L3 CCD Technology

Simon Tulloch (ING)

Low Light Level (L3) CCD technology is a recent development from E2V that opens up interesting new observational regimes. The technology allows production of scientific CCDs in which the read noise of the on-chip amplifier becomes negligibly low. Additionally, this effective zero-noise performance is decoupled from readout speeds and the almost zero noise performance holds up to frame rates of 1KHz. E2V achieve this by using an avalanche multiplication mechanism in the horizontal register of the CCD. A single photo-electron entering this register exits as a substantial charge packet; the exact gain being variable and determined by the level of a high voltage multiplication clock. At gain levels of around 500 it becomes possible to identify individual photon events in the image. The downside of L3 technology is that the multiplication process degrades the SNR at higher signals by a factor of 2^1/2 . There is also a small additional noise contribution from spuriously generated electrons within the device.

L3 CCDs should be useful in any observing regime currently limited by detector noise. Wavefront sensing is an obvious application and E2V have produced the 128×128 pixel frame transfer CCD60 with this in mind. We have purchased one of these and are currently working on an upgrade to the Naomi WFS. Figure 1 shows the kind of gains we can expect once this system is commissioned. For comparison, the performance of Naomi’s current WFS, the CCD39, is also shown.

Other applications for L3 technology are currently being evaluated on the WHT using a cryogenic test camera in which we have mounted the CCD60 (see Figure 2).

Figure 1 (left). Potential L3 gains in NAOMI. Thanks to Richard Wilson for providing Figure 1 [ JPEG | TIFF ]. Figure 2 (right). The L3 Test Camera [ JPEG | TIFF ].

 
This camera was mounted on the Auxiliary port in November where we tested its performance as a fast photometer. On the 10th we observed the Crab Nebular pulsar and were able to directly distinguish its 30Hz variability. The camera used its own data acquisition system based around a Linux PC and a slightly modified SDSUII controller. This DAS combined the functions of an acquisition TV and a science camera. This was important given the rather small 14 arcsec field of view. Once the image was acquired, the camera switched to its fast photometry mode in which it made a rapid sequence of 1024 windowed readouts at a rate of 180 frames per second. The resultant image format consisted of a ‘movie strip’ of consecutive frames, a short section of which is shown in Figure 3.

Figure 3. The Crab Pulsar indicated by the red arrows in this series of frames. [ JPEG | TIFF ]

Although faint, the pulsar is visible. The red arrows indicate the frames in which the brightness peaks. An animated GIF of these pulsar observations can be found at: http://www.ing.iac.es/~smt/WFS/CrabMovie.gif.

We currently have on order a larger engineering grade L3 CCD measuring 512×512 pixels. This will be incorporated into a second test camera and mounted on ISIS where its suitability for rapid spectroscopy will be investigated .

Thanks to Durham University’s RLGS team and to Vik Dhillon for their cooperation in the testing of this new camera.¤

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Email contact: Simon Tulloch (smt@ing.iac.es)