corresponds to that of the telescope, and we shall see in the following that another OTF may be preferred. For instance, the diffraction pattern of the telescope spider may pollute the final image and hinder the detection of faint objects in the vicinity of a bright one.

Another drawback of this OTF is that low spatial frequencies dominate the transfer function. In photon counting mode mode, the few available photons are then mainly spread in the (less useful) low frequencies.

As the limited dynamic range of detectors such as the CP40 imposes the use of neutral densities to reduce the photon flux for bright objects to only a few hundreds per frame(§4), they always work in photon counting mode. This limits the performances of the restoration methods even for bright objects. This is the reason why the next mode could be preferred in that case.

[*3.2. Masked pupil speckle imaging and aperture synthesis*]{}

10000 By inserting masks into the pupil plane (P1) (cf. Fig. ), the pupil function (and thus the OTF) can be modified as desired. For instance the spider diffraction pattern can be removed by placing a Lyot’s mask or a four-hole mask which carefully avoids the shadow 