FORS1 Wavelength Calibration Accuracy

QC1 PLOTS: LSS PARAMETERS

QC1 ASCII DATA

Year:
Item:
Grism:
Slit width:
CCD

QC1 database (expert mode)

GENERAL INFORMATION

The determination of the dispersion relation for LSS data is done as follows: The recipe fors_calib identifies reference lines on LSS arc lamp exposures, and traces the spectral edges on the associated flat field exposures. With this information the spectral extraction mask to be applied in the scientific data reduction is determined. From the input flat field exposures a normalised flat field frame is also derived. The input arc lamp and flat field exposures are assumed to be obtained quasi-simultaneously, so that they would be described by exactly the same optical distortions. Wavelength calibration is performed row by row and produces a 2-D rebinned science frame. This has the effect of removing the instrumental line curvature and allows the user to extract her/his spectra and have them wavelength calibrated without any additional operation on the frame itself.
The global list of lines currently used by FORS1 pipeline can be found here.
Due to the high number of possible grism/slit combinations, the Pipeline currently uses a polynomial with a maximum degree of 5 (for 150I without filters) in the dispersion direction, while a 2nd order polynomial is adopted in the spatial direction.

In the following figure we show an example of wavelength solution produced by the FORS1 Pipeline for grism 300V coupled to a 1".3 slit.

Chip 1

Chip 2

The Pipeline has found and succesfully identified 18 lines. From the fitted profiles it is possible to compute the FWHM of the lines and hence the resulting resolving power as a function of wavelength. In this case the estimated average resolving power is 379 (362) and the average accuracy of the model is 0.063 (0.064) pixels with the numbers in brackets refering to Chip 2.

SUPPORTED MODES

Currently (November 2006) the pipeline supports all grisms.

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