==== Absolute flux calibration ====
=== Responsible ===
Observer

=== Phase ===
Night time

=== Frequency ===
Max. once per night per instrument mode

=== Purpose ===
Determine instrumental zeropoint / flux conversion factor (Jy/cts/s)

=== Procedure ===
  * Observe a (spectrophotometric) standard star (see Appendix for a discussion about applicable standard stars)

=== Pipeline data products ===
  * Raw-reduced image / spectrum of the (spectrophotometric) standard star
  * Photometric zeropoint / flux conversion factor

=== Prepared observing blocks ===
(tbd)

=== Prepared templates ===
  * [[METIS_img_lm_cal_standard|METIS_img_lm_cal_standard]]
  * [[METIS_img_nq_cal_standard|METIS_img_nq_cal_standard]]
  * [[METIS_spec_lm_cal_standard|METIS_spec_lm_cal_standard]]
  * [[METIS_spec_nq_cal_standard|METIS_spec_nq_cal_standard]]
  * [[METIS_ifu_cal_standard|METIS_ifu_cal_standard]]

=== Pipeline support ===
  * Raw-reduce data according to instrument mode (image, spectrum, IFS)
  * Using a tbd database of known standard stars, determine the photometric zeropoint / flux conversion factor

=== Duration ===
Like a normal observation of the respective observing mode, approximately 30 min (tbc)

=== Prerequisites ===
  * clear skies
  * access to telescope

=== Required Accuracy ===
  * Relative spectro-photometric accuracy over an entire atmospheric band: 10%
  * Absolute line flux accuracy: 30%
  * Relative photometric accuracy: 1%
  * Absolute flux calibration: 5%

=== Expected Accuracy ===
Limited by (our limited knowledge of) atmospheric variations, uncertainties in the standard stars themselves.



=== Notes ===
  * Additionally there is the requirement (METIS-1203) to achieve the photometric stability with just one on-sky flux calibration per night per mode
  * For high accuracy observations, we expect that the user will demand (on his own time budget) a dedicated flux calibration, in addition to this (observatory-provided) flux calibration.
  * Open questions:
    * (How well) can we predict the change of the ADU/Jy conversion factor within a night from environmental parameters or from the science spectrum / aperture photometry itself?
    * Can we estimate the sky opacity from the atmospheric emission? And can we infer the atmospheric transmission in a certain filter (i.e. the ADU/Jy conversion factor) from this? In the absence of scattering, there should be an anti-correlation between the background level (atmospheric emissivity) and the conversion factor (atmospheric transparency).