FLUX CALIBRATION

• Copy reference table
• Compute flux correction
• Apply flux correction

Copy reference table

• Principles :
  
  The fits table containing the absolute spectrum of the reference star will be copied into the work directory; this ensures that the original will remain in a safe place, and will not be inadvertently tempered with. To be reachable, this table must be stored into directory
  {Local XOasis installation path}/oasis-4.3/user/flux_ref/
  where a few ones are already provided with the present software (the ones which had already been of interest for the Lyon team, in fact...). The following stars are already included, as of August 1998 :
  • HR 4963
  • HR 5501
  • HD 23733
  • HD 93521
  • BD+332642
  If you need another one, you must provide it. You may for instance download the corresponding ASCII file giving F=F(lambda) for this particular star from the ESO spectrophotometric standards library. With your favourite package (MIDAS, IRAF, ...), create then from that a FITS table with at least two columns, one for LAMBDA, the other for F_LAMBDA. But you may create more data columns, with different flux units, which you may use later into XOasis. The standard name of such a table is {Name of the star}abs.fits in XOasis, but you may use any denomination of your own, like Georgette or MonEtoileStandard.
  


• Use :
  
  
  • Click on Flux in the main menu, then on Copy reference table.
  • In the new window which pops up, check the table(s) you want to copy to your work directory, then click on Copy.

Compute flux correction

• Principles :
  
  A standard star observation is used to calculate the spectral transfer function of OASIS/TIGER. The total flux of the star is obtained by summation of all the spectra obtained within a certain radius, larger than the seeing disc radius. In the case where it is suspected that the wings of the star image may be lost due to CCD size limitation, it is possible to use the AOB theoretical PSF (two gaussians) to estimate the amount of light lost. The correction curve, or spectrum, obtained is stored in a Throughput spectrum.
  


• Use :
  
  Click on Flux in the main menu, then on Compute flux correction. In the new window :
  
  
  • Input reference star datacube is the preprocessed datacube of the standard star. You can type it in directly, or use the browse icon at the end of the field, or drag and drop it from the Reduction folder. This must be a FLUX standard, and the absolute spectral flux of this star must be provided as a table :
    
  • Input flux reference table is the table where is recorded the outside atmosphere absolute spectrum of the standard star; same input possibilities as above. In this table the Wavelength column and Flux column are the labels of the two columns of interest, usually Lambda and F_Lambda (case unsensitive).
    
  • Output flux correction spectrum is the name the user wish to use for the file holding the flux correction curve.
    
  • Spatial summation radius is the radius inside which all the spectra will be summed up to get the total flux of the reference star .
    
  • Summation center (X,Y) is the center of this spatial summation. This center must have been obtained before. Use for instance the Analyse, Integrate spectra function (see the Analyse data : spatial or spectral integration section of the present manual). Integrate the spectra over the whole wavelength in the cosmics-removed standard star datacube, make an image with the resulting data, choose View result, and note the cursor coordinates of the centroid of the star image, as well as the radius of this image; an eye estimate is good enough for the present purpose. This will give you the Summation center, as well as the Spatial summation radius, which should be large enough to enclose ALL the star energy.
    


• Options
  
  
  • Use PSF estimate : checking this option means that the user is willing to use an AOB PSF estimate (made from two gaussians) to take care of light from the star possibly falling outside the CCD. It validates the following sub-options :
    
  • Core sigma is the sigma you adopt for the inner peak of the AOB PSF, while Halo sigma is the same parameter, but for the outer PSF halo. Halo/core ratio is the peak-to-peak ratio of the two.
    
  • Polynomial smoothing may be used on the correction curve, with a given Degree.
    
  • Debug : this switches the program to verbose mode, and more informations are recorded into the history file (see Getting started).
    
  • Save values : All the input values (files names, coordinates) are saved, and become the new default values for the user. They can be recalled at will, and are used each time the Compute throughput window is opened.
    
  • Recall values : The values (files names, coordinates) saved by the user, are loaded into the various input fields.
    
  • Default values : The input fields are set to the general defaults values; for instance, the file names are set to blank.
    
  
  

Apply flux correction

• Principles
  
  Each spectrum in the object datacube to be flux-calibrated is multiplied by the flux correction spectrum obtained during the preceding step. Care is taken of the different integration times and air masses.
  


• Use
  
  
  • Input object datacube : the object datacube as it is after wavelength calibration, flat-fielding, and cosmic rays removal. You can type it in directly, or use the browse icon at the end of the field, or drag and drop it from the Reduction folder.
    
  • Input flux correction spectrum : the correction spectrum just generated during the Compute throughput phase. Same input possibilities as above.
    
  • Output flux-calibrated object datacube : the result will be recorded there. This will be the end product of the reduction process for this exposure.
    


• Options
  
  
  • Debug : this switches the program to verbose mode, and more informations are recorded into the history file (see Getting started).
    
  • Save values, Recall values, Default values : as in the previous section.
    
  
  

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