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CRIRES Quality Control:
image quality from science frames

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top Image quality

CRIRES science observations are usually carried out with a series of nodding and/or jitter sequences. The pipeline combines these individual images into one single frame before extracting spectra (see also the science recipe pages). The measured image quality is determined from the pipeline-combined image as the medium FWHM of the cross-dispersion profile, multiplied with the image scale (0.086 arcsec/pixel).

The expected image quality is derived from the DIMM seeing measurements that are stored in the header of the science frames when the DIMM seeing monitor is operational. The keywords "TEL AMBI FWHM START" and "TEL AMBI FWHM END" store DIMM seeing measurements at start and at the end of the exposure, respectively. The average is computed and assumed as the DIMM seeing value of the exposure. Because DIMM measurements are done at zenith and at 0.5 micron, the DIMM values are reduced to the zenith angles and wavelengths of the science observation, in addition a first order outer scale correction appropriate for a 8m telescope is applied. The formula used to obtain the expected image quality is given by M. Sarazin.

Processing of science observations was discontinued as of October 2011. Access to QC1 parameters and trending is available for historical data.

QC1 parameters

QC1 parameters are measured for each detector array separately.

parameter QC1 database: table, name procedure
measured image quality in science frame crires_sci, measured_iq FWHM of cross-dispersion profile in combined science image, multiplied with image scale (0.086 arcsec/pixel)
expected image quality crires_sci, expected_iq DIMM seeing corrected for airmass, telescope diameter, and wavelength


Observations with closed AO loop and with open AO loop are trended separately. On average, the expected and the measured image quality should be equal for open AO observations. In closed loop, the measured IQ is much smaller, except for observations of extended objects.