LOW FREQUENCY FLAT-FIELDING

• Compute flat field correction curves
• Apply flat field correction

Compute flat field correction curves

• Principles

  The spectra which are now recorded into the TIGER datacube suffer from two (related, but separated in the present reduction program) flat field effects. One is the high frequency flat field effect, due to pixel-to-pixel response variation on the CCD, the other is the low frequency flat field effect, due to non-uniform transmission of the optics, both spatial and spectral. This last one is dealt with here. To get rid of this effect, a GUMBALL continuum lamp exposure is used (usually the one which has already been used for the spectra ridges search). The continuum spectra must be wavelength calibrated, and the cosmics impacts removed. They are divided by the median integrated value of the central spectra to get them near unity. To eliminate any non-uniformity of the GUMBALL field, a sky spectroscopic frame is optionally used as a reference to spatially flatten the data. For that, each sky spectrum is summed over -almost, see below- the entire wavelength range, and the resulting scalar used to normalise the corresponding GUMBALL spectrum. All the spectra are then medianed, and a spline fit is performed on this median to eliminate any high frequency residual. The resulting spectrum is the low frequency flat field reference spectrum. A new set of spectra is then computed, by dividing each continuum spectrum by this reference spectrum. A new datacube is so created, holding the low frequency flat field correction curves (somewhat misleadingly called spectra); it will appear as Flat correction datacube in the reduction folder display.
  



• Use

  Click on Flat in the main menu, then on Compute flat field. In the new window :
  
  
  • Continuum datacube is the name of the wavelength calibrated (and cosmics-removed) GUMBALL continuum datacube,i which should be displayed as Wavelength calib continuum datacube. or Cosmics removed continuum datacube. 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.
    
  • Sky flat datacube is the name of the wavelength calibrated sky flat datacube, which should be displayed as Wavelength calib sky flat datacube. or as Cosmics removed sky flat datacube. It is used if you check the left button. Same input possibilities as above.
    
  • Reference flat datacube is not a topologist's nightnare, but the datacube to be created to record the flat reference curves ("spectra"). Same input possibilities as above.
    



• Options

  • Smoothness parameter : to be used by the spline fit of the reference spectrum. Zero means that each pixel is taken as a node; that is, there is NO filtering. Five to seven is usually OK, with the fit going nicely through the noise. Greater values increase the level of smoothing. Note that non-integer values are allowed.
    
  • Fraction of wavelength range : the central fraction of the wavelength range to be summed up to compute the sky flat spatial correction factor.
    
  • Filter radius : used in the ... phase.
    
  • Debug : this switches the program to verbose mode, and more informations are recorded into the history file (see Getting started).
    
  • Lens number : only lens/spectrum N will be treated; this is very useful as a first step, to tune the parameter values before performing the complete operation.
    
  • Output spectrum : enter here the name of the file which is to receive the low frequency flat field correction curve (somewhat misleadingly called spectrum) for spectrum N.
    
  • Save values : All the input values (files names, coordinates) are saved, and become the new default values for this user. They can be recalled at will, and are used each time the Compute flat field table 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 flat field correction

• Principle
  The low-frequency flat field correction computed during the preceding step is applied to the wavelength calibrated object datacube, and a new datacube is created, holding the wavelength calibrated and flat fielded spectra of the object. This new file appears as Flat fielded object datacube in the reduction folder display.
  


• Use
  
  Click on Flat, in the main menu, then on Apply flat field. In the new window :
  
  
  • Datacube to be flat fielded : enter the name of the wavelength calibrated object datacube. 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.
    
  • Flat correction datacube : enter the name of the datacube holding the low frequency flat field correction curves, which was created during the previous reduction step. It is displayed as Flat correction datacube in the Reduction folder display. Same input possibilities as above.
    
  • Output datacube : enter the name of the datacube where you wish to get the final result recorded. Use some significant name, where the object name, exposure number if there are several, do show up; same input possibilities as above.
    
  
  
• 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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