Data Reduction Frequently Asked Questions
Why do my flux-calibrated data not agree with independent photometric measurements?
In order to have a true absolute flux calibration, several requirements need to be fulfilled:
- All flux of both the target and the standard star needs to collected, i.e. a wide slit has to be used.
- The transparency does not change between the observations of the target and the standard star.
With respect to the first condition one should keep in mind that the flux standard stars are observed with a 5'' wide slit, while science data are typically observed with slit widths of 0.8'' to 1''. For a seeing of 0.8'' a slit width of 0.8''/1.0'' means that some 33%/24%, respectively, of the target flux are lost. This would result in a too low flux for the flux calibrated spectrum of the target.
If the second conditions is not fulfilled, i.e. the standard star or the target or both are observed under non-photometric conditions (e.g. CLR or THN) their observed flux will be lower than it should be. If the standard star is observed under photometric conditions but the science target is not the flux in the flux calibrated target spectrum will be too low. The opposite happens if the target is observed under photometric conditions but the standard star is not. CLR/THN conditions allow for transparency variations of 0.1/0.2 magnitudes.