Other extraction methods

In some cases, the source is not point-like. The spatial extent is calculated from the comparison between the PSF and the source spatial profile (see Pipeline steps). The optimal extraction of an extended source may bring a better signal-to-noise, but the flux level, the large-scale flux dependence with wavelength, and the relative strength of features at different wavelength can deviate significantly from the "real" values. For this reason, it is preferable to use another method for the partially-extended and extended sources.

Note: in version LR5 and up, the extraction method is automatically chosen and the alternative method and corresponding products are still available for comparison.

Tapered column extraction

Whether the source is found to be partially extended or not, the default spectrum is extracted with a tapered column. The tapered column extraction uses an extraction aperture whose width varies with wavelength. The following table gives the extraction window width for the reference wavelength:

SL2 6um 4px
SL3 8um 5.3px
SL1 12um 8px
LL2 16um 4.25px
LL3 20.5um 5.45px
LL1 27um 7.17px
Reminder: 1 SL pixel is 1.8", 1 LL pixel is 5.1"

The extraction aperture width also scales with the spatial extent to account for all of the source's flux. For a given module and order, the width is scaled by the extent parameter which provides the ratio of the source spatial profile FWHM to the PSF FWHM.

The flux calibration for tapered column extraction makes use of the default SSC flux conversion tables.

Note that while the flux might be closer to the truth when using tapered column extraction on a partially extended source, there is currently no satisfactory calibration for sources other than point-like sources and very extended sources (significantly more extended than the slit). When using a point-like source flux calibration on a partially extended source, the LL spectra contain more flux than the SL spectra spectra because the LL extraction aperture is bigger than the SL one.

Experimental flux calibration for partially-extended sources

For tapered column extraction, the spatial extraction window scales with the source extent so that all the source's flux inside the aperture is accounted for. However, the missing flux outside the aperture is calculated using a point-source assumption, for lack of a description of the source morphology. This is the default product provided by CASSIS. In some cases, there may be an offset between the SL and LL spectra, because the corresponding apertures have different sizes.

A somewhat more precise flux calibration can be achieved by calculating the fraction of the flux lost outside the aperture as a function of the source extent and as a function of wavelength. This wavelength-dependent, partially-extended source calibration is calculated for all CASSIS spectra but is only available on request. On first approximation, a circular symmetry is assumed for the source morphology, with a size corresponding to the source spatial extent derived by CASSIS (extent calculated for each module/order individually). An analytical PSF, convolved with a 2D Gaussian shape representing the intrinsic source shape, is used for this purpose.

Please contact the CASSIS team for more details.

Full slit extraction

For extended sources (as extended or more than the slit). The spectrum in this case should be in surface brightness units and the geometry/extent of the source well known to recover all the flux or to interpret correctly the spectrum. Not yet included in CASSIS.

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