The AO module is made up of the two deformable mirrors (DM1 and DM2), respectively placed in a collimated and convergent space through the doublet Doa1. DM1, conjugated with the pupil, is set up in a collimated space. DM2 is conjugated with an altitude that we would like to be variable in order to test different configurations. This mirror is placed in a non-collimated space. We do not want DM2 to vignet the considered field whatever its conjugation altitude.
Principle
A conventional configuration, presented in figure 1, consists in placing the mirror in altitude within a convergent beam, retaining the pupil size of the beams whatever the conjugation altitude. Thus the useful surface of the mirror decreases with the conjugation altitude. The constant envelope device, shown in figure 2, was proposed in order to benefit from the whole surface of the mirror at altitude. The marginal ray passing the edge of the pupil in the collimated entry space, corresponding to the extreme half-field, passes through the primary focal point of the doublet Doa1. A limitation of such a system is that the field has to remain smaller than the extreme field, pre-defined when the test bench is defined.
Figure 1: Principle of a conventional assembly for the conjugation of the mirror with altitude. The size of the pupil is maintained but the useful surface of the mirror decreases with the conjugation altitude.
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Figure 2: Principle of the constant envelope and pupil footprint on DM2
