All Good Cameras Have a Sensor

I spent some time choosing a sensor early in the project, as much to test the feasibility of the project as anything. The CCD sensor will make or break the whole project, so as a sanity test, I decided to go ahead and do the research to choose a chip that fits the application.

The sensor of choice for this project needs to be usable in a camera system that does not include a mechanical shutter. That requirement alone narrows the field. I found that the sensors that go into your basic digital camera would not work because they need that mechanical shutter. Specifically, they need to be covered while the captured image is read out of the sensor. Yes, you can connect your DSLR camera to a telescope, but that camera body includes the mechanical shutter. But then again, newer digital cameras have video modes, so the right sensors for this project surely exist.

We also want our sensor to have an area of around 4Meg pixels. This eliminates most cheap webcam sensors. In fact, 4Meg is large even for HD video. That pretty much rules out webcam sensor chips. We are getting into the realm of scientific imaging sensors.

Fortunately, Kodak has a whole range of sensors, the Interline CCD family, that fits the bill nicely. Not only do these chips support readout while the sensor is uncovered, they have an electronic shutter that allows for precise control of integration time. The KAI-04022 in particular is the right size (4meg) and is square (2048x2048). That is a nice feature. (Square pegs fit better in round holes then do rectangular pegs.) These sensors are indeed intended for scientific imaging. Looking good.

The pixel density of the KAI-04022 is 7.4um square, which gives approximately 1.6 arc-seconds per pixel and a total of 55 arc-minutes field of view when put behind my 1000mm focal length telescope. The Dawes limit for this 120mm refractor is 0.98arc-seconds, so this strikes me as a good compromise. I can add a barlow lens if I really want to push the resolution. If I want to increase the field of view, I'll just need a shorter telescope. There are physically larger sensors in the series, but they are getting into the 8-10MPixel range. Maybe later.

The chip is able to read out pixels at around 40MPixels/second, which gives about 10 frames per second. This is plenty fast enough for a plausible focus mode. It's not fast enough for full-motion video, but that's fine for what I'm after.

And the Kodak chips are well documented, with thorough datasheets, and evaluation boards with complete schematics. That's good.

Finally, I checked with Kodak sales, and the KAI-04022 is currently in production. There are color and grayscale versions of the chip, so I have some flexibility there, and for small quantities the chips themselves can be purchased directly from Kodak for something in the range of $1,000.00. A lot of money, but within the total budget for the camera as a whole, so that's what it will be.

By the way, it turns out that many store-bought astronomy cameras in the $3000+ price range use these or similar chips as well, so that gives me some extra confidence that I'm not too far off the mark.