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Second step-for Filippenko & Greenstein data

The most direct way to deal with data where the published values represent fitted continuum values is to fit a continuum to your observed data, interpolate the tabulated data, and divide the two resulting spectra to get a calibration spectrum.

Fitting a continuum is a task that is not easily automated. The simplest way in Figaro is to display the spectrum and then use `cfit'. For example:

   ICL> splot standobs reset accept
   ICL> cfit  output=standfit

will enable you to use the graphics cursor to indicate continuum points on the displayed spectrum and thus generate the continuum spectrum by spline interpolation. A messy job, but not one you have to do often. Remember that the messy parts of the flux calibration are connected with generating the calibration spectrum, and you only have to do that once; applying it is simple.

The published points represent a smooth curve, so just interpolating directly between them should be quite satisfactory. That is, there should be no need to edit the spiketrum generated by `gspike' prior to using `interp', although `spied' is always available if necessary. This time, since HD 84937 is an Oke & Gunn standard, let's use G158-100 instead. So the first two steps will be

   ICL> gspike spectrum=standobs table=g158m100 spiketrum=gmspike
   ICL> interp spiketrum=gmspike spectrum=gmfit

generates `gmfit' as the interpolated spectrum from the published points.

Dividing `gmfit' by `standfit' will now generate the required calibration spectrum. Note that `idiv' will not do, since the division has to allow for the wavelength range of each element, and for the exposure time. The command `caldiv' must be used instead.

   ICL> caldiv standard=gmfit spectrum=standfit output=calib

will generate `calib' as the required calibration spectrum.



next up previous 78
Next: An alternative second step-Filippenko & Greenstein data
Up: Flux calibration
Previous: Second step-for Oke & Gunn data

FIGARO A general data reduction system
Starlink User Note 86
Keith Shortridge, Horst Meyerdierks,
Malcolm Currie, Martin Clayton, Jon Lockley,
Anne Charles, Clive Davenhall,
Mark Taylor, Tim Ash, Tim Wilkins, Dave Axon,
John Palmer, Anthony Holloway and
Vito Graffagnino
2004 February 17
E-mail:ussc@star.rl.ac.uk

Copyright © 2008 Science and Technology Facilities Council