Determination of the Detector Parameters

The determination of the detector parameters is based on sets of images of a spatially uniform-illumination scene (flatfield frames) and without any illumination (dark frames), putting a plug in place of the filter, taken at a series of exposure times. We have two possibility for the flatfield scene: with the uniform illumination of a led diode inside the dewar (inner flatfield) and with an external uniform source (external flatfield). The number of frames was the same for each exposure time and not less than 10.

We interleave before and after each exposure time in the series of flatfield frames a set of reference images at an exposure time of 1 sec to determine the possible temporal variation of the illumination level. When necessary, the average of the frame medians for each pair of fixed-exposure time sets divided by the global average is applied as a rescaling factor.

The dark frames can be obtained in three different conditions: the first series after a complete reset of the array, the second and the third after a weak and a strong illumination of the detector to test for memory effects,

The mean and the standard deviation relative to the mean are computed for each stack (pixel by pixel) after having renormalized the frames to get the same median value in a selected region of the first quadrant. Therefore, the averages over 16x16 subareas are taken both for pixel stack mean and standard deviation to increase signal-to-noise for the calculation of the gain factor and read noise and to help visualize results.

Two representations are used to derive the detector parameters. One is signal vs. exposure time in which detector spatial trends are considered; these are used to determine the dark current and detector non-linearity. The other is variance vs. signal in which spatial trends can, in principle, be ignored, from these, we determine the gain factor.

For the gain analysis, the subareas were grouped into quadrants and a linear regression is performed on the spatial medians of the stack variance relative to the stack median, the gain factor is the inverse of the slope of the regression. Variation from quadrant to quadrant seems to be negligible and we see no evidence for long-term variation of the gain.

The readout noise is determined as the mean standard deviation of each pixel in the stacks of short integration times where the dark current is negligible.

The dark current is measured from dark frames: the median images are fit, pixel-by-pixel, to a first-degree polynomial as a function of exposure time with the error on the mean given by the pixel standard deviation in each stack, the first-degree coefficient gives the dark current.

In Detector Performance (in the section Results) are the tables of the last results of these tests.

Last update 10 June 96

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