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4.2.2 Flux Calibration and Atmospheric Line Correction
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For every EXES science observation, the EXES temperature-controlled black body source and a nearby blank sky field will be observed. From these, a calibration spectrum will be constructed that, after division over the science observations, will correct for response variations, and provide flux calibration. In principle, division by the calibration spectrum would also correct for telluric absorption lines (see Lacy et al., PASP volume 114, issue 792, p. 153), but this is presently not the case because of the large difference between the blackbody and sky temperatures. The flux calibration is expected to be better than 20%, but the true accuracy is currently uncertain. Experiments focused on line profile information and those that can normalize the continuum level, or use past observations for setting the continuum, will likely be more successful. Projects requesting a telluric calibration object, in particular those observing lines near strong telluric features or those observing relatively broad lines, will need to include the observation time required in their proposal. Because of the difficulty of scheduling a given telluric calibrator with the science target in a given flight, the specific calibrator will need to be chosen at the time of flight planning in consultation between the program GI, the instrument PI and the SMO support scientist. For wavelengths below 8-10 μm this will most likely be a hot, bright star (e.g., Vega or Sirius) and at longer wavelengths an asteroid. Galilean moons will also be considered, provided they are well separated from Jupiter.
For the proposal, a separate observation entry should be entered via SPT with name "Cal_target", where "target" is the name of the associate science target (i.e. "IRC+10216" and "Cal_ICR+10216"), and given the coordinates RA:12:00:00, Dec:+90:00:00. The observing time for such a telluric standard observation will depend on the instrument configuration and wavelength observed, as well as on the signal-to-noise level needed.
Proposers must use the EXES ETC to estimate this, assuming a continuum brightness of 100 Jy below 10 μm and 150 Jy above 10 μm for the High_Medium and High_Low configurations. For the Medium configuration, a brightness of 50 Jy should be assumed, and for Low, 25 Jy at all wavelengths. Proposers are urged to limit the EXES clock times on the telluric standard at a given wavelength and instrument configuration to less than about 30 minutes. Further improvement of the removal of telluric absorption features may be achieved by employing models of the Earth's atmospheric transmission.