CRIRES Quality Control: wavelength calibration

CRIRES Quality Control:
wavelength calibrations

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QC PLOTS
	CURRENT	HISTORY
cross-correlation
ThAr central wavelength
N2O central wavelength
dispersion
ThAr lamp efficiency
Resolving power
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.

Wavelength calibration for CRIRES can be obtained with several different methods: from sky lines in science frames, from dedicated wavelength calibration frames using an arc lamp or a gas cell, or from a physical model. This page describes QC checks on arc lamp and gas cell exposures.

CRIRES offers a ThAr arc lamp for measuring emission line spectra and a gas cell which is filled with N2O gas and illuminated by the halogen (flat-field) lamp for absorption line spectra. Wavelength calibration frames are measured during daytime according to the settings used during the night. They are also frequently attached to science OBs. For settings above 2400 nm, wavelength calibrations are usually not taken because the density of sky lines is sufficiently high. For health check purposes, a ThAr exposure with reference wavelength 1090.4 nm and an N2O exposure with 4092.9 nm are measured daily.

In April 2008, a fibre bundle was mounted in front of the ThAr lamp. The light is projected as separate traces on the detector. Since June 2009, also a metrology fibre is present (on the very top).

The pipeline recipe performs a cross-correlation between the observed frame and a template which is derived from the line lists. The cross-correlation value is a unitless number between 0 and 1. A value above 0.2 is regarded as a successful solution. In case of a failure, the recipe gives -1 as cross-correlation result.

Raw ThAr arc lamp frame (until March 2008)

Raw ThAr arc lamp frame with fibres (April 2008 and later)

Raw ThAr arc lamp frame with ThAr fibres and metrology fibre (June 2009 and later)

Raw N2O gas cell frame

top Wavelength calibration parameters

QC1 parameters for CRIRES arc lamp and gas cell exposures are measured for each detector array separately.

QC1 parameters

parameter	QC1 database: table, name	procedure
central wavelength	crires_wave, lambda_cent	wavelength of the central column of the detector (in nm)
dispersion	crires_wave, dispersion	linear dispersion (in nm/pix)
cross-correlation result	crires_wave, xcorr	cross-correlation result
ThAr lamp efficiency	crires_wave, lamp_eff	sum counts in raw frame between y=wl_starty and wl_stopy, subtract backgound, devide by DIT
spectral resolution FWHM	crires_wave, resol_fwhm	identify lines in ThAr arc lamp exposure and determine their FWHM in dispersion direction (in pixels); QC parameter is median of all lines
spectral resolution	crires_wave, resolution	convert resol_fwhm into nm and calculate resolving power as lambda/Delta lambda

Trending

QC1 parameters for wavelength calibration are trended for the daily health check settings with 1090.4 nm (ThAr) and 4092.9 nm (N2O). Trending consists of the cross-correlation result, of the central wavelength on each detector array, the dispersion, spectral resolution (ThAr lamp only), and the lamp efficiency (ThAr lamp only).

History

April 2008: fibres are mounted in front of ThAr lamp
June 2009: metrology fibre
2009-10-01: change of pipeline parameters: wl_starty=135 and wl_stopy=175 (instead of almost the complete detector); this gives a lower pipeline-calculated lamp flux
2010-09-30 and later: ThAr lines are weak and varying in strength, especially on detector 2
2012-01-09: ThAr lamp exchanged; line intensities are weak; fixed 2012-01-14
2012-02-01: small jump in central wavelength after changing the width of the intermediate slit
2012-04: new algorithm for lamp efficiency; extracts counts from a small area around one fibre trace in a dark-subtracted raw ThAr frame
2012-06-28: for lamp efficiency, determine dark/background directly in raw frame and do not use a master dark