Performance Testing a CCD Camera

Capturing images from the sky is a complex process. Starlight passes through the atmosphere, through your telescope, through the filters in your camera, until it finally strikes your camera’s image sensor. In the controlled environment of the camera body, photoelectrons accumulate on a CCD image sensor, your camera’s electronics read out the CCD, digitize the signal, and pass data to the driver software in your computer, until it finally reaches the image acquisition software.

Since my goal was to test the QSI’s 532ws performance, I needed to simplify the system. Rather than taking sky images and introducing a host of unknowns, I tested by taking standard calibration frames—bias frames, darks, and flats. This is the way professional astronomers test their cameras and CCDs.

The performance of the camera and the performance of the CCD image sensor are all but inseparable. In testing QSI’s 532ws camera, I had to test Kodak’s KAF-3200ME CCD image sensor as well.

The camera and the CCD inside it enjoy an intimate relationship. The camera not only provides a benign physical environment for the CCD, but it also controls the electronic environment. For top-notch performance, the camera and the CCD must work well together. Put a good CCD into a bad camera, or put a bad CCD into a good camera, and performance suffers. Only when the camera and CCD are both top-notch can you expect truly great imaging performance.

What does the camera do? Here is a list of key functions:

Provides physical connection to external optics.
Provides a clean, dry, airtight container for the CCD.
On command, moves optical filters into the light path.
On command, shutters light entering the CCD.
Cools the CCD to desired operating temperature.
Provides all voltages necessary for correct operation.
Controls CCD functions by executing internal firmware.
On command, reads, digitizes, and stores image data.
Communicates data to external computer software.

If any of these functions fail, the performance of the camera suffers. When you buy a CCD camera, a good part of what you’re paying for is the know-how that the camera manufacturer brings to the design, assembly, and configuration of the camera.

A well designed camera should be nearly transparent to the user. It should extract every bit of performance that the CCD can deliver. If you become aware of the camera’s presence in a negative way—if the camera adds noise to the signal—the camera is not functioning as well as it should. In other words, when a camera does its job properly, the CCD—not the camera—becomes the performance-limiting factor.

My approach was to test the QSI 532ws as a “black box,” that is, I explored the characteristics of the camera through the images it produced. I compared these images to the performance of a hypothetical “ideal” image sensor and against Kodak’s published specifications for the KAF-3200ME image sensor. If the images were degraded from the “ideal” CCD, or worse than the specifications for the Kodak image sensor, I held the camera responsible.

Key characteristics for the image sensor in a CCD camera are:

Readout noise
Dark current
Saturation signal
Non-Linearity
Non-Uniformity
Dynamic range
Quantum efficiency

Kodak specifies these in a performance specification document; specs for all of Kodak’s CCDs are published on the web (http://www.kodak.com/go/imagers) as a PDF documents that anyone can access. The only specification that I was unable to test was quantum efficiency, which requires calibrated laboratory equipment.

I am happy to report that the QSI 532ws met or exceeded Kodak’s performance specifications in all categories I tested.

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