OIG and SARG CCDs Characterization

CCDs characterization is the preliminary step to perform before the CCD can be properly used at the telescope. Most of the scientific instrumentation at the Italian National Telescope \Galileo" use CCDs as detectors. In particular the optical imager (OIG) and the high resolution spectrograph (SARG) use a mosaic of two 2k X 4k CCD manufactured by EEV (EEV 4280).

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The Calibration Facility

The main component of the facility is the instrumental apparatus to measure the quantum efficiency in the wavelength range 130 - 1100 nm. A complete description is given on the web page.

For uniformity and linearity measurements a 20 inches integrating sphere is optically connected to the QE measurement system through a quartz singlet (see same web page). The useful wavelength interval range in this case is 200 - 1100 nm.
The system gain measurements and the CTE analysis are performed using a camera accomodating a Fe55 x-ray (see same web page) source which is connected to the CCD dewar.

System gain measurement

The measurement of the system gain is obtained exposing the CCD to a Fe 5 X-ray source. The main line (Kalpha) emitted by the source has an energy of 5.9 keV, so the charge deposited by a single photon in each pixel is 1620 e- The measurement of the position of the line on the spectrum of the source recorded by the CCD allows to determine directly the system gain with a precision better than 1%. An example of the X-ray spectrum of the Fe 55 source is shown in Figure 1 for the OIG CCD.

Linearity Analysis

The linearity is measured illuminating the CCD with a uniform source of radiation at increasing exposure times. At each exposure time, ti, we compute the average signal, Si, in a given area and define the deviation from linearity at that signal level as:

Dark Current Analysis

The dark current is obtained through the following procedure:

The frame Difference is divided in 10 x 10 pixels sub frames (boxi) for which the dark current is computed individually. The value of the dark current is obtained as the average of these values.

Uniformity analisys

The uniformity of the CCD is measured by illuminating the detector with a uniform source of radiation at dierent wavelengths. The deviation from homogeneity in a given area is obtained as:

Values of deviation from homogeneity are given in table 1 at three different wave- lengths for the two CCDs.

Quantum Effciency

The quantum effciency is measured illuminating the CCD with monochromatic radiation (see the panel that describes the QE calibration facility) and comparing its response to that of a calibrated detector. The measurements were done between 200 and 1050 nm in incremental step of 50 nm. In Fig. 3 we have plotted the quantum effciency of the OIG CCD and of the SARG CCD.

Charge Transfer Effciency

The charge transfer effciency (CTE) is measured using the X-ray stimulation method. In particular, the parallel CTE is obtained exposing the CCD to a Fe55 source for a given amount of time. After integration, the columns are stacked together and the signal is plotted versus the number of pixel transfers. The CTE is then given by:

 Figure 4 shows the results of this analysis for the OIG CCD (Fig. 4a) and for the SARG CCD. In both cases the CTE is very good (see table 1) and within the specifications of the manufacturer.

This paper is available in pdf format.