Ccd Testing And Characterization At Fermilab

Every device to be installed in the DES focal plane will first be characterized in the CCD testing facility being built at Fermi National Accelerator Laboratory (Fermilab). Testing and grading includes an initial phase in which CCDs are exercised and characterized, and a production phase in which the production devices will be characterized to determine which should populate the focal plane. This initial phase is important to develop the infrastructure and experience so that the production phase may begin when devices first become available. The production phase will be designed to test up to 20 devices per month, with some capacity to absorb bursts of higher delivery rates. Packaging of the devices for the focal plane will also be performed at Fermilab.

The testing and grading task is required to make a comparison of the test results against the device requirements to insure that each CCD device installed meets the minimum requirements for use in DES. This is also required to determine grade, so that the highest quality devices will be used.

The electronics chosen for the readout of the focal plane devices in DES will be based on the Monsoon standard [6]. These readout electronics will also be used for CCD testing and characterization. DES is currently adapting the Monsoon readout system to accommodate LBNL devices. The electronics engineering group at DES is designing a 12-channel acquisition board for Monsoon that is required for the number of channels in the DES focal plane.

A prototype test stand exists at Fermilab, and has successfully operated LBNL CCDs. This testing station provides a variety of measurement conditions. CCDs are installed in a dewar (see Fig. 5) that is cooled to -100°C, and controlled by a PID temperature controller (temperature fluctuations are below 0.2 K). An automated mechanical shutter is used for the control of the exposure time. A broadband light source is used for illuminating CCDs inside the dewar, and a flat field is obtained by using an 6' integrating sphere. A calibrated photo-diode provides an absolute intensity measurement of the light. Exposure of the CCD to Fe55 can be used for charge transfer efficiency and gain measurements. Narrow band (10 nm) filters are used to select a wavelength for illumination onto the device to measure quantum efficiency. The production testing facility will also include the monochromator for greater wavelength selecting capabilities. The existing testing station is shown in Fig. 4. The testing facility is located in a humidity controlled room equipped with ESD safety tools to reduce the risk of damaging a CCD. In the production phase we expect to have three of these testing stations.

Figure 4. Existing prototype CCD testing station at Fermilab. The aluminum dewar is mounted on a rotating table for easy access to the removable front cover and the CCD. A 6' integrating sphere is used for illumination, and the light level is controlled in another output of the same sphere using a photodiode. Mechanical shutter and filter wheel allow for controlling exposure time and wavelength of the illumination. The container mounted in the chimney of the dewar is filled with LN2 and allows 15 hours of stable operation for 15 hours.

Figure 4. Existing prototype CCD testing station at Fermilab. The aluminum dewar is mounted on a rotating table for easy access to the removable front cover and the CCD. A 6' integrating sphere is used for illumination, and the light level is controlled in another output of the same sphere using a photodiode. Mechanical shutter and filter wheel allow for controlling exposure time and wavelength of the illumination. The container mounted in the chimney of the dewar is filled with LN2 and allows 15 hours of stable operation for 15 hours.

Figure 5. CCD testing dewar. This dewar is cooled with LN2 from the chimney. A fused silica window in the front (see left side of figure) allows for the illumination of the device. To avoid condensation in the CCD, vacuum is pumped inside the dewar. A 10 watt heater is installed in the cold finger connecting the chimney (fill tube) to the CCD to provide the heat needed for CCD temperature stability. An Fe55 source is mounted on the inside of the from cover of the dewar, to allow for X-ray exposures of the CCD. The source can be moved out of the way (Fe55 knob) and parked in a shielded garage when not in use.

Figure 5. CCD testing dewar. This dewar is cooled with LN2 from the chimney. A fused silica window in the front (see left side of figure) allows for the illumination of the device. To avoid condensation in the CCD, vacuum is pumped inside the dewar. A 10 watt heater is installed in the cold finger connecting the chimney (fill tube) to the CCD to provide the heat needed for CCD temperature stability. An Fe55 source is mounted on the inside of the from cover of the dewar, to allow for X-ray exposures of the CCD. The source can be moved out of the way (Fe55 knob) and parked in a shielded garage when not in use.

Figure 6. Full size CCD testing vessel. It will have a focal plane that can hold up to 6 CCDs and will be a test bench for many of the mechanical design ideas for the DES camera vessel. It will be equipped with a movable Fe55 source to produce X-ray exposures of the devices in the focal plane.

Also programmed as part of the CCD testing at Fermilab is a full size camera vessel prototype (see Fig. 6), with a focal plane of 6 CCDs and operated with Monsoon electronics. This will become a test for the readout electronics (10% of the full DES focal plane) and will contribute significant information for the final mechanical design of DECam.

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