If a stretcher is in air and a compressor is in vacuum, then the atmospheric pressure variation by 10-20 Torr can substantially reduce peak power due to changes in the index of refraction of the air .This effect is the stronger, the larger the stretcher dispersion, the pulse bandwidth, and the angle of reflection from the first grating.For a super-Gaussian pulse, a small dispersion has approximately three times the effect on pulse power as it does on pulse duration; at the pulse stretching by 5% the power reduces by 15%.Placing the stretcher in a hermetic chamber one can not only eliminate this parasitic effect but also control dispersion without mechanical distortions.The change in the stretcher/compressor dispersion caused by changes in n may be compensated for in all dispersion orders by varying the angle of incidence in a plane orthogonal to the diffraction plane.
Efim Khazanov (Fri,) studied this question.