Updated Flux Error Calculations for CalCOS
Abstract
Recent reports indicated that the flux errors provided by CalCOS version 3.3.9 and earlier versions may be overestimated for low signal-to-noise ratio (S/N) COS FUV/NUV spectra. An investigation into this phenomenon confirmed that the error calculation method, which was based on the analytic approximations provided by Gehrels (1986), was not implemented as intended. Starting with CalCOS 3.3.10, flux errors are now calculated using a numerical approximation of the Gehrels (1986) analytic functions, and asymmetric errors are reported for X1D, X1DSUM[N], and X1DSUM files. The new method provides significantly reduced flux errors for low S/N spectra while leaving higher S/N data mostly unaffected. Four new columns are generated by CalCOS 3.3.10 for output 1D spectra, including a lower flux error and the three terms that are used to calculate the flux errors. For consistency with previous reductions, the ERROR column remains the upper flux error. The new columns increase transparency and permit CalCOS users to apply custom error calculations when desired.
- Publication:
-
Instrument Science Report COS 2021-03
- Pub Date:
- April 2021
- Bibcode:
- 2021cos..rept....3J
- Keywords:
-
- HST;
- Hubble Space Telescope;
- STScI;
- Space Telescope Science Institute;
- COS;
- Cosmic Origins Spectrograph