Starizona's Guide to CCD Imaging

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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 like there are various types of telescopes, there are also many different types of CCD cameras. The basic difference between models is the CCD chip 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 Chip Size

A bigger CCD chip 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 chip 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.

Above: Common CCD chips shown to scale. The KAI-11000 is about the size of a 35mm film frame, 35mm x 24mm.

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. 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.

CCD Calculator Select a CCD camera and telescope from the menus below. The pixel resolution and field of view will be displayed below. You may also change the binning to see the effect this has on pixel resolution.
CCD Camera
Telescope
Pixel Binning
Resolution: Field of View Width: Height:

Above: Standard CCD theory holds that a resolution of about 1-2 arcseconds/pixel is ideal. Above is an image taken with a resolution of 3.6 arcseconds/pixel. Pixel size is really no big deal for most imaging. Image by Gary Breneman.

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. Most CCD cameras are now coming with high-speed USB 2.0 connections, and computers are becoming extremely fast, so these disadvantages are becoming less and less important. The advantages to 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. 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 chip 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 non-color 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.

Black and white 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, and great images have been taken with small telescopes and relatively inexpensive CCD cameras from suburban backyards, many of which are even better than some images taken with high-end, sophisticated telescopes, monster CCDs, and permanent observatories at dark sites.

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