Corrected grating recording by active optics compensator: case of the HST Cosmic Origins Spectrograph

Recording methods for making aberration corrected holographic gratings are drastically simplified by using a multimode deformable mirror (MDM). For this, the plane MDM acts upon one of the two recording beams. The MDM design is derived from a vase form having a constant rigidity on its clear aperture and a perimeter ring of higher rigidity linked to several external radial arms. It is shown that MDM compensators provide easily the superposition of many interesting active optics modes that we have named Clebsch-Zernike modes . These modes belong to a sub-class of the triangle aberration mode matrix. They are generated by only applying a uniform loading onto the rear side of the clear aperture and/or axial forces and radial moments onto the outer ring. In addition the spherical aberration provided by a uniform loading, the Clebsch-Zernike modes are distributed along the two lower diagonals of the triangle matrix. Several six-arm MDMs, and twelve-arm MDMs have been built and experienced. Due to the huge simplification for the design of optical systems as compensators , such a recording method is to be considered as a universal one. As an example of the method, the recording of three concave holographic gratings of the HST Cosmic Origins Spectrograph has been investigated. The result is that much higher order aberrations can simultaneously be corrected. Compared to the design of the COS team, the residual blur of monochromatic images along the spectra occupy areas in average 30 times smaller -- as well in term of pixel number -- decomposing as follows: (1) gain of 10 times in spectral resolution and, in the orthogonal direction, (2) gain of 1 -1.2 in limiting magnitude .