Error propagation

Throughout the CASSIS pipeline, two kind of error are considered, statistical errors (root mean square; RMS), and systematic errors.

RMS error

For all the steps until the spectra combination, the statistical (RMS) errors are fully propagated using the standard equation:

s = \frac{ \sqrt{ \sum_{i=0}^N w_i^2 \sigma_i^2 } } { \sum_{i=0}^N w_i },
where w is the weight and sigma is the uncertainty. Weights throughout the pipeline are 1/sigma^2

Systematic error

The combination of the spectra include an additional - systematic - error to account for the flux difference between the two nod spectra (which is usually due to the presence of other sources in the slit). The 2 errors (RMS and systematic) are given in 2 different fields in the output files.

The second main source of systematic offset uncertainty is the flux calibration itself. The corresponding uncertainty depends on the wavelength, the template used, the RSRF dispersion, etc... This uncertainty is not estimated in version LR4 of the CASSIS spectra.

Total error

LR5 version information

v5 spectra provide the total error in the 3rd column. The actual error bar is the linear sum (not quadratic) of the RMS error (extraction statistical error), systematic error (systematic differences between nod spectra), and calibration error (offset uncertainty).

LR4 version information

When downloading the VO spectrum or when importing spectra in SMART, the error bar shown will be the RMS error only. In order to get the total error, you need to add the errors manually from the FITS files.

Format of downloaded spectra

The spectra downloaded in LR4 include the following columns:

  • wavelength, flux, rms error, systematic error

The spectra downloaded in LR5 and up include the following columns:

  • wavelength, flux, error (rms+systematic), rms error, systematic error, offset uncertainty due to calibration

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