Performance of large format HgCdTe and InSb arrays for low background applications

The first VLT infrared instrument, ISAAC, was installed at the 8 meter Antu telescope in 1998. Experience and results with both InSb and HgCdTe large format arrays will be reported. Effects limiting the performance and strategies to partially overcome these limitations will be discussed. The Hawaii 1Kx1K HgCdTe array is now in routine operation in SOFI at the NTT and the short wavelength arm of ISAAC, the Infrared Spectrometer and Array Camera installed at the VLT [1]. The ultimate performance of this array is limited by the electroluminescence of the Si readout multiplexer. Even if the on chip amplifier is not used and the internal bus is accessed directly, the glow of the shift registers produces a signal of 700 electrons/hour when continuously reading the array. By imaging the bare Hawaii multiplexer during readout onto a CCD, the optical nature of the multiplexer glow has been verified. Algorithms will be given for the removal of the intensity gradient in the center of the array and for row crosstalk which is present when bright sources are in the field. For low flux applications the nonlinearity after resetting the array can be avoided by switching off the reset clock. A foundry run yielding six 1kx1K InSb arrays has been evaluated. One of these arrays has, successfully been mounted in the long wavelength arm of ISAAC. By employing a special monitoring technique using dead pixels with open In bumps, a darkcurrent as low as 14 electrons/hour has been measured at a detector temperature of 25 K. The temperature drift of the video signal for Aladdin arrays is 2700 electrons/K. A temperature stability in the micro Kelvin range is required without drift compensation. With our versatile 32 channel high speed data acquisition system IRACE the readout noise of Aladdin arrays could be suppressed to below 10 electrons rms by application of multiple nondestructive readouts and subpixel sampling of the analog signal.