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GIRAFFE Quality Control:
BIAS frames

CAL | HC | refs | QC
Trending & QC1
Data Management
QC links:
bias level | RON | structure | bad pixels
Bias level, RON, structure, bad pixels
QC1 database (advanced users): browse | plot
   Click on CURRENT to see the current trending (Health Check).
   Click on HISTORY to see the historical evolution of the trending.

2008-05-24: new GIRAFFE CCD "Carreras" operational (replaced "Bruce").
Both CCDs are identical in format and size; unless otherwise specified, the content below applies to both CCDs.

2010-01-05: description of bad pixels added

2011-01-11: description of fast read mode added

2016-04-01: the 'slow' mode is offered in VM, calibration data are processed and QC parameters are available, but they are not trended anywhere because of their very random and rare acquisition.

A set of 5 BIAS frames is measured daily as part of the routine calibration and health check plan. The BIAS level, the read noise and the structure are monitored. Other CCD parameters are monitored using the darks and the image flats.

Since October 2010, the fast readout mode is offered in addition to the normal readout mode. Its trending plots are set up the same way as the ones for normal read mode, find them under 'bias_Fast'.

BIAS frame, old CCD. There is an upward gradient of the order of several ADU which is monitored by the STRUCTY parameter. The bright vertical feature is a bad column.
BIAS frame, new CCD. The new CCD has cleaner cosmetics, there is no bad column. The gradients in X and Y are much smaller.

Find other detector parameters documented here:

dark parameters dark
detector parameters like gain, contamination etc. detector

top BIAS level

Box 1 shows the mean BIAS level measured across the whole CCD, given in ADU.

QC1 parameters

parameter QC1 database: table, name procedure
"BIAS_level": median of bias level giraffe_bias..median_master median of whole master_bias; values below 100 and above 300 ADU are clipped


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


There is a trending plot covering the full history of contamination (here). The mean bias has slowly decreased over the years.

top Read noise (RON)

The read noise (RON) is measured as rms in a single raw frame and in the master bias frame, using in both cases a small central window 100x100 pixels wide. The RON of the master bias frame is about a factor sqrt(N) lower than the raw file RON, where N typically is 5. RON is given in electrons, calculated from the ADU value using the nominal transformation factor (CONAD = 1/gain) 2.25 e-/ADU. (The measured CONAD values are close to that value, check out here.)

QC1 parameters

parameter QC1 database: table, name procedure
RON (raw) giraffe_bias..sigma_raw measures rms in a single raw frame (the first one), using a small central window 100x100 pixels wide; clip outliers below 100 and above 300 ADU
RON (master) giraffe_bias..sigma_master same, for the master bias frame


Box 2 of the trending plot shows the RON of single raw BIAS frames, box 3 the RON of master bias frames.

RON plots. Box 2 shows the RON of single raw BIAS frames, box 3 the RON of master bias frames, typically constructed from 5 input raw frames.


top Structure

The GIRAFFE BIAS frames contain some structure which is monitored here.

QC1 parameters

parameter QC1 database: table, name procedure
"STRUCT_X": structure in X direction (rows) giraffe_bias..struct_row collapse the master_bias in Y direction, measure rms of result frame
"STRUCT_Y": structure in Y direction (columns) giraffe_bias..struct_col same, in X direction


Box 4 and 5 of the trending plot show the structure of the BIAS, derived from collapses of columns (structure in X direction, left) and rows (Y direction, right).

Structure plots. The structure in X direction is derived in the master bias, collapsed along the rows (box 4). The structure in Y is obtained along the columns (box 5). The collapses are shown below.

Old CCD:   New CCD, normal read mode :

Collapsed master bias. While the master bias (both for the old and the new CCD) has virtually no structure in X direction (along the rows), it is quite structured vertically (along the columns).

Old CCD: Roughly the first 1000 rows increase from a level about 4 ADU below average to the average level. The step is due to the bad columns around column 420 starting at about row 1460.

New CCD, normal read mode: the increase is much smoother and has an amplitude of less than 2 ADU. There is no bad column.

  New CCD, fast read mode:

New CCD, fast read mode: the structure looks very similar, but not identical, to the normal read mode.


top Bad pixels

The number of bad pixels in GIRAFFE BIAS frames is monitored here. Note that the pipeline recipe delivers proper values only since 2008, for the new CCD.

QC1 parameters

parameter QC1 database: table, name procedure
"N_badpix": number of bad pixels giraffe_bias..bpm_npix pipeline recipe


Box 6 of the trending plot show the number of bad pixels which is usually between 0 and 200 in the new CCD.