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L3 Technology CCDs for NAOMI WFS upgrades |
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Project
Progress Reports and latest news
The NAOMI Wavefront sensor is detector noise limited in many observing regimes. Any new detector exhibiting lower read noise whilst retaining other performance criteria will offer fainter guide star limits and greater sky coverage. In the case of a Rayleigh laser beacon being implemented, it would mean the beacon could be observed at higher altitudes thus improving the wavefront correction.
We have recently bought a new E2V CCD that has the potential to remove the read noise limitation of the current WFS detector. It uses a new internal gain technology (LLLCCD or L3) that amplifies the pixel charge before it is measured by the CCD output amplifier. Read noise becomes negligible. The device we have bought, the CCD60, is specifically designed for WFS applications. Its characteristics are compared below with the current NAOMI CCD39 WFS detector.
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CCD60 |
CCD39 |
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Format |
128 x 128 pixels Frame Transfer |
80 x 80 pixels Frame transfer |
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Pixel size |
24microns |
24 microns |
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Peak QE |
93% |
90% |
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Number of outputs |
1 |
4 |
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Read Noise |
<1e |
5e |
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Pixel rate |
1.1Mpix/s (SDSU limited) |
1.6Mpix/s (CCD limited) |
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Summing well for binning |
Not present |
Not present |
The CCD60 is capable of much higher read out rates but we are limited by the performance of the SDSU controller which has a mimimum clock transition interval of 40ns (approximately 6 of which are required to do a single pixel shift).
Direct comparisons of performance between the two chips are complicated by the presence of a new noise mechanism in the L3 detectors. The multiplication processs is intrinsically noisy and will degrade the SNR by up to a factor of 1.4. Another potential problem is spurious charge generation during image transfer. This is normally not visible in normal CCDs as it is masked by the effect of read noise. Here, it could be a dominant noise source but can be reduced by careful choice of clock voltages.
The graph below compares the performance of two WFS detectors. The L3 detector is shown in two of the curves, one with a multiplication noise factor (‘X noise factor’) of 1.4 and the other with a multiplication noise factor of 1.0. An actual device will lie somewhere between these two extremes.
There is a clear advantage in using the L3 CCD at low signal levels. Additionally the high front end gain will give the new detector greater immunity to external interference.
The WFS Upgrade Feasability Study
This is being conducted at the University of Durham. A test camera will be built around the CCD60 incorporating a lenselet array and stable light source. It will be interfaced to an SDSU controller that incorporates an additional circuit module from E2V required to generate the stable high voltage multiplication register clock. The CCD will first be operated in normal unity gain mode where it should give the same performance (although somewhat slower) than the current WFS detector. It will then be operated in high gain mode and its performance measured and compared with the first results. Particular attention will be paid to the following areas:
Maximum read out speed and frame rate
Signal to Noise Ratio improvements at low
signal levels (see here for actual data)
Multiplication gain stability with
time/temperature/device self heating (see
here for gain/temperature relationship)
Actual excess noise factor (see here for actual measurements)
Minimising the spurious charge generation
levels
Binning performance (there is no summing well
and when binned the device could be noisy)
At high gain levels and very low photon arrival rates, these chips can be used for photon counting. This has no application to WFS but may be useful for other astronomical regimes. The photon counting performance will also be assessed using this test camera.
Further Information
ING Web Page on L3
technology (with various papers on the subject)
E2V CCD60 Drive Module Data Sheet
E2V
Paper measuring the excess noise factor in L3 chips (March 03)
Detector Group Astronomy Pages