SKY SUBTRACTION

• Define the empty sky area
• Compute the sky spectrum over the empty sky area
• Subtract the sky spectrum from the object spectra

Define the empty sky area

• Principles :
  
  Using an image of the observed field, reconstructed from spectroscopic data from a datacube of the same family, the user selects graphically a zone which is supposed to be empty, and so is able to give the pure sky spectrum. This region is then translated into a selection to be made over the spectra set, in fact into the table associated with all the datacubes which derive from the same raw object datacube. This selection will be used during the next step (Compute the sky spectrum) to restrict the spectra summation over the selected spatial area.
  


• Use :
  
  
  • Prepare an image of the field, to be used as a coordinates reference. For that, from the main menu, click on Analyse, then on Integrate spectra, and output an image corresponding to a whole wavelength range spectral integration; use a cosmics removed datacube as data base, to avoid spatial ghosts due to cosmic impacts. The datacube chosen must be of the same "family" as the datacube you will to subtract the sky from; that is, it must be associated with the same initial fits table. It is not necessarily the datacube to be sky-subtracted, although this one is usually a good choice. But you may, after a complete reduction, discover on a line-only reconstructed image that in the area you elected as empty, there are some emission blobs. It is then time to redefine the sky area on this last image, and redo the computations, starting with the sky subtraction function. For image reconstruction, see details in the Analyse data section of this manual.
    
  • In the main menu, click on Sky, then on Define sky area; the RTD interface pops up. Click on File, Open, and chose the image you just computed.
    Click on Auto set cut levels to get a better view of the low levels, and play with the zoom button to make things completely comfortable.
    Click on Select, then on Select an area. A square, with four conspicuous nodes at the summits, appears. You can move it (center mouse button depressed), drag a node (left button depressed), add more nodes (right click). Envelop the empty zone you whish to choose as the sky area.
    
  • In the RTD menu, click on Select, then on Apply selection to. In the window which pops up, enter the name of the table you whish to select. It is usually the table which has been created at the spectra extraction time, and is associated with all the datacubes created since that by the various reduction steps already done. Select the New action, which means that you are going to get rid of any previous selection. Then click on Accept.
    
  • You may want to enclose in a polygon the object itself, and define as the clear sky region the outside of the polygon. In such a case, select the Use area outside polygon option.
    
  • If you want to use as sky area the reunion of several zones, repeat the above point (no need to close RTD, just drag the previous polygon to the new location, reshape it, and reselect the table; if you think there are too many nodes on this previous polygon, erase it with the [Select]/[Clear selection] function and define a new one by [Select]/[Select an area]). But when you are about to Apply selection to, choose the Join action, so the previous selection will be kept, and the new zone added to the previous one(s).
    
  • You may combine at will New, Join, Intersect, Inside and Outside to satisfy your needs. For instance, in a field where appear only two stars, enclose one of the stars in a polygon, apply selection with New and Use area outside of polygon; then, enclose the other star in a polygon, and apply selection with Intersect and Use area outside of polygon.. Another interesting possibility is to define two selections on two different images (for instance one reconstructed from the continuum, the other from lines), and Intersect them.
    
  • To Check the final selection, it is a good idea to plot the selected table, just to be sure you did not get lost in the Inside/Outside/Join/Intersect jungle... In the main menu, use the Display item, choose Spectrum and table. Then File, Plot table. Give the name of your table, and chose X taken from column = A, Y taken from column = D. If you like to see the lens numbers, choose Label each data point using column = no, and click Plot with label instead of Plot.
    


• Important warnings :
  
  
  • RTD is unable to make a selection over a very small area, due to the fact that each polygon summit has a circular "dead" neighbouring where no point can be selected. If the summits get very close, their neighbourings overlap and no place is left for any lens to be selected. Approximately ten lenses must be enclosed within the polygon to get them selected.
  • Once the table has been selected (i.e. restricted) by the above described function, it remains so. If you want to regain access to the complete son datacubes, you must deselect the table, using the [Tools]/[Table access]/[Table select] function. Otherwise, some selection-aware functions may perform on the fraction of the datacube corresponding to the table selection. Not all functions take into account table selection, but this is a risk.

Compute the sky spectrum over the empty sky area

• Principles :
  
  The "mean" spectrum of the area selected in the previous step is computed, and will become the sky spectrum from the corresponding frame.
  


• Use :
  
  
  • Click on Sky, then on Compute sky spectrum. The input window pops up.
    
  • Enter the Input datacube, which is usually the datacube with cosmic rays removed, and the Output spectrum name, that is the name of the resultant sky spectrum. You may type them, or use the browse icon at the end of the field, or drag and drop them from the Reduction folder window. Click then on Accept.
    
  
  
• Options :
  
  
  • Method : You may choose Histogram, Median, or Mean. The two last do what you expect, and the first one (default) uses an algorithm which, to make short a long story, computes the common part of the spectra in the area, which is by definition the true sky spectrum.
    
  • 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 sky spectrum window is opened.
    
  • Recall values : The values (files names, coordinates) saved by the user, are loaded to the various input fields.
    
  • Default values : The input fields are set to the general default values; for instance, the file names are set to blank.
    

Subtract the sky spectrum from the object spectra

• Principles
  
  Straightforward : the sky spectrum, evaluated from the same frame, is subtracted from every spectrum of the input datacube. This datacube must have been flat-fielded before.
  


• Use
  
  
  • In the main menu, click on Sky, then on Sky subtraction; the Subtract sky spectrum window pops up.
  • Enter the name of the Input datacube from which you will to subtract the sky spectrum, the name of the file holding the Sky spectrum (which you obtained in the preceding step Compute sky spectrum), and the name of the output sky-subtracted datacube which will be created.
  • Click on Accept.


• Parameters
  
  
  • Normalisation factor : provided here just to allow the formerly hopeless astronomer to use a sky spectrum computed on another frame, with a different exposure time; not very clean, but as long as nobody knows ;-)... Should stay at 1.0 for "normal" operation...
    
  • Debug : the program is switched to verbose mode, and more informations are recorded into the history file (click on Getting started in the left menu for details).
  • 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 Subtract sky spectrum window is opened.
    
  • Recall values : The values (files names, normalisation factor) saved by the user, are loaded to the various input fields.
    
  • Default values : The input fields are set to the general default values; for instance, the file names are set to blank.
    

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