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Limiting magnitudes for instrument and internal wavefront sensors
GPI limiting magnitudes are determined by several components, the AO WFS (I-band), the LOWFS (H-band), and the IFS (selected filter). In addition the observing conditions add another layer of limits. Thus the brightest of the science object is limited in I band from the AOWFS, in H-band from the LOWFS (not a constraint in DIRECT mode as then no coronographic mask is used and no LOWFS is possible). It should be noted that it is NOT possible to observe without the AOWFS and Coronographic modes are NOT possible to observe without the LOWFS.
|Maximum brightness [mag]||Minimum brightness [mag]|
*Only valid in Coronographic mode, in the Direct and the NRM modes there is no LOWFS and thus no constraint imposed by the LOWFS.
The selected observing mode is strongly affecting the brightness of the target that can be observed with the IFS without saturating in the selected science wavelength by the IFS. The principal four modes (each that can be done in all the filters Y, J, H, K1 and K2) are Coronographic Spectroscopy (the ''-coro'' modes), Coronographic polarization (the ''-coron-pol'' modes), Direct (the ''-direct'') observations with either spectroscopy or polarization, and the NRM (the ''-NRM'') mode observations in either spectroscopy or polarization. Note that this limit is applicable to the relevant science wavelength.
|Observing Mode||Maximum brightness [mag]|
*Only valid in Coronographic mode, in the Direct and NRM modes there is no LOWFS and thus no constraint imposed by the LOWFS.
GPI is a XAO instrument and the stated performance is reached under IQ70 and CC50 conditions. It is possible to use in IQ85, CC70 and CC80 conditions but it should be noted that the performance is highly variable for these conditions and NO guarantee on performance can be given. Observations have been done under both IQ85 and CC80, and taking the limits into account the instrument is safe to use, the main risk is low light levels causing the various WFS systems to guide on noise which could damage the instrument. Note that ANY observations where the effective limit of the AOWFS is passed is NOT allowed.
The given magnitude limits for the AOWFS and the LOWFS (if used) should be modified in the following way for worse than IQ70 CC50 conditions:
|Observing conditions||Decrease of faintness limit [in magnitudes]**|