QC PLOTS
	CURRENT	HISTORY
noise parameters
linearity, gain
contamination
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Click on CURRENT to see the current trending (Health Check).    Click on HISTORY to see the historical evolution of the trending.

Image flats (also called detector flats) are measured as technical calibrations every 4 weeks or so, as part of the GIRAFFE calibration plan. While the ordinary, daily fibre flats mainly measure fibre characteristics, the image flats are exposures of the CCD by the flat field lamp without the fibre system. Hence, they are used to monitor detector characteristics like

• CONAD (1/gain),
• gain variations (px-to-px scale, 'fixed-pattern' noise),
• linearity,
• contamination.

Master image flats are created from a stack of typically 2 input raw frames to

• remove cosmic ray hits
• obtain a high-S/N gain map.

Since May 2008, a new template and a new recipe ("detector monitoring") is in use and has a different acquisition and processing strategy. This is applied to the new CCD.

Find other detector parameters documented here:

Two kinds of small-scale fluctuations exist in any raw frame: photon noise, and fixed-pattern (gain) noise (the third noise source, read noise, is negligible here). While the photon noise can only be reduced but not removed totally, the gain noise is constant with time and can be entirely removed from science frames using gain maps derived from image flats.

Both sources of noise are monitored. They are measured in small subwindows of 100x100 pixels size. They are always checked to be random (Gaussian shape of histogram curve).

QC1 parameters

Trending

The measured fixed-pattern noise of the old CCD is about 0.5% (Fig. 1 of the trending plot). It nicely follows a linear slope (Fig. 2 of the trending plot, and also the figure below). The measured photon noise follows a square-root law as expected.

The new CCD has a stronger fixed-pattern noise, at the level of roughly 2%. It is currently not yet trended. Apart from its higher amplitude, the following is fully applicable to the new CCD.

The noise characteristics are not only relevant for Quality Control, but also interesting for data reduction purpose. Whenever it comes to obtain a good S/N, gain maps are used to remove the fixed pattern noise.

The penalty to pay is added photon noise, inherent in the image flats. For a single raw flat file obtained with a typical integration time of 220 sec, the turnover from the photon-noise into the gain noise regime is at exposure level 22000 ADU. This is visible in the figure below. For a master stacked from 2 raw frames, photon noise can be reduced by a factor of sqrt(2), and the turnover is at 11000 ADU. For a stack made of 3 frames, the critical exposure level is at 7000 ADU. This means: if high S/N is an issue, one should take care to use gain maps having sufficiently high exposure level everywhere. In principle it makes sense to attempt a gain noise correction only if the photon noise in the map is lower than the gain noise in the science data.

Some of the GIRAFFE setups have flat fields with rather high dynamics. E.g., a single LR 427.2 flat, being exposed at 110 sec, has parts with just 4000 ADU and other parts being almost saturated.

These issues are neglected by the Giraffe pipeline which accepts whatever input master flat is specified.

Noise properties of the old GIRAFFE CCD, as measured in a series of image flats. Data have been bias subtracted. Flats have been taken in a series of exposure times between 1 and 220 seconds. Fixed pattern noise follows a linear slope (red dots: measurements, broken line: fit). Photon noise follows a square root law (blue dots: measurements in a single raw file; black broken line: fit). The intersection between the linear and the square-root curve marks the regime useful for data reduction: data with higher exposure level are useful for gain noise removal, while data with lower exposure level are photon noise dominated. The example plot here applies to a single raw file. Usually flats are combined from at least three raw files. Stacking reduces the photon noise by a factor sqrt(3), while the fixed-pattern noise is not affected by stacking. By co-adding, the intersection line between the two noise curves can be shifted towards lower values.

QC1 parameters

Trending

Detector linearity is trended here. A sequence of image flats is exposed between 1 and 220 secs. Their mean exposure level is plotted against the exposure time (top). A fitted function (broken line) is used to derive residuals which are normalized to the mean and plotted vs. exposure time (bottom). The normalized residuals are below one percent.

Linearity: Box 1 shows the measured flux levels for the sequence of image flats, and a linear fit. Box 3 shows the residuals after subtracting the fit, normalized to the averages.

QC1 parameters

Trending

The conversion factor electrons to ADU (CONAD) is the inverse gain and is also monitored in the linearity trending plot, box 2. It is measured from the difference between two identical raw frames, by comparing the square root of the signal to its measured rms. The mean CONAD value is given in the plot. Multiple values exist since usually several pairs are available with EXPTIME > 50 sec (e.g. 60 sec, 120 sec, 220 sec).

QC1 parameters

Trending

Monitoring contamination. Left: Intensity in 4 subwindows relative to central window. Right: Sketch of subwindows, same color coding as the data points.

A potential issue is contamination which is monitored in box 4. A set of four 400x200 pixels subwindows in the corners of the CCD is used to register the intensity there relative to a central reference subwindow. The fraction is trended over time here.

History

Find a trending plot covering the full history of contamination here.

Typically, contamination builds up very slowly and then more strongly. Once the contamination parameter in one of the windows is below 0.9, an intervention is scheduled (heating of the CCD) to bring the CCD efficicency back to its nominal values.

Below find a comparison between the image flats from 2003-04-28 (left) and 2004-06-06 (right, just before an intervention)). A contamination of about 7% has built up in window 4.

Monitoring contamination: Image flats from 2003-04-28 (left) and 2004-06-06 (right).