Performance Testing a CCD Camera
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 cameras image sensor. In the
controlled environment of the camera body, photoelectrons
accumulate on a CCD image sensor, your cameras
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 QSIs 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 framesbias 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 QSIs 532ws camera, I had to test Kodaks 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:
If any of these functions fail, the performance of the camera suffers. When you buy a CCD camera, a good part of what youre 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 cameras presence in a negative wayif the camera adds noise to the signalthe camera is not functioning as well as it should. In other words, when a camera does its job properly, the CCDnot the camerabecomes 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 Kodaks 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:
Kodak specifies these in a performance specification document; specs for all of Kodaks 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 Kodaks performance specifications in all categories I tested.
Copyright © 2008 by Richard Berry