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Stage 4: Flux calibration

Stage 4: Flux calibration

There are many ways to do this. This what I do for a reasonably simple case

Create a black-body model of the standard star

Look up the spectral type of your standard star. Each spectal type corresponds to a black body temperature. There's a table of these on the ukirt web pages. Have your DR software generate a black body spectrum at the appropriate temperature, on the same pixel and wavelength scale that your standard star is on.

Next, deduce the flux of your standard star at some wavelength which is on your spectrum. Usually you'd use the band centre, and you'd deduce the flux from either the known magnitude of the standard in that band, or the magnitude in some other band, determining the colour from the spectral type. Scale your black body spectrum so that it has the correct flux at this wavelength.

Create a sensitivity spectrum

Take the black body spectrum you've just made (which you've scaled to be in flux units, say W/m2/um), and divide it by the "observed spectrum" of your standard star (which is probably in counts per second or similar). This gives a sensitivity spectrum - ie the value at each wavelength is the flux needed to produce 1 count per second at that wavelength.

BUT, there's a small problem in that the standard star probably doesn't really have an exact black-body spectrum. It more than likely has some lines in it. You should be able to identify these lines in the sensitivity spectrum. Ask a local expert or your support scientist if you're not sure. Different types of star have different lines. try starting off looking for hydrogen recombination lines.

When you find a feature in the sensitivity spectrum that you think arrises from a line in the star, interpolate over it using the "continuum" either side of it.

Apply the calibration

Simply multiply your sensitivity spectrum by your "observed spectrum" of your target, to get a flux calibrated spectrum of your target.

Contact: Tom Kerr. Updated: Wed Oct 6 11:54:19 HST 2004

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