Guider camera frame transfer ccd
With a frame-transfer CCD, the recorded image charge is rapidly shifted from the illuminated area to the readout/storage area of the device at the end of each integration time. Provided this transfer is fast compared to the integration time, a reasonable extinction ratio is maintained and a shutter is not required. But, there is a limited choice of large-format, big pixel, back-illuminated CCDs with an on-chip frame-transfer mask.



E2V CCD47-20 detector


The preferred detector for the guider camera is an E2V CCD47-20 frame transfer CCD. The array is made up of two sections of 1024×1024 pixels, each independently clocked. One section is covered with an opaque aluminum mask. The detector has a split serial register with two output amplifiers, so it can be read using either output (or both). The device is backside illuminated, with 13 µm square pixels, 80% broadband quantum efficiency, and less than 3 e- rms readout noise (at 20 kpix/s). The recommended parallel transfer time for the back-illuminated E2V CCD47 chip is 10 µs, so the frame transfer time is close to 10 ms.

Another candidate detector is the E2V CCD67, a 256×256 pixel frame transfer device with 26 µm pixels, and an on-chip mask. The smaller format (ie smaller field-of-view) of this device can be traded-off for a faster frame rate.

And, there is also the SITe 1024×1024 pixel, back-illuminated SI-003A device. With an off-chip mask attached to the CCD package, it could be used in frame transfer mode, with a 512×1024 pixel imaging area. The principal limitation of this device is the parallel transfer time (180 µs) which limits the frame rate.The guider camera electronics will support various detector choices without modification. Each different detector is mounted on a custom header board. The characteristics of the three detectors are shown in the table.
CCD Detector Properties
Device Specifications E2V CCD47 E2V CCD67
SITe SI-003A
Units
Readout Noise 3 3 5 e-
Peak Quantum Efficiency >80 >80 >80 %
Array size 2048 × 1024 512 × 256 1024 × 1024
pixels
Pixel size 13 26 24 µm
Amplifiers 2 2 2
Dark current 250 1000 700 e-/pix/s

Greg Burley (burley@obs.carnegiescience.edu)
Ian Thompson (ian@obs.carnegiescience.edu)