Robert W. Leach

CCD and IR array controllers

A family of controllers has bene developed that is powerful and flexible enough to operate a wide range of CCD and IR focal plane arrays in a variety of ground-based applications. These include fast readout of small CCD and IR arrays for adaptive optics applications, slow readout of large CCD and IR mosaics, and single CCD and IR array operation at low background/low noise regimes as well as high background/high speed regimes. The CCD and IR controllers have a common digital core based on user- programmable digital signal processors that are used to generate the array clocking and signal processing signals customized for each application. A fiber optic link passes image data and commands to VME or PCI interface boards resident in a host computer to the controller. CCD signal processing is done with a dual slope integrator operating at speeds of up to one Megapixel per second per channel. Signal processing of IR arrays is done either with a dual channel video processor or a four channel video processor that has built-in image memory and a coadder to 32-bit precision for operating high background arrays. Recent developments underway include the implementation of a fast fiber optic data link operating at a speed of 12.5 Megapixels per second for fast image transfer from the controller to the host computer, and supporting image acquisition software and device drivers for the PCI interface board for the Sun Solaris, Linux and Windows 2000 operating systems.

New generation CCD controller requirements and an example : The San Diego State University generation II controller

New generation astronomical CCD controllers are being required to operate a variety of CCDs in a range of ground-based applications. These include simultaneous readout from two or four corners of the same CCD (multiple readout), operation of several CCDs in the same focal plane (mosaics), fast readout of small devices for wavefront sensing in adaptive optics systems, readout of only a small region or number of regions of a single CCD (sub-image or region of interest readout), merging the charge from neighboring pixels before readout (binning), continuous readout of devices for drift scan observations, and low contrast polarimetric or spectroscopic differential imaging. Most astronomical applications require that the controller electronics not contribute significantly to the readout noise of the CCD, that the dynamic range of the CCD be fully sampled, that the CCD be read out as quickly as possible from one or more readout channels, and that some flexibility in readout modes and device format exist. A further requirement imposed by some institutions is that a single controller design be used for all their CCD instruments to minimize maintenance and development efforts. The Generation II controller design recently completed at San Diego State University to address these requirements is reviewed. A user-programmable digital signal processor (DSP) operating as a sequencer and communications processor is combined with 12-bit digital-to-analog converters for setting all CCD voltages, a video processor chain with speeds of up to 1 MHz, 16-bit analog-to-digital converters, and a bussed backplane architecture for incorporating the control and readout of multiple CCDs by replicating the clock driver and video processing elements.

DESIGN OF A CCD CONTROLLER OPTIMIZED FOR MOSAICS

A controller for operating Thomson-CSF CCDs in a 2 x N mosaic is described. It is designed around a monolithic Digital Signal Processor, a bank of digital-to-analog converters for clock generation, a simple video processor, and a fiber-optic serial data link communicating with an instrument control computer. The controller is compact, low power, low cost, fast, and easily programmable to generate waveforms of arbitrary timing whose voltages are also software controlled. Up to 16 CCDs can be efficiently controlled, and each CCD has its own set of clock drivers and a video processor, allowing customization of the readout of each CCD device.

CCD controller requirements for ground-based optical astronomy

Astronomical CCD controllers are being called upon to operate a wide variety of CCDs in a range of ground-based astronomical applications. These include operation of several CCDs in the same focal plane (mosaics), simultaneous readout from two or four corners of the same CCD (multiple readout), readout of only a small region or number of regions of a single CCD (sub-image or region of interest readout), continuous readout of devices for drift scan observations, differential imaging for low contrast polarimetric or spectroscopic observations and very fast readout of small devices for wavefront sensing in adaptive optics systems. These applications all require that the controller electronics not contribute significantly to the readout noise of the CCD, that the dynamic range of the CCD by fully sampled (except for wavefront sensors), that the CCD be read out as quickly as possible from one or more readout ports, and that considerably flexibility in readout modes (binning, skipping and signal sampling) and device format exist. A further requirement imposed by some institutions is that a single controller design be used for all their CCD instruments to minimize maintenance and development efforts. A controller design recently upgraded to meet these requirements is reviewed. It uses a sequencer built with a programmable DSP to provide user flexibility combined with fast 16-bit A/D converters on a programmable video processor chain to provide either fast or slow readouts.

Design and operation of a multiple readout CCD camera controller

The increased number of pixels in CCD sensors has meant a corresponding increase in the time required to read out the sensor with low noise. Therefore two to four readout circuits are now being included on a single CCD. The CCD image can be broken up into sections and moved separately to each readout amplifier, reducing the overall readout time. This creates the need for CCD camera controllers that can control more than one CCD readout at a time. The system architecture of such a controller is described. The two major boards - a digital timing board shared between all the readouts, and a mostly analog readout dedicated one per readout - are discussed in detail. The timing board contains a fast digital signal processor that contains the readout program and timing information in internal memory and updates the CCD clocks at a 10 MHz rate. The performance of the analog cicuitry and a sample readout program described.

UnISIS: Laser Guide Star and Natural Guide Star Adaptive Optics System

UnISIS (University of Illinois Seeing Improvement System) is a versatile adaptive optics system mounted on a large optics bench at the coude focus of the Mount Wilson 2.5-m telescope. It was designed to have both laser guide star (LGS) and natural guide star (NGS) adaptive optics capabilities. The LGS side of the system relies on a pulsed UV laser with raw power of 30 W capable of creating an artificial laser star via Rayleigh scattering 18 km above the telescope. The LGS system can work at temporal response rates as high as 333 Hz-limited by the UV laser pulse rate-and the NGS system can work at rates up to 1.4 kHz. Each side of the system has its own highspeed wavefront sensor that runs separately, but in the LGS mode the NGS wavefront sensor is converted into a natural star tip-tilt sensor. The deformable mirror is conjugate to the telescopes primary mirror and has one of the most densely packed sets of actuators of any adaptive optics system currently in operation. This paper provides details of the UnISIS design and describes key updates we have made to the system. We show NGS AO-corrected images from the sky from the 900 nm z-band through the 2.12 mu m K(s) band. The highest NGS Strehl achieved to date is 0.67 at K(s) band.

Rayleigh Laser Guide Star Systems: UnISIS Bow‐Tie Shutter and CCD39 Wavefront Camera

Laser guide star systems based on Rayleigh scattering require some means to deal with the flash of low-altitude laser light that follows immediately after each laser pulse. These systems also need a fast shutter to isolate the high-altitude portion of the focused laser beam to make it appear starlike to the wavefront sensor. We describe how these tasks are accomplished with UnISIS, the Rayleigh laser-guided adaptive optics system at the Mount Wilson Observatory 2.5 m telescope. We use several methods: a 10,000 revolution per minute rotating disk, dichroics, a fast sweep and clear mode of the CCD readout electronics on a 10 μs timescale, and a Pockels cell shutter system. The Pockels cell shutter would be conventional in design if the laser light were naturally polarized, but the UnISIS 351 nm laser is unpolarized. Therefore, we have designed and put into operation a dual Pockels cell shutter in a unique bow-tie arrangement.

Noise and readout performance of a multiple-readout CCD controller

As astonomical CCD image sensors have incorporated more pixels the total readout time of the arrays has increased to the point where it can significatly reduce overall available observing time. The incorporation of multiple readout circuits on CCDs and multiple readout capability in their associated controllers allow several pixels to be read out simultaneously, reducing the overall readout time of the array. A controller is used to operate a single CCD containing four readout circuits that operate simultaneously, giving a factor of three reduction in readout time, and a small increase in readout noise. Another tactic for reducing readout time is described, that is easily implemented with programmable CCD controllers that reads out only selected regions of interest and discards the uninteresting portions of the image, giving a considerable decrease in readout time and no readout noise penalty.

Low-light-level performance of a TI 800 X 800 CCD

The behavior of a thinned 800 X 800 CCD for use in low-light-level detection programs in optical astronomy is discussed with respect to charge-transfer efficiency, noise statistics, quantum efficiency, and charge loss effects. It is found that charge transfer in the horizontal direction below the one-electron level is good. In the vertical direction, charge transfer deteriorates below three electrons. Down to a level of two electrons per pixel, image noise is attributable to the sum of readout noise and Poisson counting noise. It is shown that the quantum efficiency is independent of light level over a range of 0.6 to 400 electrons. Leakage effects on a time scale of four hours at a count level of 82 electrons per pixel are found to be less than 0.11 electron per pixel per hour.

CHARACTERISTICS OF FOUR THINNED TI 800 X 800 CCDS

Four CCD imaging sensors manufactured by Texas Instruments have been optimized and characterized for use from 3000 A to 10,000 A. They are 800 x 800-pixel arrays that have been thinned and UV-flooded and yield quantum efficiencies that are close to the reflection limit for silicon. Readout noise values of 6.2 to 6.8 electrons rms are obtained for three of the four devices. A dark current of 20 electrons per pixel per hour is obtainable by cooling the CCDs to -140 C, while operating them in inversion mode can give a dark current as low as 2 electrons per pixel per hour. The conversion gain, full well, and cosmetics of each sensor are discussed. The detection rate of cosmic rays and the distribution of the number of detected electrons per cosmic ray is in good agreement with theory, indicating that there are no local source of radioactivity.