark sites.