Performance Of Virgo Detectors

The operation and performance of the VIRGO multiplexer and the test results from the first science detector were reported by Bezawada, et al. [1]. Over 20 detectors have been individually tested since then and the performance of the 16 science detectors is summarized in the following sections whilst a few specific characteristics from SCA-45 are detailed.

Performance Overview of the VISTA IR Detectors 3.1 Conversion Gain

The gain is determined from the spatial photon transfer using several pairs of identical flats obtained by varying integration time whilst the input flux is kept constant. A sample box of 50^50 pixels is moved over the frame and a histogram is obtained from all samples corresponding to signal levels ranging from 20 to 70% of full well. The full well is determined from the noise roll-over point from the signal vs. variance plot (see Fig. 1). The detector transimpedance conversion gain showed a strong dependence with the signal level and varies up to about 14% with SCA-45 (see Fig. 1). A similar dependency is also observed with photon transfer from pairs of flats obtained at various flux levels with fixed integration time. This dependence can be attributed to the increase of the detector junction capacitance (hence the decrease of the node sensitivity) as the input node integrates towards saturation. For the purpose of estimating the full well, the average conversion gain obtained from the signal levels of 20 to 70% of the full well is used. For the purpose of estimating dark, read noise, persistence etc. a conversion gain estimated from low signal level (up to 20% of the full well) is used. The conversion gain is also estimated using the temporal method where the variance is calculated for each pixel in the frame from a sequence of 100 Correlated Double Sample (CDS) frames at a given signal level. The gain estimated from the spatial method agrees very closely with the values obtained from the temporal method at any given signal level. However, the effect of the inter-pixel capacitance on the conversion gain (hence the node capacitance) is being investigated and reported by Finger, et al. [2].

Photon Transfer Curve

Photon Transfer Curve

0 5.0x103 1.0x10" 1.5x10" 2.0x10" 2.5x10" Signal (ADU)

Figure 1. Conversion gain depends on signal level. (left) Photon transfer curve.

(right) Transimpedance conversion gain dependency with signal level.

Trons_Conv_Goin Vs Signal Plot

0 5.0x103 1.0x10" 1.5x10" 2.0x10" 2.5x10" Signal (ADU)

Trons_Conv_Goin Vs Signal Plot

0 5.0X103 1.0X10" 1.5X10" 2.0X10"

Signal (ADU)

0 5.0X103 1.0X10" 1.5X10" 2.0X10"

Signal (ADU)

Figure 1. Conversion gain depends on signal level. (left) Photon transfer curve.

(right) Transimpedance conversion gain dependency with signal level.

Both the conversion gain and the full well are proportional to the applied detector bias (see Fig. 2). The higher detector bias results in a slight increase in the mean dark generation as well as the non-linearity. The non-linearity is measured from 10 to 80% of a full well and is about 3 to 5% for most of the detectors.

Full Well Capacity

22 23 25 30 31 33 35 36 38 39 41 42 43 44 45 46 Module#

Figure 2. (left) Conversion gain at 0.7V detector bias. (right) Full well capacity at 0.5 V and 0.7 V detector bias for all modules.

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