SPICE User's Guide - IRSA

SPICE User’s Guide
The uncertainty defined in the input uncertainty plane is propagated into the extracted output
spectrum. The output spectrum table includes the order, wavelength, flux (in electrons per
second), propagated error, and cumulative pixel status for each wavelength. The cumulative pixel
flag is a bit-wise OR of the flags of contributing pixels.
2.5.2 Extended Source Extraction
In addition to the standard point source extraction, the Extract module in SPICE offers the users
the ability to estimate the surface flux for particular extended sources.
The calibration assumes that the source is infinitely large in the sky, and that at each wavelength
the source is uniform. Starting from the extracted spectra, two corrections are applied: one for
light losses due to the extraction aperture, and another for light coming in and out of the slit due
to diffraction. We describe each in turn here. A more formal discussion is provided in the IRS
Instrument Handbook.
In the calibration of point sources, SPICE uses an expanding aperture for the low-resolution
modules, and the full slit for the high-resolution ones. For extended sources, a straight-edged 28pixel
extraction aperture is used for the low-resolution modules (which allow us to ignore the
noisy edge pixels). The full slit is used for the high-resolution modules. Appropriate calibration
coefficients (“fluxcon values”; see §1.5) are applied to the low-res modules to account for the
large extraction window and are provided in b[0,2]_aploss_fluxcon.tbl. These fluxcon values
assume a point source centered on the slit. For a point source, the difference in e-/s between the
point and extended source apertures is at most ~10%. When the user clicks the extended source
button, the fluxcon table for the large aperture is applied to convert from e-/s to Jy. Note that no
such correction is necessary for the high-resolution modules -- they will have the same fluxcon
table for the point source and extended source calibrations.
The slit losses are a function of wavelength. For point sources, the flux lost due to the PSF wings
outside the slit is added back by comparing the signal to a model. For a source that is uniform in
flux at a given wavelength and infinitely large, the light lost from the slit is exactly matched by
the light gained by the slit. Because the point source calibration adds the flux from the wings
outside the slit in order to derive the final calibration, this calibration must be "undone". To
understand the wavelength dependent loss of light, the point spread function was simulated using
a STinyTim model with the slit sizes quoted in the IRS Instrument Handbook. The slitloss
correction was taken to be the ratio of the flux within the slit of a simulated PSF, to the total PSF
flux. The slitloss corrections are given in (b[0,1,2,3]_slitloss_convert.tbl), which have the
wavelength dependent factor by which the spectrum of the extended source should be multiplied.
To convert the resulting spectrum in Jy to a surface brightness, the user should divide it by the
area of the aperture in the sky (or the area of the extraction window, if the default was not used):
SPICE Modules 28 Extract