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GIRAFFE: Dark
DARK level | Glow

 
QC PLOTS
DARK current
QC1 database (advanced users): browse | plot

2008-05-24: GIRAFFE CCD "Carreras" operational (often called "new") (replacing "Bruce"). Both CCDs are identical in format and size.

A set of 3 DARK frames is measured approximately once per month. These frames are combined and debiassed. The DARK current is monitored. The most prominent feature in the dark of the old CCD is the glow in the upper right. It is caused by illumination by a faint diode and appears on all frames with longer exposure times, in particular on science frames. Note that the glow strength, as well as the dark level, vary slowly in time, but sometimes suddenly after an intervention. Between 2003-07 and 2004-07-26, there has been no glow at all! If you are concerned about careful background correction, you should remove the dark signal with a master dark close in time with the science data.

The new CCD has no such feature and does not require master dark subtraction.

DARK frame, old CCD. The dark current as seen in a master DARK frame. The dark shown here is simply summed from 3 input frames. These are taken roughly once per month and have 1 hour integration time each. The pipeline generates a stack which is cleaned from the numerous cosmics.

This example refers to the old CCD ("Bruce") with the strong glow.

The new CCD ("Carreras") has a very low and uniform dark, in particular no glow.


DARK level
DARK level | Glow

The dark current is defined as the mean dark level in ADU/hr/pixel. Box 1 of the trending plot shows the DARK level measured across a central window, excluding the glow. The value in the plot is the dark current and is given in ADU per hour per pixel. The DARK frames should be taken in a template of 3 exposures of 1 hr each.

QC1_parameters

FITS key QC1 database: table, name definition class* HC_plot** more docu
QC.DARK.CURRENT giraffe_dark..current_dark dark current in ADU per hour per pixel (outside glow) HC
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter
**There might be more than 1, always check the links 'HC_plots'

Trending

The median DARK level is plotted in box 1 of the trending plot.

Scoring&thresholds DARK level

The mean dark level is very low and stable. Outliers are likely to occur only if the acquisition pattern is not normal (i.e. less than 3 dark frames taken, or with much shorter exposure times). The upper threshold is set such as to trigger an alert for closer investigation of the data. For SciOps: In those cases it is best to repeat a sequence of 3 darks.

History

Before 2008 (old CCD): the mean dark has evolved over the years, see the FULL plot. For data reduction (glow removal), a master_dark not too far in time should be chosen.

Algorithm DARK level

mean of master_dark, measured in a central window


Glow
DARK level | Glow

On the old CCD (for data from 2008 or earlier) occasionally (not always) a glow pattern was visible. Its strength scaled with the exposure time. Box 2 of the historical trending plot (2008 and before) measures the strength of the glow pattern. The flux is summed in a window centered on the glow (ADU per hour).

The QC1 database for dark frames also stores the X and Y positions of the glow.

giraffe_dark_reproc..glow_dark contains reprocessed historical data (with current pipeline).

The position of the maximum of the glow was monitored for the old CCD, as giraffe_dark..glow_posx and giraffe_dark..glow_posy.

QC1_parameters

FITS key QC1 database: table, name definition class* HC_plot** more docu
QC.GLOW.LEVEL giraffe_dark..glow_dark cumulative counts in glow area CAL
QC.GLOW.LEVEL giraffe_dark_reproc..glow_dark cumulative counts in glow area CAL
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter
**There might be more than 1, always check the links 'HC_plots'

History

The trending plot covering the full DARK history (here) shows that the strength of the glow was slowly varying with time ("old" CCD). It was completely absent between 2003-07 and 2004-07-26.

The "new" CCD used since 2008-05-26 has no glow.

Scoring&thresholds Glow

No scoring is done since there is no glow for the new CCD.

Trending

Trending was done for the old CCD before 2008. Find the glow parameter trending in the FULL plot, report #2, and by clicking details for 2008 or earlier.

Algorithm Glow

Glow is the sum of all counts in region X1400..2048, Y3800..4096, in ADU per hour. For posx and posy, find the maximum of glow, measure its X and Y position.


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