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(1) Università degli Studi di Firenze - Dipartimento di Astronomia, Largo E.Fermi 5, 50125 Firenze, Italy
(2) Osservatorio Astrofisico di Arcetri, Largo E.Fermi 5, 50125 Firenze, Italy
This paper deals with the characterization of engineering grade arrays and with some problems in the reduction of infrared spectra.
We acquired dark and flatfield frames, the former without any illumination of the detector and the latter with the uniform illumination of a led diode inside the dewar. The dark frames were taken at three different temperatures (55, 65 and 78 K) with different integration times (stacks of ten frames at 1,2,5,10,20,30,60,120 and 180 sec).
The dark frames were 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.
For each series of measurements stacks of ten frames were taken with the same integration times used for darks.
The badpixels were found out from normalized dark and flatfield frames by dividing each quadrant in 4x4 subareas and defining the badpixels as those exceeding by 4 sigma the median value in the subarea. In all of the following analysis the badpixels were not considered and their value was substituted with the median over an 11x11 box.
The mean and the standard deviation were 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 were taken both for pixel stack mean and standard deviation.
The readout noise was determined as the mean standard deviation of each pixel in the stacks at 1 and 2 sec and then averaging over areas of 32x32.
The dark current rate and the bias were derived by a linear fit, pixel by pixel, of the stack mean versus integration time, with the error on the mean given by the pixel standard deviation in each stack.
In tab.1 the results of our measurements over the best 100x100 pixels subsection of the two engineering grade arrays and over the whole scientific array are presented.
TAB. 1 Measured characteristics of the arrays 1 best 100x100 2 best 100x100 Scientific Bad Pix (%) 3.0 1.7 1.0 Read Out Noise (electrons) 240 45 45 Dark Current (electrons/sec) 40 2 1
In fig. 2 the read-out noise is plotted for each subarea as a function of the detector temperature. An indicative value for RON is 2.5 ADU (about 45 electrons).
Fig.2 - Readout noise (RON) in counts (ADU) of the new engineering grade array as a function of the detector temperature.
In fig. 3 the dark rate is plotted as a function of temperature. The three different symbols at 78 K indicate frames taken after complete reset (circle), after normal illumination (triangle, like measurements at lower T) and after strong illumination (squares). In the latter case one can notice an increase of the dark current rate by about 20%. Indicative values for the dark rate are 0.1 ADU/s (2 electrons/sec).
Fig.3 - Dark rate (ADU/sec) of the new engineering grade array as a function of the detector temperature and in three different conditions: after a full reset (circles), after normal (triangles) and strong illumination (squares).
A simple object-sky subtraction is not always enough to guarantee a complete removal of the emission lines as the sky spectra in the two frames (source and sky) may be different, because of a shift in the wavelength direction due to mechanical flexures in the instrument and of variation of line intensity.
The used technique is to consider the difference between object and sky frames and choose an area of the frame where there are bad subtracted sky lines. The extra noise there is mainly due to residuals of OH lines. The factor to rescale the sky frame and the shift to apply are fixed by minimizing the noise in the previously chosen area. In the spectral region where the background is not too high one also need a dark frame. The procedure can be summarized in the following equation:
where A is the factor by which OH line varied (and this correction must not be applied to the dark frame!) and D is the shift due to grating movements. A and D are determined by minimizing the noise on the background.
This procedure is not able to correct grating shifts > 0.1 pixels during the integration, in that case the problem is still open ....
This procedure of sky subtraction becomes less and less efficient as the integration time increases, in particular one can notice how the noise gets greater than simple fluctuations of the background when integrating longer than 200 sec (fig. 4).
This problem limits the integration time and increases the limiting flux (about 10e-13 erg s-1 cm-2 arcsec-2 micron m-1),preventing us from reaching background limited performance in the J and K bands.
Fig.4 - Noise (ADU) of the sky subtracted images as a function of the integration time. The lines represent the poissonian component of the background noise, note the high noise due to bad subtraction of OH sky lines after 200 sec of on-chip integration.
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