| NACO Quality Control:
DARK frames and detector monitoring
A number of DARK frames is taken
within the NACO daytime calibrations in case NACO science observations have been acquired in the night. Each DARK consists of 3
raw frames obtained with the instrumental setting matching the
one used for science observations. The parameters matched are:
The NACO detector can be operated in normal (DET.FRAM.TYPE=INT) and in cube mode (DET.FRAM.TYPE=CUBE2). In cube mode, the detector adds two more rows at the upper rim, that the read-out size is 1024x1026 pixel instead of the 1024x1024 pixel in normal read mode. Furthermore the detector can be operated in any used-defined window read out mode. The QC monitoring covers:
Other Window options other than 512 x 512 and 512 x 514 ore not pipeline supported and are not subject of QC checks.
Since 2015 NACO is operated with a different detector. All screen shots shown on this page are from the old detector used until 2013.
The operationally most critical detector characteristics derived from dark calibration frames are monitored by the following four QC parameters:
The upper three boxes of the following plot show the temporal evolution of the read-out noise for three daily taken detector setups.
The upper three boxes of the following plot show the temporal evolution of the dark level for three daily taken detector setups.
The 8-column noise is a special feature of the NACO detector, which became dominant in Feb 2012, when the upper right 512x512 pixel detector quadrant showed an extra signal as repetitive pattern in every eightst column.
This QC parameter deserved temporarily its own HC plot in 2012. The HC was decommissioned in 2013. The QC parameter noise_UL_lf_fpn (low frequency fixed pattern noise in the UpperLeft quadrant) as well as its complements for the other three detector quadrants are continuously acquired and are available in the QC1 DB.
The trending plot shows the number of the hot and cold pixels measured in the dark frames. The data points are separated by the detector read-out mode.
Detector Monitoring (detmon)
The NACO detmon template acquires a series of flat field pairs and corresponding dark frames in order to retrieve the detector gain and detector non-linearity. Up to 2016 the following four read modes are monitored:
Data acquired in 2015 are based on a new detector, for which the template is not optimized. 2015 detmon data are less useful. In 2016-01 Double_RdRstRd_HighSensitivity is replaced by Uncorr_HighDynamic, and the monitoring is restarted again. Since the detector suffers since 2015-01 from a dead column effect in the lower left quadrant, only the upper right detector quadrant is used for the detmon analysis. Furthermore it turned out (for the period 2015 up to 2016-01) that flats with DIT=3sec and Uncorr mode show additional fixed pattern noise which biases the photon transfer curve when retrieving the gain. These DIT=3sec flats are ignored when processing detmon frames in Uncorr read mode.
The following read modes are monitored, values are for the upper right detector quadrant:
In optical CCDs there is a fixed saturation level of 65335 ADU and the small non-linear component of the response can be described by a polynomial in the count range of up the discontinuity at 65335 ADU. In IR detectors the non-linearity is stronger, starts at lower counts and the saturation level is reached more smoothly when compared to CCDs. The detmon pipeline recipe is configured to use for each of the four monitored read modes an optimised set of recipe parameters. These command line parameter sets are tuned to retrieve the gain and the non-linearity but not the saturation level. Since in 2015-10 the saturation level changed in one of the read modes, the following procedure was implemented to monitor the saturation level (see the magenta points here):
The median values of the raw flat frames is fit by the following function:
f(x) = [1/2 - atan((x-x0)/s)] * (a+b*x) + [1/2 + atan((x-x0)/s)] * c
with a linear part (a+b*x) dominating for x << x0 and a constant part c dominating for x>>x0, where x0 is the transition zone between both functions and s is a smoothing width for the transition zone.
The median values versus DIT have been fit two times, once (green f(x) ) with a free width and a second time (blue g(x) ) where x0 is fixed from f(x) and the width s is fixed to a 5 times smaller value of f(x), which results in a more stable values of c.
The non-linearity of the detector response is calculated by the detmon recipe. A higher order polynomial is fit to the dark subtracted flat frame median as a function of DIT. See:
This figure shows an extraction of the larger QC report. The third order polynomial fit (green) and a corresponding linear component is shown. The QC parameter returned from the recipe is the deviation from linearity at a reference level of 8000 ADU (for Double_RdRstRd_HighDynamic). Dividing the green line by the blue line and expressing DIT by ADU, the deviation from linearity is shown:
which means that the non-linearity is a function of count level. The figure shows that in 2016-05, the relative deviation from non-linearity at R=8000 ADU is about 0.04, which is monitored as QC parameter in this HC plot.
The detector gain value is calculated by the detmon recipe, where a lower threshold for excluding bright flats is applied in order confine the analysis (the photon transfer curve method is used) to the linear part of the detector response. The following figure shows an extraction of the QC report.
The left box shows the variance of the flat frames (minus the variance of the off-lamp frames) as a function of twice the dark subtracted flat counts.
The right figure shows the gain values for each DIT, where two flats and two darks have been used (red circles).