Properties of the KAF-3200ME Image Sensor


This part of my review focuses on the KAF-3200ME image sensor itself. The 3200ME is described in a technical paper, “A 3.2 Million Pixel Full-Frame True Two-Phase CCD Image Sensor Incorporating Transparent Gate Technology” by E.J. Meisenzahl, et al., available from (http://www.kodak.com/go/imagers), and Kodak’s technical specification, “KAF-3200E / KAF-3200ME Performance Specification.”

Before launching into technical details, I want to point out that in my tests, the KAF-3200ME was a shining star. It is roughly twice as sensitive as many front-illuminated CCDs, and it is sensitive across the entire spectrum from deep violet right into the infrared. Furthermore, its dark current is extremely low, its readout noise is very low, and its bias frames are exceptionally uniform and smooth. Having established its qualities, we’ll pick a few nits.

 

Indium Tin Oxide Gate Technology

By using microlenses in conjunction with standard polysilicon for one clock phase and indium tin oxide (a blue-transparent gate material) for the other clock phase, the KAF-3200ME attains a quantum efficiency of ~60% at 400 nm (blue-violet), peaking at ~80% at 600 nm (orange) wavelength, and falling to ~60% at 760 nm wavelength.

This means that each pixel consists of two areas, one that is roughly ten times more sensitive to blue/violet light than the other. Cylindrical microlenses over the array direct all light falling on the device to the indium tin oxide side of the pixel.

In 2005, several members of the AASVO Photometry group raised questions whether the dual gate structures might produce artifacts such as those reported by Christian Buil (see http://astrosurf.com/buil/us/fiber/fiber.htm). No further discussion or testing appears to have followed. Buil’s investigation suggests that users should avoid making undersampled images, especially in green, blue, and violet.

 

Micro-Lens Artifacts

Micro lenses are tiny cylinder lenses designed to concentrate light on the indium tin oxide half of each pixel in the KAF 3200ME. Although I can imagine scattering from one micro lens to adjacent micro lenses could account for some artifacts, they may also be the astronomical equivalent of the “urban myth.”

Because I aligned my camera precisely north-south and east-west, I was not able to attribute possible low-intensity extensions along rows and columns in my images with any certainty to spider diffraction or blooming. If the effect is real, it is very small.

 

Residual Images

When you take an image that saturates the CCD, some photoelectrons diffuse into the silicon substrate. When you shoot the next image, some of those electrons diffuse back into the charge well, appearing as a “ghost” of the first image. Figure 12 shows effect; the left image is a 60 second exposure of a bright star; the right image is a 60-second dark frame. In time the residual image fades.

I investigated residual images by making a single 60-second exposure of a bright star followed by sixty 60-second dark frames. Using AIP4Win’s Multi-Image Photometry tool, I measured the mean pixel value of the residual image. The decay curve is shown in Figure 13. In 5 minutes, the residual image falls to the level of the readout noise and becomes hard to see. Using precision photometry, I was able to follow the decay for about 15 minutes.

The moral of the story is that after making images with bright stars, you should avoid making dark frames or images of faint objects for 10 to 15 minutes.

 

Cosmic Rays and Particle Tracks

While acquiring dark frames and deep-sky images, I observed a large number of “cosmic rays” in the images. In dark frames, they appear as bright spots, tracks, and worms that make corresponding dark spots, tracks, and worms when subtracted from images. Calibrated images thus end up with a scattering of irregular bright and dark blemishes.

In a test run using dark frames totaling 7680 seconds of exposure, I calculated a mean rate of 650 x 10-9 events/pixel/second, which means that KAF-3200ME images accumulate roughly 120 “cosmic ray events” per minute of exposure in images and dark frames.

Figure 14 presents a sample section of this long-exposure dark frame and its histogram.

Cosmic rays were discussed on the AAVSO Photometry list serve; Ben Davies found that the radioactive isotope potassium-40 (K40) in the cover glass of the KAF-3200ME is the likely source of these interlopers. Kodak uses Schott D263 glass containing 6.9% potassium rather than common optical glass BK7, which contains 11% potassium.

The most relevant study of these “cosmic rays” is found in a paper, “Radiation Events in astronomical CCD images,” by A. R. Smith et al., available on the web at http://en.scientificcommons.org/6771406. This paper claims that the rare straight tracks are real cosmic-ray muons, but the curvy worms and star-like spots are caused by radioactive decays that produce alpha-particles and low-energy electrons.

Because they occur randomly, cosmic rays can be removed when multi-image exposures are stacked by using an algorithm such as a normalized sigma-clip. This removes statistical outliers while retaining statistically normal pixel values.

Copyright 2008 by Richard Berry


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