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NICS - Detector

The detector is a Rockwell 1024x1024 HgCdTe Hawaii array which, alike other such devices used in various astronomical instruments, has some peculiar characteristics which should be kept in mind when collecting and reducing the data.


Persistency and memory effects (changed on Feb06)


Evolution of the level of persistency (ghost) signal after a strongly saturated frame. Please note that the level of persistency has decreased be about a factor of 3 after the new acquisition electronics (FASTI-NICS) has been put into operation in Feb 2006.

The array multiplexer has a stubborn attitude to remember whatever strong signal was recorded in the previous frame(s). For example, the first dark exposure taken after a saturated image will still show a ghost relic of the previous frame at a level of up to 0.4% of its original intensity regardless of the time elapsed betweeen the two frames. This ghost image will also persist in the subsequent frames with intensity slowly fading down to below the noise level (see figure on the left).
Persistency was stronger and very annoying with the old acquisition electronics, also because the system remained idle when the integration was terminated. The new FASTI-NICS control electronics, on the contrary, continuosly take short dummy frames during the idle time; thus cleaning the array.
The "clear-array" procedure, which was formely mandatory when switching from imaging to spectroscopy, should be now used only in very estreme cases, e.g. when starting a very long spectroscopic integration after having centered the target using images of a field with one or more completely saturated objects


Bias


A typical 60s dark frame with
cuts along the two axes

Contrary to standard CCD devices, the "bias" of this detector is not uniform over the array, but has a distinct horizontal pattern with pronounced maxima at rows Y=1 and Y=513. Moreover its level is not constant along a row but increases with X (see for example the figure here).
The amplitude of the peaks and their slopes are not constant (sic!) but strongly vary with the level of the signal. In dark exposures, the pattern is characterized by prominent maxima with quite gentle slopes (like in this figure), while at higher levels of illumination the peaks become progressively narrower.
This effect could produce annoying features when subtracting images/spectra taken under variable sky conditions. Moreover, the variation of the bias makes it basically impossible to obtain accurate differential flats. Luckily, however, these are not necessary in case of NICS (see the section describing the procedure to get suitable flats).


Efficiency versus wavelength

The values displayed here (blue dots) were determined from measurements of standard stars both in imaging and low resolution (prism) spectroscopy. The data were corrected for the transmission of the optics, of the filters and prism and normalized to the value at 2.2 micron given in the typical efficiency curve (black triangles) which was formerly available at the Rockwell web site. Evident and somewhat unexpected is the quite rapid decrease of efficiency below 1.4 microns which translates into a significant loss of sensitivity in the J and 1micron bands.


For any comments please contact Vania Lorenzi.