FORS Data Reduction Software

automatic data reduction software for FORS 2 in Echelle mode

finished work:

• a guess of the order position and the position of the wavelengths
• centering the order position
• iterative search and centering of the spectral lines from the wave length calibration lamp and the global fit of the dispersion solution in 3 dimensions as a function of x, order and delta y
• extration of the 2D-spectra
• normalization of the flatfields
• subtraction of the night sky lines
• rebining of 2D-spectra of extended sources to the same wavelength steps
• optimum extraction of point sources with sub-pixel sampling (modified version of the FEROS DRS)
• rebinning of extracted spectra
• blaze correction, flux calibration and optimum merging of the orders for spatially resolved spectra of extended objects and extracted spectra of point sources
• high level routines - Midas context
• manual

next work:

• online help
• adaption for FORS test data ( after the grisms are delivered )

prototype version:

• EF-manual 1.1 - 19-May-1999 - Echelle DRS for FORS 2 including extraction of slits and optimum extraction of spectra like for FEROS
• EF-software and test data - 21-May-1999 - source code, documentation and test data taken with DFOSC is available as a compressed tar-file
  • ./echelle/dfosc/ - test data
  • ./echelle/dfosc/red_all.prg - reduce test data
  • ./echelle/feros/ - test data
  • ./echelle/feros/red_all.prg - demo for flux calibration of FEROS
  • ./echelle/docs/ - documentation
  • ./echelle/forsech/libsrc/ - c-libraries to be compiled
  • ./echelle/forsech/src/ - c-code to be compiled

first results:

After a simple gaussian centering the y-positions of the orders can be fit very accurately as a function of x as the input for the later data reduction routines like the extraction of spectra. Here the orders were fit with an observed star, but for FORS2 we will do the centering with the spectrum of the internal flat field lamps through the pinhole.

Now the position of the lines from the wave length calibration lamps can be estimated with the corrected position in y and the forecasted positions in x. Here we used a Thorium Argon lamp in combination with a catalog for which we have selected the brightest lines only. Here the scale is different from the earlier figures. It seam to me that the focal length of the camera is longer then the value used for the calculation.

The first step of the wavelength calibration was now to apply the global fit as a function of the x-position and the order only. After the first centering at the forecasted position and a second iteration of the line search with the new dispersion solution, we have already got a dispersion solution with a standard deviation of 0.11Å which is acceptable for almost all applications. All identified lines are marked blue, the white symbols are all lines of the input catalog. The input catalog should be better optimized for the spectral resolution of DFOSC.

The residual deviation of the fit from the laboratory wave length show the good result. Red symbols denote lines which have been rejected as blends and wrong identifications. Many lines are not taken into account, because the input catalog was not well selected for intermediate resolution spectra

The lines are now centered along the slit and again a residual of 0.11Å was obtained for the 3D-fit as a function of X, order and delta y. The parameters of the global solution in two dimensions were hold.

The residuals of the 3D chi square optimization are presented in the next figure:

With the known order positions the raw spectra of all orders can be extracted now. Note that the bad columns were not replaced yet. Here a simple linear rebin can be done because all pixels are shifted in y only

The raw flat field can be normalized for the later reduction steps like the optimum extraction and the fit of the terrestric night sky lines.

After the division by the normalized flat the sky can be subtracted either fitting polynoms or calculating the median over a window, on the part of the slit without light from the target. The next two figures show the rectified spectrum before and after the subtration of the night sky lines. The sky spectrum is calculated at the position of the raw spectrum, before rectifying the frames, similar to the normalized flat field

Now the sky subtracted frame can be rebined to the same wavelength steps. Only the usefull part of the spectra is kept again

Finally the spectra of the single orders can be extracted now using a modified optimum extraction method which considers sub-pixel sampling effects. Note that the very strong cosmic along the spatial direction in order 12 was not removed. Below you see the rebinned spectra of the WR+O binary HD 5980 which showed a remarkable outburst several years ago. The spectrum was devided by the approximate blaze function, which was calculated for the flat field normalization. Note that the fringes were unexpectedly removed from the data, although no standard star observation was reduced yet.