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CRIRES Quality Control:
flat field exposures

CAL | HC | refs | QC
Trending & QC1
Data Packages
Data Management
QC links:
flat stability
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.

For flat-field exposures, the halogen lamp is used to illuminate the spectrograph. They are usually measured during daytime. Three health check settings with reference wavelengths 1090.4, 3218.6, and 4092.9 nm are defined and executed daily. Other settings are only measured when they are triggered by night-time observations.

On flat fields measured befor June 2009, only the central ~400 pixels (in Y direction) are illuminated on the detector because of the (short) length of the slit. CRIRES flat fields significantly vary for different settings and detectors. There are also several artefacts that are typical for specific settings. For reference wavelengths below about 1500 nm, the intermediate slit vignets parts of each spectroscopic order so that the outer regions of detectors 1 and 4 are not (well) illuminated. A strong gradient is present in dispersion direction (i.e. in X direction) and there is sometimes also contamination from adjacent orders.

For high Echelle orders, an optical ghost can be present on one detector. It is the result of a reflection from the detector to the grating.

FLAT frame. Example for 1090.4nm reference wavelength, detector #1. Dispersion is in X direction

Example for 1090.4 nm reference wavelength, detector #2, with an optical ghost

flat (full slit)

Example with full illumination (3218.6 nm, detector #3, June 2009 and later)

FLAT frame. Example for 1090.4nm reference wavelength, detector #3. Reduced slit length from 23 April to 11 June 2008


top Flat-field stability and bad pixels

QC1 parameters for CRIRES flat-field exposures are measured for each detector array separately.

QC1 parameters

parameter QC1 database: table, name procedure
mean flat level crires_flat, mean_level mean level of the raw input frame 
flat flux crires_flat, mean_flux mean level divided by DIT
master rms crires_flat, rms_master the rms of the normalised master frame
number of bad pixels crires_flat, badpix the number of pixels with values outside given thresholds in the normalised master frame; thresholds are currently (0.1, 10.0)


For trending of flat field QC1 parameters, the daily health check settings with reference wavelengths of 1090.4, 3218.6, and 4302.7 nm are used. The flat field flux and the master rms measure the stability of the lamp, of the lamp shutter, of the positioning of the pre-dispersing prism, and of the intermediate slit. Because of the current instabilities for the intermediate slit, it has been decided to only trend flux and rms for detector #3.

The number of bad pixels is trended for reference wavelengths 3218.6 nm.


January 2008: Reference wavelength of one daily HC setting changed from 4302.7 to 4092.9 nm.
23 April to 11 June 2008: Effective slit length is only 12 arcsec, which is about 200 pixels.
June 2009: detectors are fully illuminated
28 May 2010 until mid July 2010: variable slit width along length of slit; rms of flats increased