Quality Control and
Data Processing

HC PLOTS
gain [e-/ADU]
lamp counts [ADU]
lamp flux [ADU/sec]
QC1 database (advanced users): browse | plot

The gain template generates two dome flats and two dark frames with the same exposure time. The statistical noise derived from the dark frames is called read out noise. The statistical noise derived from the flat frames and corrected for the read out noise is called photon noise, from which the gain is derived. The accuracy of these measurements are limited by the horizontal stripe pattern.

QC1_parameters

FITS key	QC1 database: table, name	definition	class*	HC_plot**	more docu
QC.GAIN	vircam_gain..qc_gain	gain	HC		[docuSys coming]
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter **There might be more than one.

The gain is a conversion factor between the acquired electrons (e-) and the measured counts (ADU). The unit is e-/ADU. Two dome flats of the same flux level are subtracted from each other to minimize the structural noise. From the remaining random noise and the read noise derived from the dark frames the gain is obtained.

The gain is derived from a pair of equally exposed flat frames and a pair of dark frames acquired with the same DIT. The calibration frames suffer from a non-reproducible stripe pattern. Being not corrected for, the derived noise is biased and the resulting gain values can be up to 20% higher. For a detailed description, see the section on the horizontal stripes in the plot tutorial for dark frames.

Trending

The gain measurements are acquired once a week.

Scoring&thresholds Gain [e-/ADU]

There is no scoring.

History

No remarks.

Algorithm Gain [e-/ADU]

Two dome flats of the same flux level are subtracted from each other to minimize the structural noise. From the remaining random noise and the read noise derived from the dark frames the gain is obtained.

QC1_parameters

FITS key	QC1 database: table, name	definition	class*	HC_plot**	more docu
QC.GAIN	vircam_gain..qc_counts	mean counts in flat	HC		[docuSys coming]
QC.GAIN	vircam_gain..qc_flux	lamp flux in ADU/sec	HC		[docuSys coming]
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter **There might be more than one.

Trending

We monitor the counts acquired in the flat frame in ADU and the lamp flux.

Scoring&thresholds Lamp flux and flat counts

The counts of the gain flats are monitored and scored with thresholds of 4000 and 8000 ADU. The lamp flux is not scored.

History

On 2018-08-07, M1 and entrance window was washed. Counts of gain flats (which are dome flats and pass M1 and entrance window) increased from 6500 ADU to 7200 ADU, a value with was registered the least time in late 2011.

Algorithm Lamp flux and flat counts

The counts is a basic measure from the flat. The counts acquired in the flat frame divided by ( DIT * NDIT), given in ADU/sec. The VIRCAM detector controller is configured that NDIT >1 does accumulate (sums up) counts on the chip, while for other VLT instruments NDIT >1 means averaging on the chip. Therefore we divide here by DIT and NDIT.